Battery Capacity Test Help

andyrud
andyrud Solar Expert Posts: 70 ✭✭
in Off Grid Solar & Battery Systems #1
Hello Everyone, I need some help determining how to best test the storage capacity of a set of batteries I purchased about 9 months ago. The batteries in question are 6 Trojan L16 2 volt 1110 amphour that are hooked up in series for a total of 12 volts and 1110 amphours. I have 1270 watts of panels charging them. I am becoming suspicious of the way they drop to very low voltages ( 11.5 to 12Volts) when I draw 30 to 40 amps from them for 15 minutes or so even when before starting the 40 amp draw my Trimetric battery monitor says they are almost fully charged and my controller is in absorb voltage. I want to test their capacity because their unconditional warranty it coming up.

Any suggestions and directions as to how to go about this testing would be very appreciated.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Battery Capacity Test Help

    The only practical test you can do is to check the specific gravity of each cell. Look for low SG after charging & resting and inconsistent SG between cells (greater than 0.010 difference). You really can't load test a battery that size at home.

    Some other notes. Your panels are definitely low for recharging this batter bank. Trojan wants 14.8 Volts and 111 Amps on these. That works out to over 2kW of panels. I estimate your charge rate is barely over 5%, which simply isn't enough for these high capacity tall case batteries.

    Do you have an alternate charge source such as a generator and battery charger? It will still be very hard to come up with the huge amount of current necessary to give these a proper charge, and equalization may be quite a challenge too.

    These big batteries are difficult to deal with. That's why if you really need that kind of capacity (over 6 kW hours) you should up the system Voltage and keep the Amps manageable.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #3
    Re: Battery Capacity Test Help

    you may find this link helpful, user manuals near the bottom of the list.

    I would suspect that sulphation may be at work due to low rate of charge

    http://www.trojanbattery.com/ProductLiterature/GeneralProducts.aspx
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • andyrud
    andyrud Solar Expert Posts: 70 ✭✭
    #4
    Re: Battery Capacity Test Help

    Thanks Cariboocoot for the reply. I do have a 15 K generator available but usually don't need it. I usually only use less than about 20 % of the battery capacity so the batteries charge up so fast and go into absorb mode the amount of panels is usually sufficient.

    Andy
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  • andyrud
    andyrud Solar Expert Posts: 70 ✭✭
    #5
    Re: Battery Capacity Test Help

    Thanks Westbranch. Your system has much less panel wattage as mine but approx. the same size battery. I too don't use much of the battery's capacity. I watch my system start charging up in the morning and it is fully charged before 12 noon!

    Andy.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Battery Capacity Test Help

    1055 Amp hours of AGM are not comparable in charge needs to 1110 Amp hours of L16 FLA.
    If you think your system has enough panel and recharges so quickly, why are you wondering about its performance?

    You are right to wonder about the Voltage drop. It is an indication of something being wrong. That something is incorrect recharging.

    Are you sure you need that large a bank? If you're only using 20% you might consider a small bank more deeply discharged.

    Feel free to ignore my advice. It isn't my money you're spending. :p
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  • andyrud
    andyrud Solar Expert Posts: 70 ✭✭
    #7
    Re: Battery Capacity Test Help

    Thanks Cariboocoot. Interesting question. I have read over and over on this forum and in books that the best way to make batteries last the longest is to NOT discharge them below 80%. This causes the controllers to go into absorb mode, and that starts cutting back the amps. Very confusing for a non electrical engineer!!

    Andy
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Battery Capacity Test Help
    andyrud wrote: »
    Thanks Cariboocoot. Interesting question. I have read over and over on this forum and in books that the best way to make batteries last the longest is to NOT discharge them below 80%. This causes the controllers to go into absorb mode, and that starts cutting back the amps. Very confusing for a non electrical engineer!!

    Andy

    Actually it's not below 50% for most batteries. But then there is the deep cycle dichotomy; if they're not cycled that too is bad for them. It's enough to keep your head spinning forever.

    Absorb is a normal and necessary part of the recharging cycle. It's one that's often short changed in Voltage and time by users, and that's the worst thing you can do. It's very necessary to maintaining capacity of the batteries.

    At times this solar energy stuff seems like it's part science and part art. :p
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  • andyrud
    andyrud Solar Expert Posts: 70 ✭✭
    #9
    Re: Battery Capacity Test Help

    Thanks again Cariboocoot. I think you are right about it being part art. It can be very frustrating.

    I found a company that is making 500 amphour Lithion batteries for RV land yachts. I spoke to them and their batteries can be discharged to almost zero with no ill effects. Also, no water and they expect them to last at least 10 to 15 years. A little expensive but not the headaches of batteries.

    There is a guy who lives in his land yacht and put them in his RV and is writing a blog about these new batteries. Interesting reading. Here is the link.

    http://www.aboutrving.com/rr_one.php

    Andy
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