How low is too low for a 24v bank?

[Jim Mac]

Hi,

OK, so I'm about to hook up 8 new panels, but I'm waiting till after september 27th passes (I'm a conspiracy believer)..

Anyway, my question is regarding battery bank voltages.. For a 24 volt system, how low should the voltage be before the transfer switch switches me to grid? Currently I have it set at *Drop grid at 26v for .7 hours- and *Use grid at 24v for .7 hours.

Can someone point me to some guidelines?

Thanks!

[Dean Ravin]

Hello Jim.
I use a 24 volt system.
My settings:
"Drop Grid" 29 volts, delay 1/2 hour
"Use Grid" 24 volts, delay 1/2 hour

Having just recently moved my batteries into the "electron bunker" underground, I may lower the USE Grid a bit, as the batteries will be 30 to 40 degrees (F) warmer than their previous unheated, outdoor location.

[Chris Olson]

Hi Jim,

I don't have grid power, but my inverters start the standby gen if the bank voltage drops below 24.0 for more than two hours, and bring the generator online immediately at 23.0 volts (mainly for heavy load assist from the gen).

The loaded voltage and SOC are two different things. And that's why I have the two hour delay on the gen start. In the evenings with heavy loads on before we go to bed it's not unusual to see the loaded voltage of the bank dip in the low 23's. But as soon as we turn off the computers, big screen TV, satellite receiver, lights, coffee maker, and the water heater recovers after we take showers, the voltage can recover back above 24 and stay above 24 all night. So starting the gen would be a waste of fuel just because the voltage dips below 24 with a load on the bank.

The at-rest voltage is what determines SOC, and 24.0 is "safe" for just about all battery configurations.
--
Chris