Battery life

Kefalonianman
Kefalonianman Solar Expert Posts: 60 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
Can anyone tell me if I have the following battery bank size how long should they last with no charge going In

16 * 6v 900amp/h batteries with a 600 w load

Appreciate any help in the calculations

Paul
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #2
    Re: Battery life
    Kefalonianman wrote: »
    Can anyone tell me if I have the following battery bank size how long should they last with no charge going In

    16 * 6v 900amp/h batteries with a 600 w load

    Appreciate any help in the calculations

    Paul
    Assuming no losses, 144 hours from full charge to dead empty.

    (16)(6)(900)/(600) = 86.4kWh/600W = 144 hours

    Of course this is theoretical. Losses are never zero and you never want to drain batteries completely. In the real world I'd say that's about 60 hours of usable battery.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Battery life

    Well if we extrapolate from your sig that you have a 24 Volt system ...
    And then deduce that the batteries are arrange so as to provide 24 Volts (16/4) @ 3600 Amp hours ...
    Taking them down to a maximum of 50% is 1800 Amp hours at 24 Volts is 43.2 kW hours ..
    Divided by 600 Watts comes to the very inaccurate answer of 72 hours.

    The inaccuracy is due to the fact that batteries do not discharge linearly; the lower they get the faster they go down. Just how far off would depend on the Peukert effect particular to those batteries.

    I'm sure someone else would like to go into more detail on this. ;)
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  • Kefalonianman
    Kefalonianman Solar Expert Posts: 60 ✭✭✭✭
    #4
    Re: Battery life

    I recharge at 24.76 v so therefore this will reduce the time again, how do I calculate the time fame to this voltage presuming they weren't finished a full charge

    Sorry for the questions but trying to deduce if my batteries are working correctly

    Many thanks again
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Battery life

    :confused: What sort of batteries are these? 24.76 is really low for any lead-acid battery. In fact it's dangerously close to under 75%. Keeping L-A batts at that level will cause them to sulphate in a hurry, resulting in severe lessening of capacity. Is this what you are experiencing?

    If they were being fully recharged to 25.5 resting Volts I'd expect about 54 hours realistically for that bank with a constant 600 Watt draw.

    Keep in mind that if that Amp hour rating on the batteries is the "20 hour rate" it's meant for a steady draw of 5 Amps, and you are pulling about 5 times that with a continuous 600 Watt load.
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  • Kefalonianman
    Kefalonianman Solar Expert Posts: 60 ✭✭✭✭
    #6
    Re: Battery life

    24.7 is my recharge value the system sits at about 26 all day when the sun is shining then my usage drops to about 400w over night and the batteries recharge of the sun again in the morning

    I am using my system correctly as the 24.7 is the figure I was recommend to set to recharge the batteries
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Battery life
    Kefalonianman wrote: »
    24.7 is my recharge value the system sits at about 26 all day when the sun is shining then my usage drops to about 400w over night and the batteries recharge of the sun again in the morning

    I am using my system correctly as the 24.7 is the figure I was recommend to set to recharge the batteries

    So 24.7 is the low Voltage cut-out, not the recharge Voltage.
    If you are bringing these up to proper Absorb Voltage, et cetera, and 26 Volts is your "Float" level then you are using approximately 25% of your capacity. This is much better!

    That would mean you're using 900 Amp hours @ 24 Volts or 21.6 kW hours / 600 = 36 hours.
    Note the same problem occurs: You're trying to maintain a constant Wattage while Volts go down (albeit not so much) and Amps go up. You'd still be drawing at a peak near 25 Amps.

    Here;s SmartGauge's page on the Peukert Equation: http://www.smartgauge.co.uk/peukert2.html
    You can see from that what the problem is.

    I would expect 27 hours at least from this battery bank at that rate before you were at 75% capacity.

    This is why battery monitors are such a good idea. :roll:
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #8
    Re: Battery life

    Read the Battery FAQ's then get back to the forum.

    As to load how are you measuring? Clamp meter behind the inverter? Your inverter is 94% effiecent peak, at 15% of capacity, it's likely more like 85% so your load may be closer to 700watts...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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