Powering a UPS (also) with a Car Battery - Page 1
Forum section: electronics / projects

author: ThunderZed
author_rank: Regular Contributor
timestamp: May 12, 2025, 01:23:07 pm
content: I'd like to evaluate the feasibility, pros and cons of a project: using a standard 12V lead-acid car battery (45Ah, 360A, with refillable caps) to power theCyberPower CP900EPFCLCDUPS, which is already powered by aRBP000712V sealed lead-acid battery rated at approximately 5.5Ah. The idea is to either supplement the internal battery with the car battery by connecting them in parallel or to remove the internal battery and use the car battery alone. The project idea came up as I was looking for an alternative use for my car’s battery, which I’ve just retired after 5 years of service, because it can still manage only 3-4 engine starts in quick succession, therefore it's no longer reliable enough for my car.In addition to feasibility, I’m particularly interested in understanding the possible downsides of such a setup, aside from the need to run two external wires from the UPS housing to connect the car battery, either in parallel with the stock internal battery or used as a standalone power source.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 12, 2025, 01:28:16 pm
content: Quote from: ThunderZed on May 12, 2025, 01:23:07 pmHello, the circuit is designed to endure the thermal cycles of a small battery, what will occur if you put a large battery ?

author: mariush
author_rank: Super Contributor
timestamp: May 12, 2025, 01:32:01 pm
content: In theory, car batteries are "optimized" for short high current bursts, to start up the engine.... they're not optimal for low-medium continuous discharge you'd get in an UPS.You could buy extra sealed lead-acid batteries and install them in parallel to your current battery to get more capacity. I'd suggest recycling your old battery.

author: ThunderZed
author_rank: Regular Contributor
timestamp: May 12, 2025, 01:48:44 pm
content: Quote from: Coordonnée_chromatique on May 12, 2025, 01:28:16 pmQuote from: mariush on May 12, 2025, 01:32:01 pmI’m a bit confused by your observation, as I honestly don’t fully understand how a UPS battery is charged.I hope this isn’t a silly question, but to avoid overloading the UPS’s charging circuit when using a car battery, could it be a viable solution to pre-charge the car battery using a regular automotive battery charger (which I already own)? This might be a workaround if the overload issue for the UPS circuit occurs only during the initial charge of the external battery. Is that the case, or does the UPS continuously draw and manage energy from the connected batteries?

author: shapirus
author_rank: Super Contributor
timestamp: May 12, 2025, 02:01:37 pm
content: Quote from: ThunderZed on May 12, 2025, 01:48:44 pmIt can be either cyclic charge (disconnect the charger after the battery reaches a certain voltage) or float charge (CC/CV without ever disconnecting the charger). I think the latter is more typical for computer UPSes.You can check this by measuring the voltage on a fully charged battery when the UPS is powered on on AC power. Typical values for the float charge approach would be about 13.6 V per battery (27.2 V for two in series). Cyclic charge typically goes higher, over 14 V, and then the voltage will drop to below 13 V (I think) when the charger is disconnected.Quote from: ThunderZed on May 12, 2025, 01:48:44 pmThe UPS charger will (typically) be fine. They are normally current-limited and can survive continuous load. No need to bother with external chargers, and that also kinda defeats the purpose by introducing unnecessary hassle.What can be an issue, however, is overheating during the discharge when operating on battery. If the cooling system of the inverter's output transistors wasn't designed for long operation, then it's possible that it will overheat, because with an additional battery the UPS will be able to work much longer. To be on the safe side, you may want to add a fan blowing on the main heat sink which turns on during discharging.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2025, 02:07:59 pm
content: Quote from: ThunderZed on May 12, 2025, 01:48:44 pmAn issue is that UPS's are only designed to run for a fairly short time - they may not be thermally designed for the longer running time of a larger battery

