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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/smar...ower-consumption-is-needed-for-battery-charge
 
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