24v batteries and a 12v charger

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South Africa
South Africa Solar Expert Posts: 295 ✭✭✭
in Solar Beginners Corner #1
Did do a quick search, but cannot recall seeing a post where this has been asked.

Back in the day it was 12v systems for me. So I bought a 12v reliable good charger. Today I am on 24v.

So I was wondering, lets say you have 2 batteries in series, 2 strings parallel, to get 204amp/h 24v battery.

Using a multimeter, I can see where the bank is 12v.

Can one use a 12v charger to assist a bit charging by connecting it to the 12v points on the system, that you checked with a multimeter BEFORE you connect? :-)
5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #2
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    Re: 24v batteries and a 12v charger

    Yes, you can... If you have a low battery in a series string, you can clamp the charger across that string.

    You can even put two 12 volt chargers across one 24 volt battery bank (high/low bank chargers). What gets difficult is catching failures (one charger fails), temporary battery clips (sparks light off hydrogen gas, bad clamp connection that slows down charging), and issues with protecting against shorts circuits (high bank has both leads "hot" with respect to ground) and internal grounding of battery chargers (you have to check if the negative lead of the charger is connected charger chassis ground, etc.).

    With solar charge controllers, most of them assume that the battery bank is negative ground--And if you have one charge controller connected to the "upper" battery bank, you now have shifted grounds. If you use RS 232 or other common ground signaling, you will fry the communications cables/board traces (this was a common problem with large computer systems and terminals+printers+Modem, etc.--shifted grounds in the installation would create enough ground current to burn traces, connector pins, etc.).

    So--Yes you can, but be very careful and I would not suggest making a permanent installation unless you have no other options.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
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    Re: 24v batteries and a 12v charger

    Nope. There are two points in the battery string where the difference between two terminals is 12 volt. One from the minus terminal to the midpoint and one from the midpoint to the plus.
    To keep your batteries happy you would have to spend equal time connected to each one, and the charger had better not have one terminal grounded or connected to its case!

    And you certainly do not want to rewire the two batteries into parallel just for charging.

    That said, there are a couple of products available (advertised anyway) which will connect to all three terminals of the 24 volt battery string and both give you a 12 volt output that draws current equally from both batteries AND allows you to connect a 12 volt charger to the 12 volt connection on the box and charge both batteries at the same time.

    I have no idea how much they cost or how well they work.

    To do that with your battery bank of 2S2P, though, you might have to reconnect it as two sets of 2 batteries in parallel which sets are then connected in series. Otherwise you would not necessarily be charging both halves of bank equally, since the two 12 volt points would not be connected. It all comes down to the details of the circuit.
    SMA SB 3000, old BP panels.
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #4
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    Re: 24v batteries and a 12v charger

    Not a permanent fixture.

    I waited for nightfall, disconnected the panels from the controllers, switched off the inverter, and carefully charged the batteries, well ventilated. :-)

    Mental note: If EVER any RS 232 / Cat 5 cable is connected to any solar device ... disconnect it if I assist charging.

    I took battery for battery. But I see I can catch 3 of the 4 batteries on 12v in one go.
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: 24v batteries and a 12v charger

    If you have a 24 Volt battery bank comprised of two 12 Volt batteries in series or four 6 Volt batteries in series you can, in a pinch, Bulk charge half the bank at a time with a 12 Volt charger. Two cautions: do not use a multi-stage charger that will try to do Absorb and do not use a charger with very higher current in respect to the battery capacity.
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #6
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    Re: 24v batteries and a 12v charger

    Off course it is a multi-stage charger and it did try and do the absorb thing. I mean, does anything ever works as one THINKS it would. :-)

    This is SOLAR systems, they all have caveats. :-) :-)

    Although the lights showed something is happening, nothing really did.

    It is battery for battery, or none at all.
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: 24v batteries and a 12v charger

    Some of those "multi stage smart chargers" are nothing but a box full of blinking LED's.

    I tried one once that did nothing more than charge like an ordinary automotive charger - until the cooling fan heated up and started to scream because its plastic blades warped. Now I have a couple of evil homemade totally unregulated chargers that are definitely not for use by the inexperience or faint-at-heart. They don't get used much; only in an emergency or when there's nothing to lose. Few batteries like being hit with 25 Amps of current up to 30 Volts.
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