OC & Modding Modify UPS with Li-ion/LiFePO4 battery and BMS

goDofWar_skr said:

Sounds interesting..
could you please let me know what it does and how one would learn more about it..

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There's a lot of videos on YT about doing this. Basically, you'll get much higher energy density and lifespan than a lead acid battery. With LiFePO4, it should be safer too.

The difficulty is matching up the charging/discharging voltages that the UPS uses, which are only designed for lead acid with what the LiFePO4 needs. But a 4S LiFePO4 pack will have a nominal voltage of 4*3.2=12.8V and a charging voltage of 4*3.6=14.4V. Afaik, that's pretty ideal and compatible with the UPS. The discharge cut-off voltage for a LiFePO4 is also lower than that of a lead acid battery, so you should technically be fine there too. Another problem is drawing too much current from it, because LiFePO4's are mostly limited to 0.5-1C where C is the capacity of the battery. A BMS will help manage some of these variables and protect the battery, just in case. But I've never done this, so I could be wrong about the specifics. If I do end up doing it, I'll let you know.

Anyone has any experience with these folks -
They sell LiFePO4 ups and battery packs.

These look kind of nice. 4 of them would cost 2.5k and when wired as 4S1P, you get a 12.8V 15Ah 192Wh battery that can do a continuous discharge of 2C i.e 30A. That's up to 384W. Not entirely bad for a UPS application, I think. By a conservative estimate, that should be able to run a 100W load for more than an hour or a 200W one for at least 30 minutes.

Combined with a 4S 30A balancing BMS like this one (not entirely sure about that website), that should make for a pretty good LiFePO4 battery pack under ~3k worth of parts. Although with the dimensions of these 32140 cells, it might be a tight fit into the UPS.

Hey @rsaeon as the resident battery expert here, is this a good idea?

I really like those cells and they would be well suited for a low-load UPS with the BMS you found. I considered them for a powerwall project but this being India, the really good deal is in used LFP cells - they're usually priced at under 2000 for a 80Ah cell! (India has pretty great second-hand pricing for basically everything).

I've been hooking up my series-connected-cells-without-a-bms to the Inverter during extended power cuts and recharging each cell up manually afterward, LFP's efficiency is very impressive.

Most basic BMSs have a cut-off at 3.75V so those should be avoided, the one you found has the proper over-voltage and over-discharge thresholds.

variablevector said:

Another problem is drawing too much current from it, because LiFePO4's are mostly limited to 0.5-1C where C is the capacity of the battery.

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Yes, this is the one drawback of using a LFP pack with any UPS/Inverter regardless of whether it was made for LA or Lithium batteries. 1C is usually the maximum, with some (older) packs allowing peaks at 3C to 5C.

However, some people on youtube have noticed some BMSs will block the UPS from powering on after the pack is drained completely. They got around this by installing a push button that'll bypass the BMS wiring to allow the UPS to sense that there's a battery so that it will power on. This is for UPS models that need a battery to turn on even if they're connected to the mains ac. Most UPSs will let you turn on without a battery with a long press of the power button.

rsaeon said:

I really like those cells and they would be well suited for a low-load UPS with the BMS you found. I considered them for a powerwall project but this being India, the really good deal is in used LFP cells - they're usually priced at under 2000 for a 80Ah cell! (India has pretty great second-hand pricing for basically everything).

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Great! In that case, I'll pull the trigger on them and post an update here when I'm done with the project. It sounds fun.

I've been playing around with the idea of replacing my home's current power backup which has proven itself woefully inadequate for when the power goes out for more than 2-3 hours (1x150Ah Luminous LA and a 900VA Luminous inverter). I saw your post on the FE105 Narada cells from MDS Enterprises and I was already thinking about buying 8 of them and trying to build a 12V200Ah or 24V100Ah LFP battery first. Then eventually upgrading the inverter to a 24V 2-3 KVA Solar Inverter and then maybe only later getting some solar panels. Basically doing it in stages and cutting corners wherever possible so I don't have to spend all of that money upfront.

So the idea of used cells actually being cheap in India is very interesting. Where do you get them? OLX?

rsaeon said:

However, some people on youtube have noticed some BMSs will block the UPS from powering on after the pack is drained completely. They got around this by installing a push button that'll bypass the BMS wiring to allow the UPS to sense that there's a battery so that it will power on. This is for UPS models that need a battery to turn on even if they're connected to the mains ac. Most UPSs will let you turn on without a battery with a long press of the power button.

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Ah, that's an easy/clever solution! I'll need to test the cheapo APC 900VA I bought to see if it does that or not. But based on what others were saying, it doesn't seem to power on with a dead battery. I assume that means l'll have to do this modification for it. Thanks for the heads up.

rsaeon said:

@variablevector sorry, I forgot about your PM and your reply here.

The dealer is https://rojgarbox.com/ I last spoke to them in January:

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No worries. I contacted the MDS guy and he quoted me 7.5k for a Hithium 280Ah "10,000" cycle cell and 3.5k for a Gotion 102Ah cell. I'm thinking of just getting 4x Hithiums for now and calling it a day. They're not that much more expensive than getting lead acid of the same capacity and probably more reliable in the longer term than these used cells you mentioned -- or so I would assume.

I'm sure with the recent drop in LFP prices that you should be able to get them for even cheaper from China, I see people throwing around prices of $50-$60/cell on diysolarforum but none of the Alibaba sellers I contacted were ready to ship to India either.