connect car battery to UPS

[moon]

Hi , Any electrical eng. /techie can answer these questions? I have a small UPS-APC (15min-now giving me only 5 min.) for my Desktop PC.1.Can i connect(in fact i already connected) a spare 12v car battery i am having in place of the UPS battery(12v,7ah)?2.Is this connection possibly ok only when i am burning a cd/dvd?(Then connect the normal UPS battery)3.Will the car battery get charged with the small(?) current of the UPS if i leave it connected continously(i.e. only when PC and/or UPS is on)?4.Any chance of damage to UPS in both cases(2 and 3)?5.I disconnect the +ve terminal of the car battery when the PC and UPS are Off.a.Is it ok to keep it connected?Thanksmoon

[AbeL]

Hi Moon:What you want to do sounds good. The battery used in a UPS is jelled lead-acid to keep it from leaking the sulfuric acid all over the floor, so the chemistry of it and your car battery are the same. My only concern would be whether the charge rate to trickle charge would be low enough not to cause the electrolyte to boil (gut says it will be, otherwise the "normal" battery would also have a problem). Try leaving the car battery always connected under closely surpervised conditions and with a plastic container under it. If you see no acid leaking out after 2-3 hours it should be ok. If you can spare the plastic container, leave it under the battery permanently.Good luck,Abe

[Cluttermagnet]

Hi Moon:What you want to do sounds good. The battery used in a UPS is jelled lead-acid to keep it from leaking the sulfuric acid all over the floor, so the chemistry of it and your car battery are the same. My only concern would be whether the charge rate to trickle charge would be low enough not to cause the electrolyte to boil (gut says it will be, otherwise the "normal" battery would also have a problem). Try leaving the car battery always connected under closely surpervised conditions and with a plastic container under it. If you see no acid leaking out after 2-3 hours it should be ok. If you can spare the plastic container, leave it under the battery permanently.Good luck,Abe

I think all that Abe had to say is essentially correct. My one concern would be whether the recharging circuit in the UPS is up to the strain of recharging a deeply discharged automobile battery if you run the UPS from it for many hours. I have no misgivings about temporarily running the UPS from a fully- charged car battery. If I were to try this experiment, I would tend to just assume that the UPS can't handle charging the car battery, and I'd make arrangements for recharging it using a different charger, probably a simple one I cobbled together from various parts I always have on hand. Worst case, you might end up frying components in the UPS charger circuit, rendering it useless so far as its conventional use with the smaller, internal battery. This would depend very much on exactly what sort of circuit does the recharging and maintains the small 'float charge' current across the fully- charged battery. All batteries self-discharge if not kept 'topped off' by some small, steady 'float' current after being fully recharged. I figure some UPS charger circuits might handle recharging the car battery just fine, others might be overtaxed and eventually fail. I suspect odds do not favor the UPS internal charger handling the bigger battery OK.All lead-acid batteries have fairly similar characteristics so far as recharging, but there may be some differences between the sealed gelled electrolyte types and the wet acid automotive types. A battery's size and amp-hour rating sets the upper limit to how large an initial charge current can be tolerated. I don't think there is all that much of a minimum charge current requirement, however. Given enough time, even a 'wimpy' charger circuit can recharge a bigger car battery. So long as a charger does not try to push too much current through a deeply discharged battery, and can supply an appropriate float current at the end of charging, all will be fine- but using the 'wrong' charger circuit can definitely lead to overcharging or undercharging, both of which which can drastically shorten the battery life. Often charging circuits are designed very specifically for only certain battery types within a certain limited range of ampere- hours.Remember, most UPS units are only designed to give you some minutes of 'holdup' before their outputs sag and they switch themselves off line. It is meant to give you enough time to close programs and save data, not to run all night on. That is why they all come with that auto- shutdown software and a serial port to hook to your computer. I never bothered to set up the software shutdown myself. I know if the power fails when I am not here, the UPS will just drop power after a few minutes.Yes, I have often thought about running this particular experiment myself. How I would do it is to fit the UPS internal wires connected to the internal battery with a properly polarized connector with a good, high contact current rating. Then I would put mating connectors on both the small internal gel cell battery and on the big car battery. I would generally reconnect the car battery to its own separate charger after 'running it down' and reconnect the smaller battery to maintain that brief backup capability that allows enough UPS run time to get your system shut down upon losing the mains power. Although I'm not sure, I figure I would likely maintain the small internal battery on its own charger circuit, with the car battery nearby and on its own float charger. I would have to think a little about the possibility of losing mains power during a CD/DVD burn. I guess if I found myself doing that fairly often and my mains power was pretty unreliable, I would have to give some serious thought to designing a manually- operated rapid switchover circuit to swap batteries in real time _after_ a mains failure. It can probably be done fairly simply using a relay with some beefy contacts plus a large 'holdup capacitor' to keep the DC voltage from dropping during the few milliseconds during which the relay contacts are 'in flight' between the smaller and larger batteries.I would be very hesitant to automate this switchover operation early on, until I had acquired a lot more trust and confidence in the arrangement. When things go wrong with 12V battery switching circuits, they can go badly wrong. A car battery, in particular, is nothing to mess around with casually. Set up certain types of unintentional short circuits across one of them, and you might heat conductors enough to get a nice little house fire started. No CD/CDR burn session is important enough to risk burning your house down. I doubt I personally would ever trust such a setup to the point of letting it run unattended. There is a reason why many electronics manufacturers go to all that effort and spend all the money to get their products UL Listed or Recognized. It still won't totally guarantee a product can't burn someone's house down, but it does put you in a lot better position if the survivors take you to court later. I very much doubt whether UL/ CSA/ VDE/ TUV etc. would ever sign off on this sort of 'ad hoc' switching arrangement. There's just a bit too much at stake with a battery that is powerful enough to weld metals together or vaporize short circuit conductors. Larger batteries should be shown all due respect.

