Battery voltage vs. discharge....

[Derek Rowell]

Last year I purchased a pair of 4D wet-cell deep-cycle batteries (Harbormaster) to use in parallel as the house bank. They sit on a Xantrex TrueCharge 40+ charger (running the refrigerator) during the week. All seems well - they charge up to the float voltage on the charger, and with no load start at 12.9 volts and gradually decay to 12.6 volts. The specific gravity is 1.29 in all cells (which is a little high) when charged. Now here's my problem - after an overnight trip, when I have taken about 80 a/h out of the house battery, the no-load voltage is down to 11.8 volts. This seems very low to me - my reading on the web tells me that 11.8 volts corresponds to a battery that is 80% discharged! The stated capacity of the 2 4D's is close to 400 a/h, so in fact they should be only 20% discharged in the morning, with a no-load voltage of about 12.4 volts. I've tried equalizing. I called the battery distributor. The technical support person said that he had no idea of the voltage/discharge relationship, and that they considered 10.5 volts as the discharged voltage. As far as he is concerned there is no problem. I would be very interested to hear what the collective wisdom of the group is on voltage vs. discharge - and whether I have a real or perceived problem Derek

 

[abe]

I am not expert... but have you confirmed this

by checking the specific gravity when it is "discharged" ?

 

[Stu Jackson]

You Have NO Problem

DerekWith your fridge running, it'll always be that way.Turn the fridge off and everything else off, and in a little while the voltmeter will pick up.The drain from the 5 amp fridge load is simply killing the display volts because it's on.Not a problem.Plus it's not only voltage it's amp draw. If you have X house capacity and plan to use only 50% of X, and you know your batteries are charged and in good condition, then do the math with your energy budget or a Link 10.There's a list of battery voltages for discharge, but it ONLY applies when the batteries have been AT REST for at least 24 hours. That doesn't happen in your case based on your description.BTW, you should only check your specific gravity after the batteries have been at rest, too. That may be why you're getting such a high, and meaningless, reading.Stu

 

[Seattle Scott]

I recollect that you divide in half

the rated amps to get usefull amps. That a battery 1/2 disharged is dead and a 12v battery showing 12 volts at the terminals is dead and showing 12.7 volts is fully charged. I know this doesn't solve your problem, but I think your 400 Amp bank really only has 200amps you can use before recharging.

 

[Gord May]

Stu is right

thanx stu