what to do with old battery bank

hillbilly
hillbilly Solar Expert Posts: 334 ✭✭
in Off Grid Solar & Battery Systems #1
Greetings. I'm still in a bit of a dilemma over what to do with my current battery bank when the new batteries arrive. I've been tempted to keep some or all of the "old" bank as a back up/emergency set. Just hypothetically, if I were to do this, would there be any issues with just hooking up one of those blue seas battery switches to be able to switch from one bank to the other for charging/discharging purposes? Mostly just to keep the old set charged, and probably discharge them a bit from time to time.

I'm not in any way interested in trying to "add" them together, just looking for a simple and cheap option to keep two healthy sets of batteries. I'm sort of inclined to keep them around since they've still got some life in them, but I don't really want to spend a ton or overly complicate my system in order to do so...

Details would be:
VFX inverter charger mounted on an E-panel (250a breaker)
MX60 charge controller (current), adding a second charge controller for additional PV (likely a Morningstar 45A Mppt)
~400ah (or so?) capacity on the old battery bank
~800ah on the new bank
Charging capabilities: upping the PV array from about 1200w to about 2200w, plus two generators that can be used for bulk charging, or finish charging if needed.
Daily loads tend to be less than 3kwh typically. (80ah-140ah tends to be our range)

Any ideas?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: what to do with old battery bank

    As long as the new aren't connected to the old the main issue would be keeping them all charged. Mostly a matter of paying attention and switching charging between one and the other. Remember the old batteries will have a higher self-discharge rate.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: what to do with old battery bank

    Just make sure that the extra wiring+switch does not add more to the overall voltage drop (at max charging current).

    For a 12 volt bank, I would aim at a maximum of 0.10 volt drop (0.05 volt even better).. 0.2 volts for a 24 volt bank and 0.4 volts max for a 48 volt bank.

    Those are very low voltage drops. More than 0.2 volt drop will probably start to affect your battery charging significantly.

    -Bill "IMHO" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
    #4
    Re: what to do with old battery bank

    Bill:
    I'm not quite sure that I follow you exactly there. I do understand the need to keep voltage drop to a minimum, and have 4/0 cable to run from the inverter to the battery (could add another 10-12' of 4/0 to then run to the old battery bank). When you say that any more than a 0.2V drop would start affecting the charging of the batteries, are you talking about the parallel or series connections? Otherwise I'm confused by what you meant
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: what to do with old battery bank

    Basically, the battery charger is supposed to measure the exact voltage at the battery and control it.

    If you have 0.2 volt between the charger and the battery bank, the charger may be set for 14.5 volts but the battery is really at 14.3 volts (because of voltage drop of the heavy current + long/small gauge wire run)--And that amount of reduction of voltage at the battery will make the battery charger switch over to absorb mode where the battery starts limiting current flow (earlier than really needed).

    For a solar system, this may be lost of charging current before the sun sets (you have limited hours per day to charge).

    For a generator, this means you may have more hours of run time because of the lower current flow into the battery towards the end of the charging cycle.

    You could program the controller to 14.7 volts to get 14.5 volts at the battery under heavy charging current... But as the charging current falls, the voltage at the battery will start rising above 14.5 volts--possibly leading to overcharging/more gassing and water usage.

    There are a few charge controllers that have remote battery voltage sense leads (MorningStar TS solar charger line--both PWM and MPPT lines--and the Rogue 30 amp MPPT controller are two that I am aware of) which you attach directly to the battery terminals (or battery bus). That eliminates most of the voltage drop issues during charging.

    The other help is a remote battery temperature sensor... Some charge controllers read "hot" if you don't use the remote sensor--Which also causes the controller to drop the battery charging voltage (hot batteries need lower charging voltages).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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