[jimupnord]

There may be a chance of damage to the UPS if it is running off of the battery for too long. It could possibly overheat. I know when you get a 300 Watt inverter to do essentially the same job, it will most likely have a cooling fan in it.

[FuzzButt]

We used a UPS with a similar design when I worked for a local Drug Store Operations center.In essence it was 20 large Ford type automotive batteries. Run through a high strength 240VAC invertor. The batteries were run in series to get the voltage from 12 to 240 and I think they were also wired in parallel to get the amp/hour rating up higher. They would need to run a bunch of equipment (IBM Mainframe and AS/400 and the communications equipement and storage array) for 15 minutes till the generator kicked on and also be able to supply constant voltage incase of sags.Your issue is you have a 12V car battery that has a non sealed case is water based instead of the safer gel based. So compaired to the small battery in your UPS it is more prone to leaks. Also the amp hour rating of your small battery is 7a/h where as the automotive might be 80a/h (might be off by a factor of 10 depending on the battery)(The CCA or cold cranking Amps is not the measurement I was thinking about but the reserve time). The Automotive and/or marine type battery will require much different charging than the UPS battery. I would not worry about overcharging so much as undercharging or total failure of the charge circuit. The UPS battery may only need an hour of charging after being drained and charges at maybe 1/2 an amp where as the automotive will need 2-10 amps to charge back from discharge and require 12 hours or more. The Deep Cycle Marine batteries are even more of a beast. They may reuire even more complex charging requirements and do not like total discharges at all. I have killed a deep cycle battery from improper charging and total discharge. Finally we come to heat. How effective the invertor is to converting the 12VDC to 120VAC is not as high as you might think. There is a lot of waste in the form of heat. This is ok for a 5 to 15 minute rundown but I bet the temperature without active cooling for long periods are enough to kill it. Before you burn your house down or cause serious injury to your self go and purchase a fire extinguisher and a much large capacity UPS for your needs. If you really want total system keepup I would build a poor mans power station. Using several Marine Deep cycle batteries wired in parallel (to keep voltage at 12VDC) connect them to a 1500W or better DC-AC power invertor (a good Marine supply store will have them for large boats and sailboats for $500 or so). Build an enclosure for them and add the approiate charging circuit and fans to keep it all cool. Now the output of your new invertor can be run to a switching device to allow it to apply power when/if the utility drops. It is complete overkill but that is what I would do and is similar to what Solar/Wind/Generator homes are equipped.Good luck.Chris

[Cluttermagnet]

Before you burn your house down or cause serious injury to your self go and purchase a fire extinguisher and a much large capacity UPS for your needs. If you really want total system keepup I would build a poor mans power station.  Using several Marine Deep cycle batteries wired in parallel (to keep voltage at 12VDC) connect them to a 1500W or better DC-AC power invertor (a good Marine supply store will have them for large boats and sailboats for $500 or so). Build an enclosure for them and add the approiate charging circuit and fans to keep it all cool. Now the output of your new invertor can be run to a switching device to allow it to apply power when/if the utility drops. It is complete overkill but that is what I would do and is similar to what Solar/Wind/Generator homes are equipped.Good luck.Chris

