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:
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.
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:
- "Small" (~25-35 AH ) Lifepo 4 packs - con, rather expensive
- 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
- Keep using SLA and use a DC-DC converter, possibly with a bypass when the voltage is low enough
- 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.
- Some combo of the above such as 18650 but use a DC-DC
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.