48v Lifepo4 as backup/main battery for UPS, thinking outloud

[Mithril]

Bit of background:
Ok, so I have a few server rack UPS units that take 48v SLA (4x12v SLA) batteries. I've noticed that sometimes they refuse to switch to back to "AC" input from my AIO inverter or take and extended period of time to do so. So that has me a bit annoyed, and paranoid. They are extended run units that can take external packs, and do not have a timed shutdown, just a thermal/overload one and a proper fan.
One of them UPS has ~1year old batteries, one has very old batteries that barely work, and the other I got from ebay with no batteries.

The problem with having SLA and Lifepo4 in parallel is the voltages are *compatible* BUT... fully charged Lifepo4 is above the "trickle charge" voltage of LifePo4, and a host of other fun gotchas.

I'd like to make use of my LifePo4 battery stack to *supplement* the 1 or 2 units in my actual server rack, which is within 20 FT (as the wire would run). Each unit has a 1200 VA inverter so it's 30A or less (depending on voltage), in practice ~400VA before load shedding starts. Now, having the LifePo4 AS the battery is simple and I've tested it. A properly rated diode or "ideal diode" (mosfets controlled by an opamp) keep the UPS from trying to trickle charge the batteries.

However, I'd like for the UPS to still have some level of battery, to survive being unplugged from the lifepo4 stack for say 5-10 minutes.

A list of potentially dumb ideas:

1. "Small" (~25-35 AH ) Lifepo 4 packs - con, rather expensive
2. Use 18650 or similar protected cells with a BMS, aim for ~4-5Ah, so likely 2p or 3p (technically the same as the usable capacity of SLA 7AH made for UPS). - con, safety? Different voltage curve. In theory I could have those on the UPS side of the idea diode and let them float charge, however I'd need to handle inrush when reconnecting. Since theres a giant metal compartment for the SLA batterys it could go in there for a little more safety
3. Keep using SLA and use a DC-DC converter, possibly with a bypass when the voltage is low enough
4. Keep using SLA and use a disconnect circuit that measures the voltage(s) and has a current sensor to measure when the SLA is discharging.
5. Some combo of the above such as 18650 but use a DC-DC

I don't anticipate or expect a DC-DC (if used) to supply 30A since it would be supplemental only.
I appreciate that this is a bit of a strange thing to do, and yet here we are. This is for the moment a thought experiment, and on my all too long list of "maybe I'll do it" project ideas, lol.

 

[venquessa]

The simpliest, IMHO is option 3. But... It depends on how long this is going to go in service and how much money you want to spend on it. What power efficiency loses are acceptable etc.

I use it in a temporarily permanent setup for a vaguely similar reason.

I run the office network infrastructure off a 12V solar system. However, as the weather didn't play ball I ended up having to switch it back to mains. After going back and forth a few times, each time rebooting the network. I put an old lead acid in the middle as the UPS!

I have a little boost/buck converter and I have options. If I'm leaving the house or going to bed, I tend to turn it off and let the SLA discharge into the loads while the PSU rests. When I return I can let the PSU supply up to 5 Amps of current. I pick the voltage depending on how and when I want to manage the lead acid. If I haven't done so in a day or so I might make that 14.40V until 1 Amp, if I just want it to pick back up as close to passthrough as possible I might set it to 12.80V or when I can't be bothered fiddling I just leave it set to 13.0V and the SLA consumes about 300mA in addition to the loads. At 12.80V that comes down to 200mA after it settles. Which honestly isn't that bad for an SLA with a good few thousand cycles on it. Unfortunately if you put it under any significant load or expect any more than about 30% stated capacity it starts to show it's service life.

If you want to use a DCDC "Buck" only, which support higher currents and are cheaper you will need at least a 5S lithium to charge the SLA. You "can" get up to 40A boost converters, or expensive charges which will contain same of course. However boost is less efficient than buck.

You could replace the SLA with 4S LFP + BMS

SLAs are absolutely happiest while wasting several watts of heat in float. As soon as you take them off float they are unhappy again. Leave them without float long enough and they get trashed. You makes you choices and pays your dues. If you want to keep those SLAs happy you have to spend the watts somewhere.

 

[venquessa]

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