Some great comments, FuzzButt-I enjoyed reading them. I especially like the suggestion about the marine deep cycle batteries, inverter, and charger setup. We lose power fairly often in my neighborhood, usually because of trees or branches falling across power lines during storms. After this years east coast hurricane, we lost power for around 102 hours. I was going nuts from internet withdrawal. Tried to cobble together 5-10 minutes of 'uptime' with various smaller UPS, but the whole thing came crashing back down just before Windoze had finished loading. I'm too cheap to buy a gasoline generator, and anyway, I hear the the waveform out of those beasts more resembles a ragged square wave than a sine wave, and can be hard on the power supplies of various electronics. As I understand it, one of the main issues to look at with both generators and inverters is whether or not the units output something vaguely approximating a sine wave. These are more expensive and fairly scarce, but they do exist.I'd have to research it, but I figure some electronic gear would handle lousy waveforms just fine, others might experience component failures in power supplies, especially if they are running various filter capacitors a little 'toasty' near their max ripple current ratings to begin with. Maybe some smart person could figure out how to cobble together a workable, high-current waveform conditioner ('line conditioner') from locally- available parts to recover a reasonable sinusoid from some of these nastier generators. Line conditioners are a little pricey. Can they even do this job? They are designed more to smooth out spikes and sags on 'dirty' power lines. The issue would be increased filter cap ripple currents due to increased energy at harmonics of the 60Hz line frequency. I paid 105.00 for a line conditioner rated for 5A at 125VAC. I'd sure not be willing to run my expensive radio communications gear off a generator without one. Ditto for my computers, I guess. OK, maybe I could run for a few minutes here and there, but running this way for hours potentially does raise the problem of overheated power supply components giving out on you.

[FuzzButt]

Ya I agree I too am a bit too cheap to go and buy a generator. I paid for some of one for my parents back in late August. 6250W for $700. A bit pricey and it will only help on extended outages like the 3 that I have had this year so far. If I ever build a house I am seriously looking into alternative power and heating for it. Geothermal heat/cooling and at least some solar power with maybe a fuel cell. Problem with the outage back in August was even if I could get power I still had no internet as all the fiber amplifers/switches and headunits around town were dead also.Chris

[AbeL]

Clutter & Fuzz:I'm puzzled by your comments regarding the waveform delivered by generators being anything but sinusoidal. If you're referring to gasoline powered "generators", they are in fact "alternators" (despite the advertising calling them "generators"). An alternator, being a rotating machine, will inherently output sinusoidal waveforms (assuming its used within its design limits).Generators, and electronic inverters, can (will?) produce some odd-shaped waveforms unless they are well filtered. Yeah, I know that a generator is also a rotating machine. but it also includes a commutator and brushes for the purpose of providing DC at its terminals. If you "look" at the voltage across any electrical pair of commutator segments you'll see full-wave sinusoid. A simple generator consisting of only one winding, will have the full-wave across the commutator, and half-wave rectified across the output terminals. With a more practical generator consisting of mutiple windings, the valleys between half-waves "get filled in".Remember when all cars had generators, then in (I think) the '50's the switchover to alternators was initiated? For all you fellow old-timers why wasn't the change made earlier? Abe

[Cluttermagnet]

Clutter & Fuzz:I'm puzzled by your comments regarding the waveform delivered by generators being anything but sinusoidal. If you're referring to gasoline powered "generators", they are in fact "alternators" (despite the advertising calling them "generators"). An alternator, being a rotating machine, will inherently output sinusoidal waveforms (assuming its used within its design limits).Generators, and electronic inverters, can (will?) produce some odd-shaped waveforms unless they are well filtered. Yeah, I know that a generator is also a rotating machine. but it also includes a commutator and brushes for the purpose of providing DC at its terminals. If you "look" at the voltage across any electrical pair of commutator segments you'll see full-wave sinusoid. A simple generator consisting of only one winding, will have the full-wave across the commutator, and half-wave rectified across the output terminals. With a more practical generator consisting of mutiple windings, the valleys between half-waves "get filled in".Remember when all cars had generators, then in (I think) the '50's the switchover to alternators was initiated? For all you fellow old-timers why wasn't the change made earlier? Abe

