I recently got a very good deal on a refurb APC SMT2200RM2U battery backup. The unit seems to be in perfect working order, however the seller either gave me really cheap or old batteries.

The batteries go from a 100% to 70% charge in less than a minute. From there, it drops slower, but still very fast. At a 20% UPS load, I get about 20 minutes of runtime. I have done a runtime calibration.

What I am strongly considering is getting four 35Ah 12V SLA batteries, wiring them in series externally and connecting it to the APC unit. I'm specifically looking at these: https://www.amazon.com/35AH-Sealed-Battery-LEISURE-Wheelchair/dp/B00A82FCEE

Is there any reason to worry about this configuration? Is it going to overload the APC's charging circuit? Overheat it? Anything else that I would need to consider?

Four of these batteries would provide 3.5 times the capacity that come with the original battery packs when you buy this unit new.

  • \$\begingroup\$ Do the battery chemistry and number of series cells match up? Will they fit, safely? \$\endgroup\$
    – jonk
    Mar 29, 2019 at 18:38
  • \$\begingroup\$ As far as chemistry, all I know is that they're SLA, and so are the originals. The original battery pack has two banks of four 5Ah 12V batteries in series for a nominal output of 48V at the terminal. Four of these 35Ah batteries in series would also provide 48V nominal. They definitely will not fit inside the unit, they will need to be external. \$\endgroup\$
    – Mike C
    Mar 29, 2019 at 18:40
  • 1
    \$\begingroup\$ I'm not a battery expert. Let me start there. I'm just a hobbyist. But I'd give this a try if it were me. However, I'd have a separate charger for the batteries and would pre-charge them to about 90% before testing them with the refurbished unit (so that the situation is "near normal," so to speak, for the best chances.) It otherwise sounds like it should work. Dispose of the old batteries safely and figure out a safe way to attach these new ones. \$\endgroup\$
    – jonk
    Mar 29, 2019 at 18:49
  • \$\begingroup\$ Thanks for your input. I've come to pretty much the same conclusion, but am far from an electrical engineer so I thought I'd see if anybody could tell me something horribly wrong in my logic! I will definitely attach safely. I was going to use 4 AWG cabling (this UPS can power a 2000 watt load), crimp proper lug terminals, use a fuse, etc. \$\endgroup\$
    – Mike C
    Mar 29, 2019 at 19:05
  • \$\begingroup\$ Lead-acid batteries are worth money. Not enough to be worth the paperwork for a single battery, but anyone who deals in old batteries in quantity will be delighted for you to add your old lead-acid to the pile. That's any auto-parts store and many mechanics. It's lead. In the right waste stream it's worth a few bucks. In the wrong waste stream, it's toxic and nasty. Throwing one in the trash is unforgiveable. Lead-acid batteries have over 99% recycle rate for these reasons. \$\endgroup\$ Mar 30, 2019 at 1:35

1 Answer 1


First of all, batteries in parallel are much easier to work with. Batteries stacks in series often need a battery management system. The original batteries may have one hidden in there somewhere (could be as simple as some resistors). This is because over time, subtle differences between the batteries will mean that their respective charges might differ wildly (this is assuming you charge the stack of batteries as one unit). In parallel, all the batteries are forced to be at the same voltage, which eliminates the problem. So, either charge the batteries from the stack separately, or periodically make sure they all are charged to the same amount to make sure they don't stop working/catch fire ...

Second, make sure that the voltage of your stack matches whatever it was supposed to be, (Like Jonk says) by charging the batteries before you first install them.

Lastly, you need to make sure that the equivalent series resistance of the new batteries is less than or equal to the original. Given that they are the same chemistry, and of larger capacity you should be good.

And one more thing, lead acid batteries do not like being fully discharged. If you do that once, the batteries storage capacity is ruined.

Disclaimer: Be careful not to shock yourself and you'll have to take full liability for anything that happens.

  • \$\begingroup\$ I would say its capacity is lightly to seriously injured, but you'll get a couple of run-flats out of any lead-acid before it kicks the bucket, and dozens more out of deep-cycle batteries made for the treatment. Note that this is far, far short of the thousands you'd reasonably expect from something that calls itself a "battery"... \$\endgroup\$ Mar 30, 2019 at 1:37
  • \$\begingroup\$ Also +1 for warning about how bitey DC gets over 30V. You wouldn't think it, DC is our friend right? \$\endgroup\$ Mar 30, 2019 at 1:43

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