LiFePo4 to replace external battery for rackmount UPS?

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Mithril

Active Member
Sep 13, 2019
354
106
43
Wondering if anyone has done this already.

LiFePo4 is Lithium Iron Phosphate; it's fairly compatible with SLA voltage range and used as "drop in" replacements for that type of battery in RV, solar, offgrid, etc. You trade off storage density and a need to be above 0C for charging for better safety than typical Li-ion. You can even buy rackmount units that have the BMS breaker, etc all built in.

So, my thinking is you make sure you use correctly constructed and fused cables, use either a properly sized diode or solid state relay for isolation so the UPS doesn't try to charge the big pack, use the correct connector for the external pack, plus a charger for the LiFePo4.

Seems fairly straightforward. If you wanted to it would be easy to add an pure sine inverter for other critical things that could be manually switched over. Personally I also already have a few things that can run "directly" (DC-DC converter) off of the existing batts so I'm not going DC-AC-DC.

Yes, I do have a small generator, but if I could avoid ever needing to mess about with it in the middle of the night ever again... :)
Plus, even inverter generators have an efficiency floor, and for longer outages need downtime (unless you spend $$$) and need refueling, and even with CO detectors in the house and running it outside as far as I can manage, not running it ever at night would be nice.
 

Istria

New Member
Feb 4, 2022
9
1
3
I have not done it myself, but I have built a small LiFePO4 battery before and gained some knowledge about it along the way.

First a disclaimer: Opposite to SLA batteries, lithium based batteries can smoke, catch fire and even explode when mistreated or used incorrectly. So always be careful of what you are doing when DIYing with (any) lithium battery chemistries. Having said that, of all the different lithium chemistries, LiFePO4 is by far the safest one. It does not explode and hardly ever catches fire (even if you try). But it can generate a lot (really A LOT) of toxic smoke and heat. So still be careful indoors. The larger the battery, the more smoke and heat released. So even though the battery itself might not catch fire, the heat might set a wooden box it's in on fire.

About the UPS battery. I agree using LiFePO4 instead of SLA should be no problem as long as you make sure that the battery you buy or build has a BMS included. If not, add it. It's a safety device you put between the battery and the load which can disconnect the battery if it senses something goes wrong.

If you choose to go yolo and do it without the (10-20usd) BMS, then AT LEAST make sure you add short circuit and overcurrent protection (a fuse). But I wouldn't sleep comfortably with that battery in my house while it's connected to either the UPS or it's charger without a proper BMS.
 

Mithril

Active Member
Sep 13, 2019
354
106
43
I have not done it myself, but I have built a small LiFePO4 battery before and gained some knowledge about it along the way.

First a disclaimer: Opposite to SLA batteries, lithium based batteries can smoke, catch fire and even explode when mistreated or used incorrectly. So always be careful of what you are doing when DIYing with (any) lithium battery chemistries. Having said that, of all the different lithium chemistries, LiFePO4 is by far the safest one. It does not explode and hardly ever catches fire (even if you try). But it can generate a lot (really A LOT) of toxic smoke and heat. So still be careful indoors. The larger the battery, the more smoke and heat released. So even though the battery itself might not catch fire, the heat might set a wooden box it's in on fire.

About the UPS battery. I agree using LiFePO4 instead of SLA should be no problem as long as you make sure that the battery you buy or build has a BMS included. If not, add it. It's a safety device you put between the battery and the load which can disconnect the battery if it senses something goes wrong.

If you choose to go yolo and do it without the (10-20usd) BMS, then AT LEAST make sure you add short circuit and overcurrent protection (a fuse). But I wouldn't sleep comfortably with that battery in my house while it's connected to either the UPS or it's charger without a proper BMS.

I think my current plan would be to use a rackmount LiFePO4, there are several reputable ones that include a known and tested BMS, (with charge regulation, thermal cutoff for high and low temp, and precharge resistor for large inverters etc) and some have UL certification. I'm aware of the need for proper fusing and that when fully charged (this is also true for SLA) the voltage IS high enough to be fatal if it crosses the heart. The battery (and rack) are also in the garage which is finished but fairly well vented (perhaps a bit too well vented :) )

The current challenge is finding a proper but not over priced charger, I honestly don't need anything over 1Kw, but part of the intended use is recharging from a generator for longer power outages. Most people working with these making their own systems are doing them with solar, sothe chargers or all in ones they use are generally intended for solar.

Also, I'm fairly sure I'll need either a decently rated (5-10A) low drop diode to keep the UPS charger from killing itself on the big external pack (and protect the pack from over voltage if the UPS charger gets too aggressive). I may need to go with a full on SSR. Need to measure the charger behavior of the UPS. Planned nominal load for long term outages on the UPS is 100-200W, and it's a 1200VA pure sinewave unit, so I'll be far under capacity there. I don't strictly NEED the external pack connected 24/7, since the internal batteries are less than a year old.


Also I killed one of my good DC-DC converters by being an idiot, so now I have to source a new one in the current market, yay...