Battery bank with undersized panels question?

arby
arby Solar Expert Posts: 108 ✭✭
in Solar Beginners Corner #1
Just wondering about my first small system at my cottage. The panels are 50 watt and 80 watt running through a a fairly good small charge controller. The batteries are 4X6V sams club golf cart batteries. The only thing this is now used for is powering a car radio/cd player and amp, and charging cell phones. This system has now worked flawlessly for at least 5 years and whenever the sun is out, it isn't long before the controller is flashing, meaning fully charged. The hydrometer readings always show in the green.
So, although the panels are not nearly sufficient for the size of the battery bank, it appears to me that if almost all of the time the batteries are fully charged or very close that they will last a long time. Best input I have seen is 7 amps.
Will batteries last a very long time if kept at or near full most of the time?
3310 watts panels, Classic 200 controller, 8 Surette S530's, Xantrex 5548 inverter, Honda EX5500 backup Genny.
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Battery bank with undersized panels question?

    What you're doing here is matching panel production with power consumption. If you were taking those batteries down to 75% SOC, well they'd already be dead by now.

    If you can maintain this balancing act they'll probably last another 5 years. The minute you start to draw significantly from that 440 Amp hour reserve the party is over, as 7 Amps is a miniscule 1.5% charge rate. Okay for keeping batteries up, but no place near enough to bring them up from any deep discharge.

    The 50 Watt panel is probably Imp <3 and the 80 Watt <5 so that 7 Amps is probably maximum output.

    I'm not sure why you'd have that much battery if you don't use it. You'd probably get the same results with 1/2 the number.
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  • arby
    arby Solar Expert Posts: 108 ✭✭
    #3
    Re: Battery bank with undersized panels question?

    If I added more panels and then used the bank more heavily would I find that the batteries are in fact not as good as they appear to be?
    3310 watts panels, Classic 200 controller, 8 Surette S530's, Xantrex 5548 inverter, Honda EX5500 backup Genny.
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  • arby
    arby Solar Expert Posts: 108 ✭✭
    #4
    Re: Battery bank with undersized panels question?

    I guess that what I am wondering about is how long batteries will last if kept fully charged with little to no use?
    Are lead acid batteries destined to die no matter what state they are kept in?

    Sorry for the dumb questions.
    3310 watts panels, Classic 200 controller, 8 Surette S530's, Xantrex 5548 inverter, Honda EX5500 backup Genny.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Battery bank with undersized panels question?

    Almost certainly.
    Batteries sulphate over time no matter what you do. You're using probably 5% of the capacity, which give you a long time before their capacity is too low to supply the power needs.

    When we talk about peak charge current it isn't so much a matter of "you must attain this current" as having it available in case the batteries are deeply discharged: it give the system the ability to recharge them quickly, which is always desirable and limits the time they spend discharged (which hastens sulphation). And of course when you pick a battery bank size it is expected the capacity is needed to supply the load needs.

    So a 10% peak charge current and 25% DOD works for most circumstances, but it's not the only way to do it.

    And yes deep cycle batteries do want to be used, so you'd get the most $ value per Watt hour from using them and recharging them quickly and completely.

    Or in simpler terms you'd be getting about the same battery performance with a smaller bank discharged a bit deeper, and possibly for less money (although not always as cost per Watt hour is far from linear!)
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  • arby
    arby Solar Expert Posts: 108 ✭✭
    #6
    Re: Battery bank with undersized panels question?

    As I understand you, using the system the way it is now being used, there would be no advantage in adding any panels.
    3310 watts panels, Classic 200 controller, 8 Surette S530's, Xantrex 5548 inverter, Honda EX5500 backup Genny.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Battery bank with undersized panels question?

    Look at it this way: if it ain't broke, don't fix it. :D

    Suppose your daily usage is <30 Amp hours. Now if you apply the "usual rules of thumb" you'd pick a 120 Amp hour battery for that (which would be discharged 25%). Now what's 7 Amps to that? A peak charge rate of 5.8% - just above the recommended 5% minimum. A battery that size (if available) might cost $160 and have 1440 Watt hours, or 9 Watt hours per dollar. Meanwhile a set of two golf cart batteries costing $160 (warehouse specials :D ) would have 2640 Watt hours, or 16.5 Watt hours per $.

    Either way your 7 Amps over 4 hours is 28 Amp hours. The larger batteries may deteriorate faster, but they have 80% more capacity to lose. As such they might well outlive the "proper" sized battery.

    If the SG and Voltage is staying up, there's nothing that needs fixing. :D
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