author: coromonadalix
author_rank: Super Contributor
timestamp: May 12, 2025, 02:09:35 pm
content: a car battery does not have the same charging circuit as other battery types, an ups need to take that into account ... your car battery circuit tooit is not just a matter of putting a car battery ...my ups does battery charging cycles, and i think some conditioning too once a montha car battery is mostly seen as a supercap for brief and powerful discharges, not long term discharges ...an ups is not an power supply to keep your things running for days lollan good or well configured system should be : the ups send a warning message to say windows or *nix computers or servers a shut down will occur in X minutes, you have x time left, time to save your work ....you have similar ups batteries with much bigger current capacities or battery banks for 1 Kw and up to 15 Kw systems, just a matter of filling them up and connecting them to the upsno need to try things like car batteries who contain acid, and ARE dangerous to manipulate i would not even try thiseven in a standard usage, something bad can/could happen, i had an APC 1500 ups who decided to overcharge its batteries, they almost exploded, so inflated, almost impossible to pull them out tossed all in the bin / recycled

author: Xena E
author_rank: Frequent Contributor
timestamp: May 12, 2025, 02:17:31 pm
content: It will work,However:Keep the batery in a well ventilated area.Don't parallel with the internal battery.In any case provide suitable overcurrent protection,near the external battery.The comment about the suitability of different types of battery is because the car battery will not be able to tolerate many total discharge cycles, it's not what they're designed to do.The internal charge circuit will be optimised for the requirements of the SLA battery and may not fully charge the car battery... however the charge circuit in the ups will normally be protected against fault current and is not likely to be damaged.X

author: Simmed
author_rank: Frequent Contributor
timestamp: May 12, 2025, 02:40:52 pm
content: found a pic on the webthis pic is model 600the OP model is 900but both units look like similar size (entry level models)of concern is the tiny heatsink like mike said

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 12, 2025, 02:59:04 pm
content: Quote from: Simmed on May 12, 2025, 02:40:52 pmHeat capacitor, the dissipation ability is very week, but it store the heat and dissipate it slowly.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 12, 2025, 03:42:06 pm
content: Quote from: Coordonnée_chromatique on May 12, 2025, 02:59:04 pmYeah.That looks a very minimal ally block!Would be important to de-rate for that, the design would only allow for it to be sufficient for the design period at full load.Extra cooling would be a pain to do without a different case.X

author: CaptDon
author_rank: Super Contributor
timestamp: May 13, 2025, 01:56:19 am
content: The downside is that it could take days to recover a full charge if the battery has run very low like 10VDC and in fact due to the higher leakage current of an old car battery it may never fully charge from the UPS current limited supply. I would make sure the charge remains above 13.4 volts to avoid sulphation although the battery is pretty much junk already. You will probably need an external charger like perhaps an intellegent motorcycle battery charger. The idea can work. I have two U1 batteries externally replacing the pair of 12volt 25AH gelcells that used to be inside the unit. My UPS is a true sinewave 1500VA unit.

author: johansen
author_rank: Super Contributor
timestamp: May 13, 2025, 04:41:59 am
content: Quote from: shapirus on May 12, 2025, 02:01:37 pmare you willing to bet half a million dollars on that when it burns down OP's home?they really do light on fire because the charging circuitry is so cheap, they bet on the thermal budget of the circuitry when recharging a small lead acid down to 10.8 or so, which immediately floats back up to 11.5 when the power turns on.a larger battery doesn't. it stays at 11volts and that extra half volt makes for double the amps from the charger.i wish i was making this up. i've never seen this happen in person but it happened to multiple people ive read of on the internet for years.. as well as someone i know in real life. their ups really did catch on fire and was destroyed.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:44:55 am
content: New idea, sell the old batteries for scrap, put the money towards a correctly rated replacement.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 13, 2025, 07:33:13 am
content: Quote from: Xena E on May 12, 2025, 03:42:06 pmI will not write that all EE are genius, but they are extremely clever in their domain. if you perform some basic maths with the quantity of metal and the exchange surface with the air the ally block is designed to smooth the heat exchange in order to match the convection abilities of the small closed box.It a cost effective appliance, It would be less expensive to use a standard dissipator, the thermal envelope seems to be very optimized and at your place i will investigate more on the thermal strategy of the circuit IMHO.

