Libera IRC Channels Sorted by Number of Users

Furthermore, all the lithiums have a built-in BMS that'll dehance the charge-path FETs if a single cell goes over its cutoff voltage. So even if my power supply were to lose its regulation and attempt to overcharge them, they'd just disconnect themselves.

The lead-acids in such an overcharge state would simply convert more of their electrolyte to hydrogen gas and eventually deplete the liquid, but because the power supply is so weak in the first place, it couldn't force that to happen at a problematic rate. The regular drafts and air changes through the house would keep the hydrogen below its LEL unconditionally.

Vastly.

> are you assuming everything has the same float voltage

Yes. Both 6S lead-acid and 4S LiFePO4 are happy at 13.8 to 13.9v. Lead-acid has a temperature coefficient but my basement temperature is nice and stable so I just set for 13.85 and it's fine. They're all in float state at this voltage.

> as soon as you go off mains/float voltage, and actually use this battery system, some batteries will deplete faster than others since they have less capacity. ... won't nearby large capacity batteries spread/smear their voltage with other batteries on the rail?

No, they're all in parallel. All their voltages are precisely the same at all times, modulo a few microvolts for drop across the wires and fuses if there's a lot of charging or discharging happening. If it's actually floating in steady-state, they're identical within my ability to measure.

> are you using it for purely backup purposes only, and not recharging?

Purely backup. When I add a new battery to the pile, I charge it to 13.85 with a separate benchtop supply first, so there isn't significant current flowing through the fuse block when I plug it into the parallel bank. I fuse each tap at 2A since there's never a reason to move more, though all the wiring could do 20, I'd rather know about a problem early. Some are probably 3A because the baggie of fuses didn't have enough 2's in it, but whatever.

> do you have something that disconnects batteries when their voltage drops too low? and does it charge them too?

It's never run long enough to reach a low-voltage disconnect point, but if it did, the LiFePO4's BMSs would disconnect them around 10.5v, and the lead-acids would continue down to about 5.6v, which is where the MC34063's can no longer control their internal Darlington switch and they fall out of regulation. It's anybody's guess how long the Pi stays online after that, but probably not long.

When power came back, the RS25-12 would try to put 13.85v onto the bus and promptly exceed its overcurrent protection, and go into hiccup-retry mode, burping out little bits of charge every few seconds. It's possible that this would eventually bring the lead-acids up to 11-ish volts where it would restart charging in earnest, and this MAY happen fairly quickly because there's very little useful capacity below there, so it might not take many burps to get there, but I haven't tested it. It's also possible that the remaining loads would keep it down and prevent this from working, so I'd have to add a low-voltage disconnect to avert that. But that adds another possible point of failure, in anticipation of an event that should be exceedingly rare.

Or, I could add a proper charger or a CC/CV-limited PSU that would recover such a state in a sensible and expedient way, or if I add the solar MPPT it'll know what to do with an overdischarged bank. But realistically if power's out long enough for this to die, I probably don't care about the internet anymore.