Learning about DC Power

poppy
poppy Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
My question is it possible to charge a 48 volt battery bank in 12 volt segments. I am trying to come up with a way to start small before I go big.
· Share on Twitter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
    Re: Learning about DC Power

    Are you talking about disconnecting the batteries and charging them as 12 volts each? Or leaving them connected and charging each battery with a 12 volt charger that you move from battery to battery?

    In theory, you can do either. The practical matter is that disconnecting batteries is a pain in the behind and/or clipping a charger every night or three to four or more batteries to recharge is not something that you would want to do long term either (wear and tear on battery connections, possiblity of miss-connections of the charger, that a battery to fully charge could take 8 hours of charging x 4 batteries with one charger would be 32 hours to recharge in 24 hour day, accidents with all that handling--drop tools on battery connections, spark causing hydrogen gas to explode, etc.).

    If you start out at 12 volts and want to go to 24 or 48 volts--typically you would need a new inverter and AC battery charger (usually only available in one voltage).

    The MPPT solar charge controllers are typically available in multi-output (12/24/48 VDC battery bank support) with a simple controller reconfiguration. With the solar panels and a MPPT type charge controller, you can wire the panels up for Vmp>62 VDC and pretty efficiently charge any battery bank from 12-48 VDC without rewiring the solar panel strings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Learning about DC Power

    there could be possible trouble if he uses multiple chargers onto a connected battery bank. one charger charging one battery at a time should be ok. i'd recommend fusing the charger output just in case.
    · Share on Twitter
  • poppy
    poppy Registered Users Posts: 5 ✭
    #4
    Re: Learning about DC Power

    I was going to leave all connections permament but have power taps at every 12 volts.I thought I could do load diversion controls from a12 volt Wind Generator.
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
    Re: Learning about DC Power

    Generally, it is a bad idea to have 12 volt "taps" on a 24 or 48 volt battery system.

    The batteries typically only work well if all the cells have balanced current flow (Amp*Hours in = Amp*Hours out).

    If the battery charging/loading is unbalanced, you can very quickly get into problems (one 12 volt battery is undercharged and another in the same string is over charged).

    Say 24 volt system, one battery at 100% charge (that wind turbine was working well) and the other battery is at 50% (well pump used more than normal because of dry spring). Now, you turn on your 24 VDC inverter and start discharging the whole battery bank... You go from "100%" to 70% because you measured the specific gravity in only the charged battery bank.

    Battery A is still at 70% capacity, but battery B is as 20% capacity and if you draw down any more, Battery B stands a chance at cell reversal in one or more cells (battery cells do not always balance/match capacity exactly). You continue to discharge, and one cell in battery B reverses polarity--and Battery B is now dead (if it was not already killed by staying at 50% state of charge for weeks at a time).

    Some people have tried a variation of the 12 volt tap on a 24 volt system (plane, truck, boat)--for example to run a 12 volt radio. Problem is that when the 24 volt generator recharges the battery bank, it has to over charge the non-tapped battery in order to recharge the tapped battery--causing sever stress on the overcharging battery.

    If you need 12 volts (say because you have multiple wind turbines)--then you are probably better off running the entire battery bank at 12 volts rather than trying to balance "12 volt" chunks on a 24 or 48 vdc system.

    It is difficult enough to keep track of one bank voltage/state of charge. Much more difficult with 2-4 interconnected banks that that have varying charging and loading profiles.

    If you are stuck with 12 volt wind turbines and need 48 vdc power levels--perhaps something else needs to be done (12 volt 48 VDC battery charger?).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    #6
    Re: Learning about DC Power

    I think your fundamental problem with a setup like that is unbalanced charging. Especially if you are switching a single RE source (wind turbine or solar) between each bank. You aren't going to be able to guarantee consistent charging, so if you switch on time you will have one set with less charging than the next. If you switch based on AH, if you have a low-wind or cloudy day you may not get around to charging one of the sets for quite some time.

    Do you plan to have the batteries in use all the time? Or are they a "backup" set that will primarily sit fully charged all the time and only need maintenance charging? If for backup, this isn't such a big deal. But if you have a load on the bank, I think you'll wind up with significant imbalances between batteries in the string in short order.

    Another concern for an automated switching system, what happens if one of your relays sticks closed? You could conceivably put a dead short across one of your batteries. Definitely use suitably sized fuses just in case.
    · Share on Twitter

Categories