author: BradC
author_rank: Super Contributor
timestamp: May 13, 2025, 08:09:03 am
content: Nobody has yet mentioned the thermal mass of the transformer. These devices are not particularly efficient and the transformer generates considerable heat. Like the driver semiconductor heatsinks, the transformer is designed to absorb the generated heat for the run time of the original battery. If you extend the runtime, you will need to provide external cooling for the transformer, and I guarantee you won't be able to run it at the maximum output and keep it cool.If you really want to extend the runtime on a UPS I'd go with the APC SmartUPS series. The XL models are designed to run effectively continuously. They have additional cooling and larger transformers (more surface area, more thermal mass) designed for it. The conventional non-XL series will run continuously if you de-rate them (ie don't run an SU2200 at 1600W, which is its rated output). I usually stop at about 50%.The last observation I'll make is most of these devices rely on a very low impedance to the battery, so if you are going to add storage, leave the internal battery and cabling in place and just jump in parallel with it.Based on the photo posted in the thread, extending the run time of the mentioned UPS is probably not the best idea. Start with a more robust unit that at least has some active cooling.

author: shapirus
author_rank: Super Contributor
timestamp: May 13, 2025, 10:20:21 am
content: Quote from: johansen on May 13, 2025, 04:41:59 amThis is no kindergarten here. Of course it is assumed that one must know how to measure and what parameters to measure, have the equipment, understand and/or do the research on what can potentially go wrong and test the modified device in various scenarios before considering it ready to use.Opening the case of the UPS can be lethal, too. No user-serviceable parts inside, you know. Take everything to a certified repair shop™.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 15, 2025, 08:29:23 am
content: This should work fine just use the car battery alone, its not a good idea to parallel batteries of different capacities together. You will get much longer usage using this new 35Ah battery, but the charging will also take long. Car batteries are designed for highly pulsed discharges, that is true, but it is also true that they are capable of providing several amps worth of current over sustain periods, which is perfect for our use. Even my late 2000s car has a bulb in the cabin, it takes ~4A (really should replace it with an LED sometime), power windows an additional 8A/motor.There is concern about overloading the charging circuit with a much lower impedance higher capacity battery. These UPSes are the "default" designs across countries and brands, they handle everything from charging the battery to generating AC, by using the transformer. With a lower impedance battery, the transformer will heat up when chargingCar batteries are not sealed, I would suggest keeping your battery somewhere outdoors, or at least well ventilated.

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 09:34:14 am
content: Quote from: BadeBhaiya on May 15, 2025, 08:29:23 amI'd be nice to elaborate on this. What possible issues are you referring to?

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 09:43:44 am
content: Quote from: BadeBhaiya on May 15, 2025, 08:29:23 amThis is not true if the charger's output current is limited, which I expect to always be the case. There's no way that any modern charger would work as a voltage source and let the impedance of the load alone determine the resulting current (unless there is also some heuristics and/or protocol to estimate the capacity of the battery and adjust the current accordingly, but that's a different story).And once the output current is limited, it doesn't matter what the impedance of the battery is. If the charge current is set to be limited to, say, 0.5 A (which is the case, for example, with my ancient Powercom KIN-1500AP), then it will be the same no more than 0.5 A regardless of the battery. The only difference it will make is the time required to reach the end of charge condition.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 15, 2025, 10:14:30 am
content: I guess i misspoke, i meant it's not a good idea to parallel older and newer batteries of different capacities together. Its only a minor concern, but one which I've come to be wary of. Paralleling batteries leads to uneven wear.Old batteries don't just degrade in their capacities but also their internal resistance increases. This leads to situations where an older but larger capacity battery is unable to provide as much current as it should, and hence the smaller battery must provide more current than it's supposed to, discharging it more.When the heavy load is gone, the older battery may actually slowly rise back to its actually voltage (aka battery chemistry recovers), but now it's going to have to charge the lower capacity battery which was drained more. This charge shifting results in loss of energy.All in all, you'll be fine, everything will still workThat's quite true, if the charging circuitry is current limit it will work fine without any problem. The battery will charge slowly and without excessive heating. The problem is, these UPSes are quite basic. I don't recall seeing much intelligence in them. I don't recall seeing any current limiting either, it's just a transformer with fixed output and rectifier to keep the battery topped up. I may be wrong here, it's been a few years since i last dived deep into UPSes

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 03:14:39 pm
content: Yes, that's a legit concern in case load is high enough, "enough" meaning too high for the new battery if we leave it alone without the older one in parallel.If, however, the load is low, then it's a "who cares?" situation: adding a worn-out battery (as long as it's not shorted internally) simply adds some on-battery run time to the system. At least as long as I'm understanding the processes correctly enough.