Hi, Abe-Some good points. My family's first car that I actually remember was a 56 Pontiac. I assume it had an alternator, but I'm really not sure on that point. Of course, I know about the generators. In WW II, they still used a lot of them. One particular type I was well familiar with was those surplus 'dynamotors' which were generators- rotary 12VDC to +HV DC converters. You could get one with, say, 600VDC output and use it for the plate supply in mobile vacuum tube radio gear. Next came the transistor inverter designs, and the dynamotors became so much scrap metal overnight (1960's).Now, I was assured some years ago by a colleague who researched mobile 'inverters' in depth that the vast majority of them had a lousy waveform that far more resembled a square wave than a sinusoid. His credibility was good with me, and I took him at his word on this. He did mention one manufacturer who was producing sinusiodal output inverters at the time (early 80's). Wish I could remember the name, but that may not matter over 20 years later, as markets have such a habit of completely re-arranging themselves over the years.Regarding the gas- powered 'generators' (yes, I'm sure we are all talking about basicially the same thing here), I had also formed the impression that there were issues about the waveforms, and that once again it became a narrower field if waveform purity were an issue for you. Again, I find that I have relied heavily on second hand information and not formed my opinions based on direct investigation. So that puts me on shaky ground here, and your reasoning regarding what sort of output waveform we should expect from an alternator is persuasive for now. Yet I have it strongly in the back of my mind that there is some good reason why we should be more cautious if we intend to power expensive electronics from these beasts. I would simply have to dig deeper and try to recall how I formed these reservations.It would be an issue if you chose just the 'wrong' combination of electronics with marginal power supply designs and then presented them with a more harmonic- rich input waveform that ended up increasing filter cap ripple currents. It would all depend so much on which particular combinations of equipment designs got hooked together, and very much on duration of use. Most anything should stand up to relatively short periods of abuse, but run it for longer durations and you might start to see some power supply failures. I have some industry design engineering experience behind these concerns, but little practical experience as to how they are designing them today. Many or most power supplies were still linear designs back when I was active, with switchers making rapid inroads at the time. From what I read in my industry journals, they are cutting it a lot closer in their designs today. Cutting costs is the prime directive in the consumer sector. The little margin we used to see a generation ago may be entirely a thing of the past. I suspect there may be some little 'gotchas' that would never be suggested by a careful reading of the equipment documentation. Is the manufacturer making the assumption that your source of power for their unit is the commercial mains? Run them from a 'generator' and we may not be in Kansas any more. It all depends on how worst- cased and overdesigned a particular unit is. Does it even matter whether we have a clean input waveform? I don't know any more.

[FuzzButt]

Clutter & Fuzz:I'm puzzled by your comments regarding the waveform delivered by generators being anything but sinusoidal. If you're referring to gasoline powered "generators", they are in fact "alternators" (despite the advertising calling them "generators"). An alternator, being a rotating machine, will inherently output sinusoidal waveforms (assuming its used within its design limits).Abe

Abe,Quite a few cheaper electronic UPS's have cheaper circuitry in them that do not have a true sine wafe output. They have a steped sine wave which instead of looking like a hill it looks like a stairway. Some electonics do not like the fake sine wave though I have never seen any. My APC Smart UPS's all have the full sine output on battery and on regular power. Remember unlike the spinning of an armature in a magnetic field an invertor has to make AC out of DC with no moving parts. It is quite a complicated action. Also unlike a rectifier circuit in a little transformer that takes the AC and uses a diode bridge, a capacitor and a transformer (used to step the voltage down only) to convert the sine wave to a steady DC voltage there are quite a few parts in the Invertor. I smoked one back in April and took it apart. There are 3 large transistors mounted to the case and I burned down at least one of them. I guess replacing a 15A fuse with a 30A fuse will do that.Chris

[AbeL]

Clutter & Fuzz:I'm puzzled by your comments regarding the waveform delivered by generators being anything but sinusoidal. If you're referring to gasoline powered "generators", they are in fact "alternators" (despite the advertising calling them "generators"). An alternator, being a rotating machine, will inherently output sinusoidal waveforms (assuming its used within its design limits).Abe

Abe,Quite a few cheaper electronic UPS's have cheaper circuitry in them that do not have a true sine wafe output. They have a steped sine wave which instead of looking like a hill it looks like a stairway. Some electonics do not like the fake sine wave though I have never seen any. My APC Smart UPS's all have the full sine output on battery and on regular power. Remember unlike the spinning of an armature in a magnetic field an invertor has to make AC out of DC with no moving parts. It is quite a complicated action. Also unlike a rectifier circuit in a little transformer that takes the AC and uses a diode bridge, a capacitor and a transformer (used to step the voltage down only) to convert the sine wave to a steady DC voltage there are quite a few parts in the Invertor. I smoked one back in April and took it apart. There are 3 large transistors mounted to the case and I burned down at least one of them. I guess replacing a 15A fuse with a 30A fuse will do that.Chris

Fuzz:I totally agree. One of the specs that drives the cost of UPS is purity of the output waveform, and it is probably the most difficult for designers to control. Two other cost factors are; watt-hours of operation, and output regulation.

