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Possibility of using car battery on an APC Battery Backup system?

I have a bunch of APC Smart UPS 1500 Battery Backups and I was wondering how dumb of an idea it would be to wire it up to a car battery?

I was going to make a special adapter for the plugs so that everything fit slick and "looked proper" (as in, nothing exposed).

I have a degree in electrical, and maybe I should know better, but I'm wondering why i would spend $150 for batteries for these things when I can spend 50 bucks on a higher capacity battery?

Cosmetics are of no concern. Fire or explosions would be of concern.

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I believe the charge voltage for a car battery is slightly lower than the sealed lead acid batteries,  around 1 volt difference.  This leads to hotter recharging which leads to the reason sealed lead acid batteries are used.  Car batteries produce hydrogen gas during charging, especially if over charged.  So you need to have  ventilation or you really could be at risk of explosion and/or fire.

Of course if a battery leaks it also will make a nice mess.

I usually get GS Battery replacement units through distribution and find the prices are a lot better than 'retail' and the batteries are high quality.


There are a lot of real crap batteries on the market.

Avatar of pdandrea

You probably have a meter to measure the output voltage of the DC voltage to the batteries for the UPS.  A car alternator usually charges the battery at around 14VDC and the battery is rated at 12DC, so a little over 12VDC is good for charging the battery, same as the batteries normally in a UPS.  Also you need to take into consideration the current draw the car battery would draw from a fully discharged state and whether the UPS can take the load, might need to enhance the heat sink of the power components for the UPS.  Need to be careful not to create a fire hazard too, it is nice to have longer up time when power is out, but you don't want to burn your place down with or without house power.  It might be safer to just double the total batteries that the UPS has, matching the same type of battery, typically each battery is a 12DC 7AH (amp hour) battery in a UPS, you can get these at most battery supply and some electronic supply stores.
Avatar of aleghart
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1. Explosive hydrogen generated in sulfuric acid solution is vented during charge cycle...which is pretty  much all the time

2. Overflow tube on car battery will vent acid solution all over your floor when the battery heats up or when jostled/moved

3. Newer, "smarter" UPS systems may interpret your high amp, higher volt car battery as a defective or spent unit, and will shut down.

4. If your DIY equipment causes damage, fire, injury, death, your insurance may not pay any claims.
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P.S. I realize that a "degree in electrical" means you know a lot of this stuff (like what a short circuit is, and what a series circuit is), but since the purpose of this website is to make good solutions public, much of the intentional redundancy is for the benefit of anyone else who doesn't happen to have a similar degree. :)
Avatar of reswobslc

A couple more observations about the APC UPS 1500 I have, that is hopefully the same as yours:

1.  My UPS originally came with two 9 Ah batteries built inside.

2.  My UPS also has an expansion port on the back, for additional power.  I purchased the expansion pack.  That expansion pack has FOUR more 9 Ah batteries inside (wired two parallel sets of two series, so it remains 24V).  Conclusion: the UPS was clearly designed for at least triple the capacity.

3.  The connector that connects the internal batteries also mates the expansion connector on the back.  So basically I am taking out the battery packs, chopping the cable, and once I've created cables for the marine batteries, I connect them to the expansion port.

4.  This particular UPS does just fine with batteries connected to the expansion port with nothing connected to the internal battery port.

5.  The remaining batteries are relatively worthless in comparison to the big batteries I'm hooking up... but they are good for recycling and avoiding the $10 or whatever per-battery "core charge" that battery retailers charge you if you buy a new lead acid battery without exchanging one.  (They don't seem to care that you're buying a huge one and returning a small one).
Avatar of kode99

Just as a cost comparison, $72.00 for a replacement unit here


Or a pair of 9 AH GS batteries for $38 each,


Actually cheaper than a pair of car batteries.  Certainly less run time but also no maintenance and no hazards.  Do watch out for shipping charges on batteries though.

The inline fused cables are a excellent idea.  Probably check what the maximum amperage the unit can handle with extra packs and fuse it to no more than that.  I would think the UPS has internal fuses for fire safety on the battery circuit  though they may not be easily replaceable.
Avatar of rid
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You pretty much have it all in the post from reswobslc. One little detail that I would like to add, is that the gas that you get from a lead-acid cell/battery is not just hydrogen, it's hydrogen AND oxygen in the perfect mix for making water (through combustion...). The gasses come from electrolysis of the water in the cells (during charging), hence the perfect mix (H2O). That's why the ventilation is so very important. Note also, that most marine (and car) batteries are nowadays said to be "maintenance-free", meaning that they are not easily opened and the electrolyte reservoir will not need filling up for the life of the battery. That is probably just so, but it is under the assumption that the charging never drives the cell voltage over 2.4 V (14.4 V for a 12 V battery) for any substantial period of time. The fact that they don't run dry, even after several years of use, is that elements are added to the lead platters that keep up the "decay voltage" of the water in the electrolyte, thus minimizing electrolysis and water loss. If impure water is added, this feature will be destroyed and rapid electrolysis will soon cause trouble. Ions of Iron, Chlorine, Manganese and Platinum are especially destructive in this context.
Avatar of reswobslc

I agree with everything said by rid.

I might note that my APC UPS's appear to hold my 2-battery series pack at 27.2 volts, or 13.6 volts per battery, or 2.27 volts per cell.  That's probably why it does a good job of not losing water.

For anyone else trying this, "your mileage may vary".  Specifically, a signficantly higher voltage when your batteries are fully charged directly equates to water loss and production of flammable gas.

I have come to believe that the reading at "BatteryUniversity.com" regarding lead-acid batteries (specifically part 1 section BU13) is great for anyone wanting a better understanding of how this sort of thing works, and increased my confidence in attempting this project.
Avatar of kurian2z5
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Does anyone know the polarity of the pins on the expansion port for the external battery? I want to wire up inverter batteries to it.

This is a picture of the back. The expansion port is the vertical one at the bottom left. Which do I connect to the positive and negative terminals of the battery? Is there a name for that particular socket so I can get a ready made plug?


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