Simultaneous 12 volt / 24 volt battery bank.

nemonemo Posts: 3Registered Users
Here is a hypothetical question.  Can I maintain a 12v and 24 volt bus off of the same battery bank simultaneously?  To be clear, this would be two separate busses wired independently off of the same bank.  Conceptually, it seems like it would work.  I would imagine you could charge the bank off of either bus.  I have a small cabin with some 12v lighting, but might like to add some 24 volt appliances without having to use electronics to bump up the voltage or convert to 24 volt and bump down for the existing lighting. 

Comments

  • BB.BB. Posts: 27,432Super Moderators admin
    The generic answer is no... If you draw power from a 12 volt "tap" off the 24 volt battery bank, the upper 12 volt and lower 12 volt batteries will go out of "balance". The only way to bring up the one 12 volt tap (typically the zero-12 volt bank" is to "equalize" the battery bank--Basically pushing a lot of charging current through both battery sets--And overcharging the "already charged" battery set.

    The specific answer is yes it can be done. HOWEVER, the standard way to do this (not uncommon on 24 volt bus conversions that want to run 12 volt appliances) is to to get a battery charger that "matches" voltage between the high 12 volt and low 12 volt sets. This is, basically, a battery charger with isolated outputs. It can both draw and supply current on both battery sets (i.e., pull current/power from the "higher voltage battery" and supply current/power to the lower voltage battery.


    These type of charge controllers are not (usually) "cheap". And many times it is simply easier to figure out alternatives.

    For example, sending power any sort of distance at 12 VDC. It usually requires very heavy and expensive copper cable to keep voltage drops "manageable" (any distances over ~10+ feet).

    Since the cost of a 12/24 volt "battery balancer/equalizer" is not much different than a 24 VDC to 120 VAC AC inverter (and roughly the same conversion efficiencies/losses), converting the 12 volt lighting to 120 VAC (and any 12 VDC appliances too) is about the same costs, and also allows you to now send significant amounts of power upwards of 100's of feet using standard house wiring techniques.

    And there are alternatives like using 2x solar charge controllers on each 12 volt section of the battery... But that ends up with one controller having a +12 volt "ground" connection (and other drawbacks too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Posts: 1,746Registered Users ✭✭✭✭
    If you are thinking along the lines of having a 24V bank, using a 12V  center tap, then that would definitely end in dissapointment, half the bank would be discharged and the charger would not recognize the depletion, resulting in premature failure of the batteries. Some say there are battery balancers which allow this, but it's best not to venture down the path of unknowns, my opinions others may differ. 
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • 706jim706jim Posts: 201Solar Expert ✭✭✭
    My place is wired for 12 volts and 120 volts. Two completely separated systems. The only thing I ended up using the 12 volt system for was cell phone chargers. Eventually, I bought a 12 volt power supply to feed the 12 volt circuit. So I invert 24 volts up to 120 volts and then transform that back to 12 volts. This keeps the battery bank equalized and despite the obvious inefficiency of this conversion, it actually works better than trying to take 12 volts directly from the battery bank.
    Island cottage solar system with 1400 watts of panels, Trace DR1524 MSW inverter, Trace C40 PWM controller 8 Trojan L16's. My 25th year.
  • nemonemo Posts: 3Registered Users
    BB. said:
    The generic answer is no... If you draw power from a 12 volt "tap" off the 24 volt battery bank, the upper 12 volt and lower 12 volt batteries will go out of "balance". The only way to bring up the one 12 volt tap (typically the zero-12 volt bank" is to "equalize" the battery bank--Basically pushing a lot of charging current through both battery sets--And overcharging the "already charged" battery set.

    I wasn't wanting to tap just one of the 12v batteries, but all of them in parrallel so they would all discharge together at the same rate to keep them in balance.
  • BB.BB. Posts: 27,432Super Moderators admin
    I am not sure of your answer of "I wasn't wanting to tap just one of the 12v batteries, but all of them in parallel so they would all discharge together at the same rate to keep them in balance."...

    There have been times when we have had questions about "wiring a battery bank to supply both 24 and 12 volts at the same time and parallel/balance the loads across all batteries". The answer to that has always been no--That is not possible.

    You could, for example, install a switch that lets you charge the battery bank at 24 volts, and switch to 12 volts for your loads--But that almost never make sense for long term needs (complex, needs to be switched for charging vs loading, and you cannot charge+run loads at the same time).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nemonemo Posts: 3Registered Users
    Can you explain how the 12v bus wired in parrallel would get out of balance?
  • BB.BB. Posts: 27,432Super Moderators admin
    The right hand battery has a dead short across its + and - connections. See if I can explain in words from my phone:

    Right battery + red wire up to left. Red wire down to left battery + terminal. Green wire to right battery - terminal.

    Dead short.

    Same thing with left battery. Another triangle short. Left battery - terminal to Black to Black to Green to left battery + terminal.

    Both batteries have dead short.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Posts: 1,746Registered Users ✭✭✭✭
    The attempt to link the two batteries to create 24V, the green line between battery 1 & 2 is what creates the short circuit across both batteries. Be very careful when working with batteries, they can provide very high current in the event of a short circuit.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

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