[AbeL]

Clutter:Your last post and my comments in red:Hi, Abe-Some good points. My family's first car that I actually remember was a 56 Pontiac. I assume it had an alternator, but I'm really not sure on that point. Of course, I know about the generators. In WW II, they still used a lot of them. One particular type I was well familiar with was those surplus 'dynamotors' which were generators- rotary 12VDC to +HV DC converters. You could get one with, say, 600VDC output and use it for the plate supply in mobile vacuum tube radio gear. Next came the transistor inverter designs, and the dynamotors became so much scrap metal overnight (1960's).You bring back memories. Car radios in the 40's and 50's were tube-type and required 150+volts for the plate (as it was called then - the later name was "anode"). The high voltage was derived from a mechanical vibrator which chopped the battery voltage (either 12 v or 6v) at between 400 to 1000 Hz, transformed and recified that.Now, I was assured some years ago by a colleague who researched mobile 'inverters' in depth that the vast majority of them had a lousy waveform that far more resembled a square wave than a sinusoid. His credibility was good with me, and I took him at his word on this. He did mention one manufacturer who was producing sinusiodal output inverters at the time (early 80's). Wish I could remember the name, but that may not matter over 20 years later, as markets have such a habit of completely re-arranging themselves over the years.Mobile inverters might be either electronic or mechanical. In either case, the DC from the battery was chopped to resemble a square wave AC which could then be transfromed to the level required, but it remained square; considerable filtering was required to suppress the harmonics. Regarding the gas- powered 'generators' (yes, I'm sure we are all talking about basicially the same thing here), I had also formed the impression that there were issues about the waveforms, and that once again it became a narrower field if waveform purity were an issue for you. Again, I find that I have relied heavily on second hand information and not formed my opinions based on direct investigation. So that puts me on shaky ground here, and your reasoning regarding what sort of output waveform we should expect from an alternator is persuasive for now. Yet I have it strongly in the back of my mind that there is some good reason why we should be more cautious if we intend to power expensive electronics from these beasts. I would simply have to dig deeper and try to recall how I formed these reservations.At the risk of being accused of trying to be clairvoyant, let me try a SWAG at why you have the reservations. Gasoline powered generators (alternators as they really should be called) suffer from lack of regulation. This shortcoming can be as detrimental (or more) to electronic equipment as poor waveform. Five or ten years ago these portable "generators" were intended to allow a homeonwer to ride out a power failure by keeeping the furnace, refridge, and few few lights going. None of these devices is as sensitive to voltage fluctuation as the electronic stuff we now find so indispensible, so the generator suppliers kept the cost down (as you state in the next para.) by minimizing regulation.It would be an issue if you chose just the 'wrong' combination of electronics with marginal power supply designs and then presented them with a more harmonic- rich input waveform that ended up increasing filter cap ripple currents. It would all depend so much on which particular combinations of equipment designs got hooked together, and very much on duration of use. Most anything should stand up to relatively short periods of abuse, but run it for longer durations and you might start to see some power supply failures. I have some industry design engineering experience behind these concerns, but little practical experience as to how they are designing them today. Many or most power supplies were still linear designs back when I was active, with switchers making rapid inroads at the time. From what I read in my industry journals, they are cutting it a lot closer in their designs today. Cutting costs is the prime directive in the consumer sector. The little margin we used to see a generation ago may be entirely a thing of the past. I suspect there may be some little 'gotchas' that would never be suggested by a careful reading of the equipment documentation. Is the manufacturer making the assumption that your source of power for their unit is the commercial mains? Run them from a 'generator' and we may not be in Kansas any more. It all depends on how worst- cased and overdesigned a particular unit is. Does it even matter whether we have a clean input waveform? I don't know any more. Amen

[Cluttermagnet]

