Battery Bank Getting Old

My 24V system at my summer cottage is made up of 6 x 130W Kyocera panels,
a Xantrex C40 controller, 8 x S460 Surrette deep cell batteries (350AH new @ C20) and a Xantrex (Trace) SW402 inverter. THE BANK WILL BE 10 YEARS OLD NEXT SUMMER (2011).
I understand that if my bank is fully charged and it has a SG range of 1.255 to 1.280
it is in excellent shape.
a) I'm told that if I do a load test (capacity test?) and the bank can reach 80% of the
capacity at C20 my batteries are still good. Would you please explain to me what
that means in simple language. What do I have to do?

b) What would the SG be at 80% of capacity at the C20 rate? Thanks.

Lone Ranger


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Bank Getting Old

    The short and simple answer: :D

    Is it still working well enough to meet your needs?
    If "yes", don't worry about it. If "no", it's time to replace the batteries.

    I just think people get a little too wrapped up in numbers and testing sometimes, and ignore the basic issue of function. Batteries are expensive. Are you going to replace them because load testing shows they're below 80% capacity? I wouldn't. Drain 'em 'til they drop! :p

    I know that's not the answer you were looking for, but someone will be along shortly to explain load testing in excruciating detail! :p
  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    Re: Battery Bank Getting Old

    A couple of battery FAQ's:

    Deep Cycle Battery FAQ
    Trojan Battery 20 page Maintenance FAQ (PDF).
    Trojan Battery Maintenance FAQ in español (PDF).
    Smart Gauge Battery Wiring Guide

    Battery manufacturers tend to say when a battery falls to 80% of its rated Amp*Hour capacity, it is at the end of its life for warranty purposes (check your vendor--not all are the same).

    However, if you normally only discharge your battery bank to 75% or 50% state of charge--there is still 30% of "head room" that you can certainly let the battery age further to get longer life (i.e., replace battery when it is only at 50% of capacity).

    If I understand the battery chemistry correctly, your SG will fall to where you can only charge it to 80% (or 50%) of specific gravity. The sulfur (of the sulfric acid) is locked into the lead sulphate crystals of a "sulphated" battery bank.

    In the end, it sounds like you should continue whatever your standard practices for maintenance and charging is--A lot of people kill their banks before 10 years (Surrette being a good quality brand that should go 10-15 years with care).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Getting Old

    what a typo bill!:p
    "...a god quality brand..."

    lone ranger,
    the test is a controlled discharge at a set rate until the batteries reach the typical dead voltage spec of 10.5v. if for example one were to discharge a 100ah battery over a 20hr period with 5a until it reached 10.5v then it is 100% for 5a x 20hrs = 100ah. if you reaped 80ah by the time it hit 10.5v and was a shorter time period than 20hrs then this battery is borderline by most manufacturers. to be certain you should check what the manufacturer suggests for testing.
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