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Ok, so I managed to pick up a hefty rack mount UPS for extremely cheap. Specifically, its an APC SURT10000XLI with optional 3 phase input and can kick out 8000W at 240V at full load.

It currently has 4 battery modules which contain 8x BP5-12 (vrla, 12v 5Ah/C20/1.75VPC/25 degrees C) batteries in series for a nominal 96V at the output or 118.4V when fresh off the charger (assuming 14.8V at full charge).

I am yet to establish if the 4 modules are in series or parallel. The manual says the modules are hot-swappable, so I'm expecting them to be in parallel but cannot be sure.

I have also found that the UPS is capable of taking additional battery banks (up to 10 I think) plugged into the rear of the unit in daisy chain fashion. These additional units also take 4 of the same battery modules so i'm assuming that the charger within the ups is capable of kicking out some fairly serious current.

My issue is, I appear to have 2 dead battery modules, which means that the UPS won't power up. So, do you good people think that I could convert this beast over to lithium power successfully? I was planning to use 18650 cells which means 4.2v per cell = 23 cells in series with 3 strings in parallel to make a 6Ah pack.

Eventual plan for this UPS is to have it living in the shed/garage and to run an armoured cable back to the to run a ring main circuit off it (via a separate consumer unit), to run "critical" kit such as fridge, freezer, boiler control etc. I may also look to get another one the same and then use them as the basis for a solar system.

Am I best going BMS per cell, or looking for a BMS that can handle 96V input? Alternatively, should I just let the UPS charger handle the charging and look to adjust the internal cutoffs for low volts cutoff (any recommendations for this point. Maybe 3.2v per cell?)? Have I missed anything else critical?

Thanks in advance people.
 

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Should be possible, but keep in mind these machines usually have a high 'no load' consumption, in this case 185W or 4,4kWh per day... 1600kWh per year.

So before doing lots of work, think how much backup-power you need, and you might be cheaper buying a (used?) inverter/charger like a Victron Multiplus.
A 3kVA Victron inverter/charger has a 25W standby-load...

http://forums.apc.com/spaces/5/smart-ups-symmetra-lx-rm/forums/general/10561/surt10000xli-power-consumption-is-needed-for-battery-charge
 

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for a nominal 96V at the output or 118.4V when fresh off the charger
Well you need to match the max voltage it's feeding the batteries, not their nominal.

You'll need 29 in series, which will be 4.08 volts per cell, a healthy amount for long-term use.
 
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