Replace APC UPS SLA Batteries with Lithium?

IreallyLikeHam said:

Thanks guys, that's a shame. Oh well!

For this usage, I think it might be a little overkill to move up to an MPP, the cost of this unit with everything included looks to be about the same as just one of those MPP's

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IreallyLikeHam said:

I have an APC SRT3000RMXLA which is a 2u, 2.7kw double conversion UPS

Instead of buying more SLA batteries. Could I replace them with Lithium to get longer lifetime?

The stock configuration is 8 x 12v 5ah SLA batteries, connected to make a 96v pack. Is there a drop in replacements for a 5ah battery, or would it all need to be completely DIY?

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nbtesh said:

Hi @IreallyLikeHam ,
Thats a great question.
over the past year, I have specialized in making Lifepo4 replacements for UPS.
The one cavet is, you need High discharge LFP, 26650 cells, S4P2.
To discharge 500 watts from that configuration, 12.8v you need Aprox 40Amp discharge from the pack continuous. it will be 10C continuous discharge from each cell. That is very difficult to source.
I use A123 26650B cells. Second life.
Ups float is 3.45v per cell. it should give me 2-3000 cycles or 5 years of life for the battery pack.

I have made 12,24,36 & 48v battery packs for ups using this method. > 20 packs for family and friends and no issues yet.

Now Im entering Car starting Batteries with same cells. Going great, have 4 cars running, with 0 issues.

Hope my journey will help some out there, same as others stories helped me when I was a Newbie.

Regards
NB

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Supervstech said:

He needs 96v
Most 12v bms cannot series that high a voltage...

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nbtesh said:

Hi @Supervstech
I dont use BMS on Lifepo4.
The UPS does the max and min Limiting.
Just have to parallel each Cell, then each pack before stringing them in series to make sure they match the voltages.

This, plus Using High Quality A123 B series Cells you eliminate the risk trying to mitigate using a BMS.

Ref:: Do it yourself Lithium Batteries. By Micah Toll.

I have overcharged testing these cells and they pop at >4.6volts, by releasing their vent valve. No fire, explosion, smoke. Just sound and liquid electrolyte smell.

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Supervstech said:

Well... that is another option...

Not a recommended option... to each their own. Lfp will wander a lot in series... and without a bms, 4V isn't the max a cell could hit...

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nbtesh said:

Yeah, well. I do understand that in this forum, the BMS is considered a hard necessity.
I also understand that because you’re using Large Amp. >50ah ,prismatic cells, volt drifting during discharge an extremely long discharge >1hr , can cause a voltage drift on a extremly flat discharge curve like lfp has. Sometimes even Bell shaped.

Also taking into account Prismatics are not as safe as cylindricals design wise. The jelly band design has its limitations. and 26650 only have 2.2 -2.5 Ah capacity.
The risk balance changes completely from my point of view.

These ups packs usually are very low Ah, since we are looking for <20 mins Runtime until the generator kicks on.

BMS will add >50% to the cost of the pack.

Please challenge my views, that is why we are here at the end. Thinker and Learn.
Rgds

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Supervstech said:

I tinker... and I have several packs of 32650 lfp cells arranged in 24v packs.
Voltage drifts significantly. I have opened up the packs to find two sets at zero, with sets over 8V... still reaching 24V no venting, still happily operating... obviously the banks will fail rapidly to produce rated capacity this way...
Imagine this drift across 32 or 64 sets? Yikes!

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nbtesh said:

Yes, Quality cells are of outmost importance.

Do you know the brand of your cells?
Bought them in Alibaba or US?

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Supervstech said:

Old military packs. Junk.

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mjcother said:

Any thoughts on connecting one of the 24V 'EG4' battery units to my APC 1400XL as an external battery? Assuming I would need to use the COM port to change the charging profile, but I sure would like a simple rack mount solution for my UPS battery that only lasts 45 min...

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