Tapping 12 V from a 24 V Bank

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Is it advisable to tap 12 volts (one battery) from a 24 volt bank of two 12V batteries in series? The system will be charged by solar panels.

TIA,
tsp

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,471 admin
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    Re: Tapping 12 V from a 24 V Bank

    No, it is not good to do this... This will "unbalance" the batteries... One will be fully charged while the other is no yet charged.

    To fully charge the battery with the 12 volt load, you will have to overcharge the other battery--If it is a flooded cell, you will have to add a bunch of water... If it is an AGM or Gel Cell, the overcharged battery will be quickly destroyed.

    You will have to do something else to create you 12 volt power from a 24 volt system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Brianellul
    Brianellul Solar Expert Posts: 95 ✭✭
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    Re: Tapping 12 V from a 24 V Bank

    No it's not!

    How much current will you need at 12v?

    If you're not looking at 100s of amps, you can buy a simple ready converter or else if you are familiar with electronics you can easily build one using the voltage regulator 7812 IC. I have done this and am powering my phones and router.

    Brian
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    You can also obtain at many Truck Shops, a 24v - 12V DC-DC converter. They come with different ratings for power output. This allows your battery bank to stay properly balanced, and still get 12V when needed.

    The converters have about 20% loss when in use, and some amount of drain when the 12V load is shut off, and the converter is on. Be sure not to let an idle converter drain your battery bank unexpectedly.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • System2
    System2 Posts: 6,290 admin
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    Re: Tapping 12 V from a 24 V Bank

    I will look for a 24V to 12V converter. I don't have much to drive with the 12V. At this time it is two radios and landscape lights. I want to perhaps upgrade to 24V and connect a large true sine wave inverter for some of my household stuff.

    Thanks to all for the advise.
    Tomas
  • James
    James Solar Expert Posts: 250 ✭✭
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    Re: Tapping 12 V from a 24 V Bank

    if the landscape lights are 12 volt halogens, the amps can add up quick...try to get a load reading with a correctly rated dc amm meter before you go shopping.
  • System2
    System2 Posts: 6,290 admin
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    Re: Tapping 12 V from a 24 V Bank

    If the yard lights are 12 volt and you have an even # of them, IE, 2;4;6;8, the simplest and most economical thing, both for money and power consumption, would be to wire them 2 at a time in series, so you end up with several pairs, each being 24 volt. Problem solved.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    As for unbalancing battery loading.

    Why couldn't you pull two ~equally loaded ciruits off each pair (or whatever configuration you have) so that while the bank is 24vdc, you are pulling equally 12vdcX2. For example you could have X amps worth of lights on one side, and a similar X worth of load on the other. I'm not sure that batteries would care in this situation. (Do inanimate objects have feelings?)

    I know there are people on this forum way smarter than me on this, and perhaps they will chime in and tell me I am full of it.

    Icarus
  • BB.
    BB. Super Moderators, Administrators Posts: 33,471 admin
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    Re: Tapping 12 V from a 24 V Bank

    Yes, batteries have feelings and I have cringed when I have seen them tortured (even wrote an ECO to fix overcharging of NiCad standby clock batteries on a CPU board--several times, although that is another couple decades old boring story)....:roll:

    If the load it balanced (across two 12 volt batteries)--I guess you can do that. Although, loads are most often not accurately balanced for a variety of reasons.

    Placing lights in series is OK for filament bulbs--for others, like fluorescents and others with electronic or other types of ballasts could have problems running in series.

    The other issue gets back to the problem discussed in another thread--Most DC devices are not really designed to work on the wide range of voltages that can be seen on a deep cycle storage battery system (10.5-15.5+ volts)... Usually, 12 volt electronics are defined as 12VDC +/- 10% (10.8-13.2 VDC).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    Feelings,, you bet. On two occasions, I have decided to replace a vehicle with another. Within days, the old vehicle developed a fatal illness and died. Once in a Gmc truck, once in a Porsche 911. (I never speak ill of my hardware, unless of course it desrves it!)

    As to the question at hand: So there is no technical reason that you couldn't run 2 seperate circuits? Understanding unbalanced loads, and the parameters of low voltage stuff?

    Icarus
  • BB.
    BB. Super Moderators, Administrators Posts: 33,471 admin
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    Re: Tapping 12 V from a 24 V Bank

    I would not do it (tap off "balanced" or unbalanced loads off 12/24 volt batteries).