Very good, Abe-Your comments all well-considered, and I think you must have it substantially right. I've been thinking about buying a modest generator this coming year, as we do have numerous outages around here. I will study the regulation issue and such and make my decision- and I will check back later and post as to what I found out. I love threads like this one. They wake you up and make you think about things a bit more. Oh, BTW, one of the many boat anchor items I was given in recent years is a 500VA (~1/2 KW) Sola constant voltage transformer. Unfortunately not the harmonic- neutralized type, but that may not be an issue with 'alternators'. It will at least do pretty good voltage regulation, although it can't help with frequency regulation. Might help a lot to make a cheapie gas generator safer to use with delicate and expensive electronics. I'll look into all that. Oh, and of course some brute force filtering could not hurt, so I might get inspired and get a little help to design the magnetics and wind my own, and of course get the proper capacitors rated for power line service and roll my own line conditioner. Yeah, there's about a year of dorking around on the test bench, eh? Like I have time for that. I guess that's why most folks will shell out the bucks and buy one off the shelf.

[AbeL]

I dunno about that CV transformer, I guess I'll have to dig out some old text books to refresh myself on how they work. The little I do remember says they hold down over-voltages by partially saturating the core. If that's right, it won't do much for maintaining waveform purity. . If I do find my old texts, and learn that I have faulty memory (yeah right!! ), I'll pass it along.If you proceed with that project, I wish you good luck - more like a project and a half .I'll be interested in hearing how it turned out.Abe

[longgone]

Well, I have nothing to add here except that this particular thread has been very educational. I have in my current line of financial income (owner/operator trucker) had the dubious involvement only once of a semi that had just a generator to take care of the charging system. You can get some really strange indications watching the volt meter/amp meter. But the experience you get from coming across a vehicle that is positive ground instead of negative can enlighten your day if you are not watching where you hook up your brand new fandangled CB radio or AM/FM/Cassette player.

[lewmur]

Some older "electronic" gear might have problems with noisy or non-sinusoidal input waveforms, but your computer is not one of them. All PC's today have "switching" power supplies that rectify the AC input to DC and then use the 150V DC to power a higher frequency muti-vibrator to produce a square wave output that is more efficient to "transform," than 60hz "household" AC.For a "once-in-a-while" situation, hooking a car battery to a UPS is fine. Just unplug the UPS from the wall so that when the power comes back on, you have time to disconnect the car battery before the UPS even tries to recharge it.However, as a replacement for an emergency generator, it would be better to put an in-expensive inverter between the battery and the UPS. And don't rely on a car battery that has been "sitting around" or been on long-term "trickle charge," for your power source. As any boat owner can tell you, leaving a battery on trickle charge for months, doesn't work very well. A lead-acid battery needs to have a highamp load applied (gentlemen, start your engines) to it on a regular basis or a "film" developes on the plates. (That's why your UPS battery only gives you five minutes now instead of the fifteen it did when new.)

[olamoree]

On the practical side.....Where I live (Costa Rica) a replacement UPS battery (gel) costs upwards of US$40 almost regardless of size, so I went the "car battery" route, extending the wires outside the case with some big terminal clips putting the car battery in parallel with the now, 3 minute gel cell. I found that if the unit was to RECHARGE the car battery, it almost Smoked! UPS case got so hot I was in fear of melting the plastic! Solution: I DO keep the car battery connected to the UPS, BUT, if the power outage is long, like 2-3 hours, after the power is restored, I disconnect the car battery (leaving the gel cell connected), connect it to a regular car battery charger until it is fully charged, THEN reconnect it to the UPS and let the UPS trickle charge it until the next power outage. If the power failure is only for 5-10 minutes, the UPS recharges the car battery before it (the UPS) gets too hot for comfort. I have three old UPS's set up like that, one for the monitor, one for the CPU and one for the VCR as it is a pain to reprogram every few days when there is a power outage. Just be careful that you don't rely on the UPS for recharging the car battery, but just trickle charging it to top it off should work well --mine have about 3 years running with car batteries, only about US$22 for a small one (35Amp/Hr). And even tho the CPU is shut down, there is still a significant power usage --almost 1 Amp flowing to the CPU. The monitor draws a half amp on stand-by as I NEVER turn it off to avoid "thermal cycling" of the components --been running 7 years without a glitch --Sony 20se. Good luck.

[Cluttermagnet]

But the experience you get from coming across a vehicle that is positive ground instead of negative can enlighten your day if you are not watching where you hook up your brand new fandangled CB radio or AM/FM/Cassette player.

