Deep cycle battery full state of charge

Hi,

I am a newby to this forum and have a limited knowledge of electrochemistry.

I recently installed a small battery bank of two Exide hybrid marine group 31 deep cycle batteries 205 reserve minites,115 AH each(will be upgrading to three in 2 weeks).

The purpose of these batteries is to be used as household backup system for power failures.

The batteries are wired parallel and hooked up to a Xantrex 1875(1500W) modified sine wave inverter.

Charger is Statpower Truecharge 20 capable of charging up to 400 AH bank, automatic bulk, absorption and float, with a manual 15.6 V desulfation (mine will only be 345AH).

I just installed and charged the battery bank, and applied a 20A load (about 300 W AC from inverter).and noticed that the voltage only started at about 12.22 VDC (which I believe is already in a state of about 40% of discharge).

My intention is to not allow the batteries to go below 50% DOD (12.06 VDC), unless of an emergency.

I allowed the bank to drop to 11.6 VDC (80 DOD), and recharged.

I have done this for several consecutive days and I cannot for the life of me not understand why the bank does not start out at 12.6 to 12.7 (full charge).

On my previous bank of group 27 batteries, the voltage after (allowing the batteries to settle down from the float charge at 13.34 VDC) was always close to 12.6+ (for nearly 10 years).

The specific gravity in 5 of the 6 cells is 1.270 or better, and one cell is around 1.255.

Would it be a reasonable assessment that desulfation at 15.6 volts for 6 hours or so help to get the initial voltage up to 12.7 volts.

The float charge is 13.34 volts, the electrolyte levels are all above the plates.

The mfg dates on the batteries are May and June 2009, and charged only only now, can this be the culprit?

I hope I can get some assistance with my problem on this forum section, if I goofed and posted in the incorrect section, please let me know and I will transfer to the appropriate section.

Best regards,

BrianJ

 

Deep cycle full charge

Thanks for reply,

My real problem with this battery bank is that at 77 degrees farenheit I need to know the starting voltage.

I am conversant with the peukert effect (non linear and the Peukert coefficient), but the self proclaimed expert at Canadian Tire states that full charge is 12.2 volts (this in reality is about already 40% DOD).

I believe that the full charge state should be 12.65 to 12.8 VDC.

When I apply a load of 20 Amps on battery side, the banks drops from 13.34 volts to 12.20volts, but I am of the opinion that after the residual charge is expunged, the remaining voltage would be full charge at 12.65 to 12.8 VDC.

I max out at 2 hours from a two battery bank wired in parallel from 12.20 volts down to 11.57 volts (80% DOD).

The batteries were at a Canadian Tire outlet in Quebec, with a manufacture date of May and June 2009, with the <charge before> stickers removed.

I got in touch with Exide (private label manufacturer of Canadian Tire Nautilus brand) and have agreed to exchange the two batteries already purchased (about a few weeks ago, along with the third I ordered at the same time),

As well they assure me they will have a new wet date of around January of February 2010.

Would you know if a deep cycle battery sitting around for 9 month off charge could cause this dramatic drop in voltage ( I would expect a DOD of 80% from a 2 bank battery set) to be at lease 4.5 - 5.5 hours, not the two hours I am now getting.

Any thoughts,

Best regards,

BrianJ

 

By all means, I really need help.

I am by no means an electrochemist.

BTW, I die desulfate at 15.6 VDC for 6 hours twice in one week, with almost no improvement.

Incidentely, the good guys at Exide are taking care of the Canadian Tire "Expert" real well.

Thanks again,

BrianJ