Battery bank feeding two motors

[burningfuel]

I currently have an off grid system powered by a bank of 4 Trojan T-105 batteries in series. I am planning on upgrading to a Lifepo4 bank but I'm looking for some advice on some behavior I'm seeing on the motors running off this bank.

The two motors are 1hp 24v DC motors with a max 39amp draw for about 6 minutes 4 times in a particular day. They run at the same time but I typically stagger the start of each motor by a second or two. One motor is about 30' away from the batteries while the other is about 4'. What I am noticing is that when both motors are running they run about the same speed yet when I shut a motor down the motor that is still running spins faster. I'd imagine the single motor is benefiting from the lower draw on the battery bank resulting in a higher voltage. I load tested the batteries individually and they seem to be in ok condition. I wouldn't think the 78 amps would be too much for the batteries but I'm no expert. The two motors do connect to the batteries through one fuse. Would love to hear any thoughts of the behavior or if this is to be expected.

That being said - While I'm planning the LifePo4 system I'm trying to figure out if I'll be better off with two 24v smaller banks of batteries vs one larger bank. The two banks does make it more complicated to keep things charged by the solar panels as I have limited space making it tough to add more panels. That's of course in addition to having double the components.

Any feedback is appreciated.

 

[sunshine_eggo]

Sounds normal.

78A draw is a very large current for the capacity of T-105s. At 75A draw, it's only a 143Ah capacity battery (ref) vs. its C20 rating of 225Ah.

Your conclusion concerning higher battery voltage is correct. Removal of motor one removes 39A draw. Voltage raises.

Staggering is a good idea. Initial surge is probably much higher than 39A.

Assuming 2awg cable, you have a 1.5% wiring loss due to voltage drop to the 30' away motor. That equates to a 1.5% power loss, which I'm guessing is no big deal to your application. I would elect to start that motor first.

Running thicker wire is probably more practical than having two separate banks.

Conversion to LFP should show an immediate improvement. LFP has a higher operating voltage and a reduced voltage sag due to improved internal resistance. You say "smaller", but make sure you keep cell and BMS discharge limits in mind. While they may pull 39A running, they may pull several times that during startup. I'd test with a DC clamp ammeter that can record MAX current. 200A initial surge isn't out of the question.

 

[burningfuel]

Thank you for your thoughts. I meant to note that I did run 1awg for the 30' run to try and reduce the voltage drop. That is a higher surge than I though so I'll definitely watch for the surge before deciding on any components.