Ouch! Been there, done that. Wish I hadn't been reminded. If you take a positive ground radio out of an 18 wheeler and hook it to a test fixture that supplies 12 volts negative ground, the radio gets real excited and lets out the smoke from inside a lot of electronic components. Ugh! No, no, don't ask me how the factory gets that much smoke into such tiny components, because I really don't know- but boy is there ever a lot of it! BTW computers should be fairly non-fussy about the shape of the AC waveform supplied to them (within reason). My earlier comments were addressed more to the concerns of folks like me who want to run expensive radio communications gear this way.

[Guest LilBambi]

My Jim has a couple car batteries in parallel for backup power in outage situations, as well as a plethera of other car and other batteries to be used during outages and when we go camping. He would love this topic! He loves batteries and making things work on DC. He's been doing this sort of thing since he was 8-10 years old with old train transformers.Yeah, it does appear that it is important that the UPS is not just a power back up but that it ALSO conditions the line.Sure, computers have switch-mode power supplies like TVs these days, but like any sensitive electronics today, they are still somewhat persnickity about their voltages. May not kill it today, but apparently if you are not careful it could ultimately shorten the life of the computer.Inverters between the battery/batteries and the computer make a great way of making sure the right voltages are delivered.Jim took an old TripLite UPS and some additional parts and made a very nice inverter, actually two of them. He added a fan for cooling too. Don't ask me how, I am not the electronics tech around here. LOL!

[ChrisP]

Having read through all of the interesting replies to this thread, I see that one very important point is not brought up -- fire safety from out-gassing.Lead-acid batteries typically release hydrogen in gaseous form during normal operation. In the normal charge/discharge cycling of lead-acid batteries, hydrogen is generated as a by-product of the chemical reactions present.There are two basic type of lead-acid batteries:

• vented
• sealed or valve-regulated lead-acid (VRLA),

with the sealed types being:

• gel
• absorbed glass mat. (AGM)

In the sealed types, the free hydrogen released during normal operation is trapped in the case and can thus be recombined in subsequent operations. In the vented types, a fair amount of the free hydrogen is released via the vents.All older automotive batteries had vent caps -- one per cell -- to provide for periodic testing of the battery's specific gravity and for refilling of the battery with distilled water as needed. It is important to recognize that even the so-called "maintenance free" automotive batteries -- the ones with flat tops and no individual cell vent caps -- are still vented. Typically, the venting system is a manifolded multi-cell vent with its outlet on the side of the battery near the top. As such, these batteries will still release hydrogen gas to their surroundings during operation.Under normal automotive conditions -- both atmospheric and vehicle operational -- the vented hydrogen is not a major problem, as it is quickly dissipated in the ambient air. Under unusual circumstances, the vented hydrogen can and does become an explosion and/or fire hazard. One such condition is that of a battery with a cracked internal bus bar. Arcing at this bus bar can cause a battery explosion, as sometimes happens (especially in heavy-duty applications) during engine cranking under extreme conditions. I vividly remember losing a 1979 Peterbilt in just this manner one cold morning in Minnesota... :(In the case of a UPS used indoors, and with the battery potentially installed in an enclosed area, it is an extremely unwise idea to use a vented battery. The spark in the mechanical relay used in some UPS models is enough to ignite any free hydrogen present. Although many UPS models use sealed relays, there are also some that have vented relays. Do you really want to gamble on it? BTW -- static sparks can also ignite free hydrogen -- remember the Hindenburg? True -- there was much more hydrogen there, and the spark was larger, but the action is the same.In short, I would never recommend that any user replace any sealed battery with a vented one. The sealed battery was chosen for the application for a specific reason. If you feel that you need more capacity, and if you are certain that your charging apparatus is up to the task of recharging them, you could connect multiple sealed batteries in parallel to increase the overall capacity.

[onederer]

Poor man's UPS.Get a AC car battery charger for 12V DC output, that can auto sense when the battery is fully charged. It will shut itself down to idle when it detects a fully charged battery, to prevent over-charging. Connect the battery charger to one or more (depending how much current you want to get out of the system, and how long you want it to operate) marine batteries, connected in parallel. Connect an inverter large enough to handle the rated wattage that you will need, to the battery terminals. Now, plug in your computer or whatever AC electrical appliance that you need to be constantly on, into the inverter.The battery charger is always plugged into the Power Company's grid, and automatically keeps the battery(s) fully charged. The battery(s) also serves as a good filter to smooth out the varying DC fluctuations to the inverter. You are always running off the batteries and indirectly off the Power Grid. When the power fails, the battery charger naturally shuts down, but your battery(s) should still be fully charged. Depending on how many batteries you have connected in parallel, will determine how long your system will keep on running, before the batteries run down. You need no special switching equipment for power failures, since all the components are hooked up in-line together in series. Your computer will not even blink when the AC power is removed from the battery charger, since it is always running off the batteries.110V ACin > batt.charger> 12V DC out>> <12V batt. bank>> <inverter>> <110VAC out>> <computer, ect..With this setup, there is no need to buy any commercial UPS. You can make your own as big or as small as you need. The only thing missing is the software smarts that the UPS's have to tell the computer to shut down. But, if you're smart enough, you can also add this circuitry to your kluge, and what you have is beefier that most systems that you would buy off the shelf.ONEderer

