questions on charging multiple batteries seperately

notsobright
notsobright Solar Expert Posts: 247 ✭✭
in Off Grid Solar & Battery Systems #1
I want to take advantage of my RVs off grid system.

using several old car batteries to power various lights and other electronics in and around the house I plan to charge them up by using the aux output on my onboard charge controller.

to get the most out of my PV I plan to charge as many batteries as possible each day but Im not sure when they are actually fully charged.

I know that the full duration of the bulk charge stage is needed but does each battery need the full duration of the absorption stage? I think the float stage is not required for a full charge, is that correct?

Im thinking I'll just pull the battery that is charging off when it hits the float stage to put the next one on charge but on my controller the time to float is adjustable. I think that this is setting is basically the duration of the absorption stage but not certain.

I have available many, various, old but good car batteries and since I have this off-grid RV system that is essentially not in use most of the time I'd like to make use of it and save a few buck on my utility bills while Im at it.

if you wanted to get the most batteries possible charged each day with 345w of PV how would You do it?


thanks
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    Re: questions on charging multiple batteries seperately

    I think to get past the sulphation point, you have to spend some time in absorb.
    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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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: questions on charging multiple batteries seperately

    You're going to have some trouble here as you want to charge several batteries of different types, no? As in "automotive" batteries. Your solar power was set up to charge a specific battery; probably a deep cycle with 'X' Amp hours capacity. A typical automotive battery is about 100 Amp hours, and is not meant to be "deep cycled". Rather they are designed to have some heavy current drawn briefly (starting), and then the alternator kicks in and tries to keep everything at 13.8 +/- Volts.

    So you know you won't have ideal charging situation here, no matter what.

    345 Watts of PV could put out 18 Amps - maybe 20. Pretty respectable current. You could probably do two automotive batteries at once, but they won't be balanced because they won't have been discharged or aged equally. What type of charge controller do you have? If it is a simple, PWM one I'd keep a close eye on the battery while charging as you have the potential for over-charging. If it's a fancy Outback MX60 you can reprogram it to suit the battery - and the Absorb stage will take care of itself. Can you at least monitor the current draw? If it goes below 2 Amps, say, and the Voltage is reading 13.8 or better you're probably done. Are these batteries sealed or can you check the SG? Nothing like a stable SG reading to tell you what's going on inside.

    It's a pretty sensible thing to attempt, in my opinion. You've got the PV, use it! :D
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  • notsobright
    notsobright Solar Expert Posts: 247 ✭✭
    #4
    Re: questions on charging multiple batteries seperately
    mike90045 wrote: »
    I think to get past the sulphation point, you have to spend some time in absorb.

    any idea how to determine that "time"?

    Cariboocoot wrote: »
    If it goes below 2 Amps, say, and the Voltage is reading 13.8 or better you're probably done.

    that helps! yes, I have a PC connected to the controller. I can monotor and program.


    whats the average ah ratings on most car batteries? many have CCA ratings on the labels but none except one that is a deep cycle has AH ratings indicated on them.

    thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: questions on charging multiple batteries seperately
    notsobright wrote: »
    any idea how to determine that "time"?

    The amount of time it takes for the battery to reach "ABSORB" from when charging starts is a good rule-of-thumb for Absorb time. In other words, if it hits "14.2 Volts" in one hour, leave it absorb for one hour.
    whats the average ah ratings on most car batteries? many have CCA ratings on the labels but none except one that is a deep cycle has AH ratings indicated on them.

    thanks

    That's because car batteries aren't designed to be used this way. They give Cold Cranking Amps because that's what they're meant to do: supply a large amount of current to get the motor started when it's cold. You'll also see "reserve capacity" which is a kind of "voodoo" rating of what you can expect after failed starting attempts.

    Mostly I'd change the parameters so that peak Voltage is 14.2, then settle back to 13.8 for the "ABSORB" mode as there isn't going to be any "FLOAT" (or vice versa, depending on your POV). The trick is to get the solar charge to mimic what would happen in a car.
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