Battery Charge Rate C/8 or??

ywhic
ywhic Solar Expert Posts: 621 ✭✭
in Off Grid Solar & Battery Systems #1
Ok I read the NAWS page on batteries..

For MOST flooded batteries it says C/8 rate and uses a 220ah battery for an example. and would be 27.5 amps..

Is that per hour of charge??

I ask because if the stars align my array will be pumping the VMP of 40.25 amps per hour (8.05 VMP * 5 panels).. (currently seeing 6.50-7.60 amps in testing)

I am planning on a 330 AH battery bank of 12v batteries.. (I know, I know...)
(MS TS45 PWM Controller Absorb is set to 14.4VDC)

Which if this is correct, I will be at C/8 more or less.. 330/8 = 41.25 amps..

Is this OK (with the 40.25 amps coming in) and correct??

Should I bump my bank to 440 AH and be at a C/10 rate or just start with 330 AH and see what happens??

Just nailing down the details..
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Battery Charge Rate C/8 or??

    I would not bother adding batteries unless you have some specific need for more batteries (and then, you usually need more panels anyways).

    Use the system and see how it runs your loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Battery Charge Rate C/8 or??
    ywhic wrote: »
    Ok I read the NAWS page on batteries..

    For MOST flooded batteries it says C/8 rate and uses a 220ah battery for an example. and would be 27.5 amps..

    Is that per hour of charge??

    No, it is a peak rate of charging. Not the same thing as a quantity replacement (Amp hours).
    I ask because if the stars align my array will be pumping the VMP of 40.25 amps per hour (8.05 VMP * 5 panels).. (currently seeing 6.50-7.60 amps in testing)

    I am planning on a 330 AH battery bank of 12v batteries.. (I know, I know...)
    (MS TS45 PWM Controller Absorb is set to 14.4VDC)

    Which if this is correct, I will be at C/8 more or less.. 330/8 = 41.25 amps..

    Is this OK (with the 40.25 amps coming in) and correct??

    Should I bump my bank to 440 AH and be at a C/10 rate or just start with 330 AH and see what happens??

    Just nailing down the details..

    40 Amps * 100 / 330 Amp hours = 12% charge rate. No worries.
    Remember that is a peak potential charge (or current) rate; it is not the current that will flow to the batteries throughout the charge cycle. As battery Voltage comes up its internal resistance rises and the current will go down. When the Absorb and Float stages are reached the controller will begin to pulse to keep the Voltage at the proper level, and again the current will diminish. Put a load on, more current is needed to keep the Voltage from dropping, the controller will try to make up the difference from the panels.

    It's pretty rare to come across an instance of too much current going to a battery. Between the battery itself and the charge controller things usually work out fine.
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #4
    Re: Battery Charge Rate C/8 or??
    Cariboocoot wrote: »
    No, it is a peak rate of charging. Not the same thing as a quantity replacement (Amp hours).

    40 Amps * 100 / 330 Amp hours = 12% charge rate. No worries.
    Remember that is a peak potential charge (or current) rate; it is not the current that will flow to the batteries throughout the charge cycle. As battery Voltage comes up its internal resistance rises and the current will go down. When the Absorb and Float stages are reached the controller will begin to pulse to keep the Voltage at the proper level, and again the current will diminish. Put a load on, more current is needed to keep the Voltage from dropping, the controller will try to make up the difference from the panels.

    It's pretty rare to come across an instance of too much current going to a battery. Between the battery itself and the charge controller things usually work out fine.

    If I started with just 2 batteries and a total of 220ah and pumping the same 40 amps I'd be at an 18% charge rate..
    That probably wouldn't be good, would it???
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Battery Charge Rate C/8 or??

    It is not the end of the world. Just watch water level and battery temperatures. If the batteries get too hot (start worrying if over ~100-110oF or so), then you are possibly pumping in too much current (or have too warm of room). Batteries last longer if kept cool (around 70-80oF is ideal, but they will last longer if kept cooler than that range too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #6
    Re: Battery Charge Rate C/8 or??
    BB. wrote: »
    It is not the end of the world. Just watch water level and battery temperatures. If the batteries get too hot (start worrying if over ~100-110oF or so), then you are possibly pumping in too much current (or have too warm of room). Batteries last longer if kept cool (around 70-80oF is ideal, but they will last longer if kept cooler than that range too).

    -Bill

    Thanks..

    That would explain why my VERY dead/old (black + terminal) car battery I am using for testing felt a little warm when it was 89'F outside and I was pumping 7.6 amps into it from just 1 of my panels...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Battery Charge Rate C/8 or??
    ywhic wrote: »
    If I started with just 2 batteries and a total of 220ah and pumping the same 40 amps I'd be at an 18% charge rate..
    That probably wouldn't be good, would it???

    You'd only be at that rate if that much current could flow to the batteries. This would depend on how deeply discharged the batteries are.

    One of the other things about charging from solar panels is that the panels don't put out their peak current unless the sun is "square on". In the meantime the batteries will begin to charge with the lesser amount of current available from the panels under indirect sun conditions. By the time the panels are able to put out their maximum, much of the charging may already have been done.
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #8
    Re: Battery Charge Rate C/8 or??
    Cariboocoot wrote: »
    You'd only be at that rate if that much current could flow to the batteries. This would depend on how deeply discharged the batteries are.

    One of the other things about charging from solar panels is that the panels don't put out their peak current unless the sun is "square on". In the meantime the batteries will begin to charge with the lesser amount of current available from the panels under indirect sun conditions. By the time the panels are able to put out their maximum, much of the charging may already have been done.

    Thats what happend today I think in my test setup.. the battery was PWM/BULK from 10am on.. then by 12pm was in just BULK.. nothing really moving per se..

    I was getting a low 28 watt reading on the TS45 meter at 10am.. with no sunlight hitting the panel.. just daylight so to speak..

    By noon with some sun on it, it was saying the 101w and 7 amp thing and BULK..
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