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 03:23:38 pm
content: Quote from: BadeBhaiya on May 15, 2025, 10:14:30 amYes they are basic, but limiting current can be easy, too. Well, I can't speak about the UPS in question, but all that I've worked with (not too many I must admit though) had current-limited charger's output. My Powercom, for example (and all the similar models), has a generic datasheet "typical applications" charging circuit based on LM317 with a current sensing resistor in the feedback loop. Others had switch mode converters, again, current-limited, sometimes with user-selectable current.Otherwise, really, the circuit designer would have to rely on the output impedance of the transformer (plus maybe some resistors in series) and assume that the battery has a very specific internal resistance to limit the current flowing into the battery. That's not impossible, of course, but... has anyone here seen anything like that in any UPS? I'm genuinely interestedYou actually mentioned some that had a transformer+bridge directly into the battery... that would be atrocious if true, lol.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 15, 2025, 03:39:43 pm
content: Quote from: shapirus on May 15, 2025, 03:23:38 pmWhy ? the car batteries are constantly charging and the energy pulsation can slow down the sulfatation, no ?

author: johansen
author_rank: Super Contributor
timestamp: May 15, 2025, 06:54:25 pm
content: Quote from: shapirus on May 15, 2025, 03:23:38 pmdo you think i just made up what i said in my last reply?even if the circuit is current limited, the heat capacity of the circuit is designed for the battery as shipped in the unit.i pulled lots of 300-900 watt ups' apart 20 years ago because people would throw them out instead of buying new batteries. many of them simply had a block of aluminum for a heat sink and no thermal protection.My former boss who has a son about my age, 35 ish.. really did hook a car battery up to a UPS 20 years ago and it caught on fire while charging the battery.

author: johansen
author_rank: Super Contributor
timestamp: May 15, 2025, 06:59:25 pm
content: Quote from: ThunderZed on May 12, 2025, 01:23:07 pmI may have a very similar unit, and there is no fan inside, it likely has square blocks for heat sinks on all the active components.you'll probably be fine, but you will need to add a fan to the case to keep the temps down.also the battery may be floating at a high dc/ac voltage relative to ground...

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 07:00:09 pm
content: Quote from: Coordonnée_chromatique on May 15, 2025, 03:39:43 pmThe point wasn't about constant charging but about not limiting the current. We discussed UPS situation rather than what happens in a car. The latter is interesting in and of itself, btw. I actually never thought or searched information about it. A quick search suggests that there is no current limiting taking place, the battery with draw as much current from the alternator as it can provide (given the resistance of the battery and the wiring and the voltage difference).In the UPS however... maybe that's just me but I don't see how a sane circuit designer would implement charging without current limiting. What if the battery goes bad and develops an internal short? What about the variations in transformers in batteries? Okay we can have a fuse, but still.

author: shapirus
author_rank: Super Contributor
timestamp: May 15, 2025, 07:12:09 pm
content: Quote from: johansen on May 15, 2025, 06:54:25 pmNah I don't. I haven't seen ones like that but it doesn't mean that they don't exist, of course. It means then that this is yet another point to be taken into consideration and verified. If there's no current limiting, then of course changing the battery type or capacity (without adding a current limiting circuit) shouldn't be done.Quote from: johansen on May 15, 2025, 06:54:25 pmIn any case it shouldn't take less than say 60 minutes to charge the battery it came with from empty, right? The heat sink(s) should be designed to handle at least that. If so, then that's enough time for the thermal equilibrium to be reached, and so it shouldn't make any difference if charging takes longer.Quote from: johansen on May 15, 2025, 06:54:25 pmThat's not necessarily bad in all cases (except that it's always good to have a thermal fuse or other means). Going back to my current UPS example: passive cooling, 0.5 A charging current, 7.2 Ah stock batteries, so it'll take no less than 10 hours to charge them from empty. In this specific case it won't be different from the thermal perspective if it takes 10 hours or 30 hours, if an additional battery is connected.