Looking for ways to charge my batteries from the wall since the threat of snow looms and I already need to beef up my panels. I ran some tests today with a 20A, 24V power supply and want to run this by you folks.
So to begin with, I'm using 24V Tesla packs. Each pack connects to a set of 1000A bus bars via 250A breakers on the positive side and through a 250A winch disconnect.
Similarly the Victron Energy MPPT charge controller connects to the bus bar via a 100A breaker on positive, and there's a 60A breaker that the panels go through into the charge controller, allowing me to disconnect the charger from the system on both ends, and a 100A shunt between the negative bus and the charger.
I have a native 24V, and stepped-up/down 12V, 48V DC fuse panels, which connects to the positive bus bar via a breaker, and the negative via another 100A shunt. I've made it easy to isolate components, measure performance, and any appliance is run behind several safety mechanisms.
So to begin my test I wired in the power supply by connecting the 24V positive output to the 24V fuse panel, via its own 20A fuse. It shouldn't exceed that, but to guard against the batteries/solar backflowing current into the supply/etc. I then put the negative from the power supply upstream of the solar charge controller's 100A shunt, so that the load on the power supply would show up as charge current.
I then disconnected the batteries and charge controller from the bus bars. System went dead. Took a while for the voltage to drop to zero in the meter output with nothing running- with a light on or something it cuts out immediately. Once out, it did not come back- there's no rogue device supplying power.
I then popped in the 20A fuse for the power supply and plugged it into shore. System came back up, 20W usage. Lights came on, etc. Good.
So lets see what blows up when I reconnect one of the batteries... nothing. Whew. 380W! The power supply is charging~! Hmmm, still kind of nervous. Disconnect the battery, back to 20W. As a kid I always thought that battery chargers were doing something magical- some special waveform or whatnot to convince a battery to charge. Thus, a charger for one chemistry wouldn't charge another battery, etc. This is of course nonsense, although some (lead acid?) chargers use some "magic" like this to try and "reawaken" or "bring back to life" cells.
Anyway, I was worried that the power supply would just continue to pump in electrons at full tilt without regard to the state of the battery. So to test this I adjusted the voltage on the power supply to closer to that of one of the batteries in isolation. 23V ish. Reconnect the battery- 50W. Seems like, as the battery charges, the power supply output will taper off.. and I assume at whatever voltage I set the supply to- stop. Its what is beyond that, which worries me.
Next test, I reset the voltage to 24V and reconnected. 380W. Lets connect the solar charger.... alright, 780W. Great. Seems like they wont interfere with eachother, at least not yet.
Then I pulled the plug on the power supply. Well, it has a fan and an "I'm on" LED- neither shut off. Charge current dropped to whatever the solar output was (I disconnected the 20A fuse to verify). I assume that the fan and LED are DC and tap off the 24V output, which is still hot because there's 24V coming from the charge controller/batteries.
Anyone see a problem with me leaving a power supply connected in parallel with the batteries to charge them? Any pitfalls there? I do wonder if it should be cut off altogether after the batteries reach the set voltage, I probably could do this with my MPPT charge controller.
https://www.amazon.com/gp/product/B01HTF1Q06
I also wondered if it would be possible to put the power supply "north of" the MPPT charge controller, and have it manage it like a panel, which connects only at night when plugged in.