    If you have flooded cells (as opposed to AGM), you can probably withstand a few percent unbalanced load as you will just have to equalize a bit more and add more water to the over charged battery...

    But, the question still is--how much... So, a little thought experiment.

    From an article
    with information supplied by Trojan Battery...

    7. Correct the readings to 80 °F:

    o Add .004 to readings for every 10° above 80°F
    o Subtract .004 for every 10° below 80°F.

    1. Compare the readings.
    2. Check the state of charge using Table 1

    The readings should be at or above the factory specification of 1.277 (± .007). If any specific gravity readings register low, then follow the steps below.

    * Check and record voltage level(s)
    * Put battery(s) on a complete charge.
    * Take specific gravity readings again.

    ....more snips....
    % charge / Specific Gravity
    100 1.277
    90 1.258
    Assuming that we can use the 1.277 as a good battery and 1.270 as a "bad cell or battery"...

    100% - (100-90% / (1.277-1.258 )) * 0.007 = 96.3% capacity difference between "100% good" and "96.3% bad" cells (or 3.68% differential).

    If you have a 235 amp hour battery bank (4x 6 volt golf cart batteries), the amount of "extra current" that you can discharge from one 12 volt pair vs another 12 volt series pair would be:

    235 amps*hours * 3.68% = 8.7 amp*hours
    8.7 amp*hours * 12 volts = 103 watt*hours

    So, using a totally unjustified guess, any cell/battery with an unbalanced load of 3.68% of capacity between full charging cycles, would just meet the definition of a "problem cell/battery"--requiring additional charging / noting of falling performance.

    Is this the above mathematics any better than just waving my hand and saying "don't do it" or "keep the unbalanced load differentials low"?

    Does this mean that you can run a load "safely and reliably" on one battery subset of a parallel string to 3.68% state of differential discharge?

    Your guess is as good as mine. But using only 3% of a battery bank's capacity does not seem like a good use of money.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    icarus,
    be careful as 2 identical items may not actually draw the same. sometimes differences, although usually small, can be present and over time will create too much of a differential in the batteries.
    as to if batteries have feelings or not i don't know, but they sure as heck can cause grief.
    this was a request. "I know there are people on this forum way smarter than me on this, and perhaps they will chime in and tell me I am full of it."
    ok, you are full of it. now how do i know i'm qualified to say this? simply because i'm being a smart*ss.:roll::p:D
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    Neil,
    Touche!

    I understand what you are saying about differential loading. Since the original post posed the question, it's up to him to do it or not. My bank is 12vdc and will stay that way for the foreseeable future.

    As to the voltage spec of 12vdc stuff. I had never thought about that over the years, as I blithly wired 12 vdc stuff into my system, radios, laptops etc. Never had a problem though,,,, The only 12vdc stuf I have are an automotive type radio that should carry 16vdc, a pump that is 12-24vdc, and a some fans that are also 12-24vdc so I guess I'm now safe.

    Icarus

    PS I've never been accused of being a smart ass!
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    "PS I've never been accused of being a smart ass!"
    ok i'm won't accuse you.:-)
    kidding asside, items placed on individual batteries in a series battery bank even if each battery has that item individualy attached can have differentials crop up. if said items attached to the series battery bank are also seriesed before attaching to the battery bank this would even things out.
  • System2
    System2 Posts: 6,290 admin
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    Re: Tapping 12 V from a 24 V Bank

    All the landscape lights - at this point - are LED therefore I do not think I can wire in series which would be hard anyway since they are daisy chained for the most part.

    tsp
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Tapping 12 V from a 24 V Bank

    Neil and Bill's admonitions and warning included. Since the loads are LED and therefore very small. I wouldn't have any problem pulling 2 curcuits off the bank of equal loading. I don't remember how many you have, (if you have told us), but a bunch of LED can't draw very many amps, or Milliamps for that matter. Keep an eye on the sg of the batteries and see what happens.

    Good luck

    Icarus
  • System2
    System2 Posts: 6,290 admin
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    Re: Tapping 12 V from a 24 V Bank

    Just a note, I wire LEDs in series regularly, In fact my in house and outside night lights are strings of 3 in series, run off a 7809 regulator to even things out. Powered from 12 system. Been using the same ones for about 5 years now.