[lewmur]

There isn't a battery charger made that won't ruin a car battery, or even a marine battery in a matter of months. Neither battery is designed to be "trickle charged." Anyone who has tried to keep a 'boat battery' fresh over the winter month can testify to this.

[onederer]

Lewmur: that may be, that a trickle charge could ruin a battery, but don't forget that in the setup that I mentioned, that(those)battery(s) are constantly being used. They get drained, and charged and discharged constantly. It is not like a battery that just sits there under a steady state situation. In a standard UPS, the battery is constantly being trickle charged, because as long as the city power grid is supplying the power, the reserve battery in the UPS circuit doesn't even come into use, until the relay kicks in when there is an AC loss. In my setup, all elements are always active. I know that my setup is a kluge, but a very effective one, that can grow or shrink as needed. One is not constrained to the dictates of industrial standards.ONEderer

[jimupnord]

Lewmur: Sounds like there should be a market for chargers that also exercise the battery with a periodic discharge.I suspect people who live off the grid have strategies for prolonging their storage batteries life.ONEderer: If the charger is always online, it will have to supply current for the (computer) load at the same time it is charging the battery. The charger will likely never stop supplying current for the load. So that special avoidance of overcharging might not work either. Consumer battery chargers may not be beefy enough to handle a constant relatively high current load. They may overheat. Remember: Don't let the smoke out!Don't miss the point about the fire hazard from hydrogen gas escaping from a car battery. Not a good idea to have this setup in your residence.Jim

[Cluttermagnet]

In the case of a UPS used indoors, and with the battery potentially installed in an enclosed area, it is an extremely unwise idea to use a vented battery. The spark in the mechanical relay used in some UPS models is enough to ignite any free hydrogen present. Although many UPS models use sealed relays, there are also some that have vented relays. Do you really want to gamble on it? BTW -- static sparks can also ignite free hydrogen -- remember the Hindenburg? True -- there was much more hydrogen there, and the spark was larger, but the action is the same.In short, I would never recommend that any user replace any sealed battery with a vented one. The sealed battery was chosen for the application for a specific reason. If you feel that you need more capacity, and if you are certain that your charging apparatus is up to the task of recharging them, you could connect multiple sealed batteries in parallel to increase the overall capacity.

ChrisP's point is well taken. Unless there is some safe and reliable method of guaranteeing that any hydrogen release is vented outside in a timely manner, you would be playing with fire to try to use car batteries indoors. Avoiding any hydrogen buildup is the critical point, as it is virtually guaranteed that there will be sources of ignition indoors, probably plenty of them- and the static charge warnings are also quite valid. We hear from time to time of gasoline pump fires at service stations caused by people discharging static charges at just the wrong time. In our area, there are warnings now posted right on the pumps describing how to safely discharge static before handling the pump hose.If you can guarantee 100 percent that you will never experience the buildup of hydrogen gas from batteries indoors, they might be relatively safe to use. But as has been pointed out, any accidental short circuit across a large battery can also cause some very unfortunate and unwanted results, unless the setup has been very professionally designed and all appropriate, reasonable, and necessary safeguards have been deployed (fuses or overload cutout relays, proper insulation methods for the high-current 12V conductors, appropriate connector designs and ratings, etc.) I would not be at all surprised to hear of home insurers weaseling out of paying for fire damages if they learn that an unsafe indoor battery setup was the cause. I suspect the answer to this whole issue is a setup where the battery and electronics all live outdoors and only the 120VAC output line is brought indoors. The tradeoff there would be environmental stresses on the electronics, both from thermal cycling (hot days, cold nights, etc.) and also from metal corrosion problems of all sorts due to an often much greater outside relative humidity.

[onederer]

Your point is well taken. Thanks for your input.Onederer