Charging other batteries from solar

Shadowcatcher
Shadowcatcher Solar Expert Posts: 228 ✭✭✭
in Solar Beginners Corner #1
This is a fairly simple question, but a long tale. I was in the process of checking out a DC to DC buck converter stability and hooked it up to a small 12V battery from the lawn tractor and to check stability hooked up a battery charger which has a digital gauge. When started the voltage will hit 13.98 volts according to my VOM and sure enough the buck converter worked fine with the voltage staying at the set 8V. and then the charger clicks out of the 13.98 and down to 12.35 while reading 13.6V. What this means is that all the other batteries, car battery for the Saab Sonett, and 75 AH AGM that is half dead are not getting much at all and this charger is not working they way it should.

I have a tap from the 150AH AGM battery in the teardrop to power what ever, and this is fed by a Morningstar MPPT converter. now the question, can I use that tap to charge these other batteries?
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: Charging other batteries from solar
    Shadowcatcher wrote: »
    This is a fairly simple question, but a long tale. I was in the process of checking out a DC to DC buck converter stability and hooked it up to a small 12V battery from the lawn tractor and to check stability hooked up a battery charger which has a digital gauge. When started the voltage will hit 13.98 volts according to my VOM and sure enough the buck converter worked fine with the voltage staying at the set 8V. and then the charger clicks out of the 13.98 and down to 12.35 while reading 13.6V. What this means is that all the other batteries, car battery for the Saab Sonett, and 75 AH AGM that is half dead are not getting much at all and this charger is not working they way it should.

    I have a tap from the 150AH AGM battery in the teardrop to power what ever, and this is fed by a Morningstar MPPT converter. now the question, can I use that tap to charge these other batteries?

    Hmm. There is no reason not to use a single CC to charge a varying array of different battery types added on in parallel to the main battery as long as you supply the necessary backflow prevention, fusing, etc. But for long term use rather than occasional manual connection, you will be up against all of the common problems of charging batteries in parallel, only worse if the various batteries have different chemistry.
    The biggest problem may be the sudden rush of current when you parallel a nearly discharged battery to your fully charged main battery. I would look at providing some sort of current limiting!
    SMA SB 3000, old BP panels.
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  • Shadowcatcher
    Shadowcatcher Solar Expert Posts: 228 ✭✭✭
    #3
    Re: Charging other batteries from solar

    Thanks Fizz
    The batteries are mostly charged but not fully charged, from what I understand not a good thing. The connectors are 40A capable power poles and I have a 20A inline fuse. The plan is to leave them hooked up for at most a day. It is cloudy today so the panel is only putting out 33V at the controller and the reading at the battery is 13.9V I will check periodically as the day progresses.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Charging other batteries from solar
    Shadowcatcher wrote: »
    I will check periodically as the day progresses.
    You are welcome, and I am glad that you picked up on the most important thing, which I forgot to say, which is keeping an eye on it. Watching the voltages is good, and if you have a DC meter you can use, watching the current is equally important.
    SMA SB 3000, old BP panels.
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