will this work?

System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
I have several boat (starting) batteries that I would like to keep float charged over the winter by a small 15 watt solar panel. Because each battery is a different size and age, I assume hooking them up in parallel would be a bad idea? If so, would installing inexpensive shunt type charge controllers for each battery all using the same panel work?
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  • System2
    System2 Posts: 6,290 admin
    #2
    Re: will this work?

    How many are "several"?
    You probably won't need a controller, depending on how many batteries you have, as you only have 15 watts, which would only give you somewhat less than one amp max.
    You could isolate each battery with a 1 amp diode, from the + of each battery. The cathode of the diode would go to the + on the battery. One diode per battery. Connect all the anode terminals of the diodes together, to the + from the solar panel.
    All the battery negative terminals would connect together to the negative of the solar panel.
    Keep in mind though, that even with the batteries isolated with the diodes, if one battery is bad and sucking up all the charge, there will be none left over for any of the other batteries, so in time, they will be in trouble.
    Keep an eye on them with a volt meter.
    Good luck
    Wayne
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: will this work?

    i'm going to put out a word of caution when using small pvs to maintain larger batteries. keep in mind that yes if the current is very small it should be fine, but i have found that because the voltage is not regulated, some batteries could be damaged. the reasonning here is that bulk charging (80% of the charge) is basically wide open charging with nothing held back until it reaches the maximum voltage point. it is that point that can't be exceeded and is now in the absorb stage of charge that allows the voltage to stay at one point while the charge current reduces. if maintaining with 1% of the battery capacity, damage is a possibility to occur without voltage regulation being present. if at .1% of charging capacity i think it's safe to allow it to be unregulated. wind-sun i believe has addressed this elsewhere and i haven't read it, but i hope it says the same as i have. the exact crossover point for no voltage regulation can be a sticky issue as this could depend on time factors allowing this type of maintenance to occur. ie., just put on for a day as opposed to always.
    btw, in reference to the wording i used for damage, i use this in a very broad sense in that it could be just excessive boiling of the electrolye. this can't be viewed as damage as higher battery maintenance can avert actual damage, but in this sense is more damaging than normal and could cause premature failure.
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: will this work?

    I'm not asking if I can leave the batteries charging unregulated. My question is if I can get away with one charge controller or three (for 3 batteries). I will not be able to check on the system for 6 months+ at a time, hence the need for a float charge.

    The diode idea seems like it would work (placing them after a single charge controller) to avoid any battery from depleting the others.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: will this work?

    i see what you're saying here. 3 batteries and 3 controllers with one pv. simply said, you can do it as the seperate controllers will isolate each battery from the other. isolation diodes will do the same, but with about a 1/2 volt drop. in either case the current outputted by the pv used will be divided by 3. if using with one controller into 3 isolating diodes this affects the settings of the controller down by that 1/2 volt drop. an example would be 14.4v out of the controller, but after the diode it will show about 13.9v. does this cover your question and any others that you would've come up?
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  • System2
    System2 Posts: 6,290 admin
    #6
    Re: will this work?
    niel wrote:
    either case the current outputted by the pv used will be divided by 3.
    Maybe yes, maybe no. It would depend on the state of charge of each of the batteries. if only one is highly charged to start with, it will have a higher voltage, so the very limited current supply from the PV will go elsewhere. Likewise, one very low battery will take it all.
    I know you are aware of that, it just didn't come across in your post.
    Cheers
    Wayne
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #7
    Re: will this work?

    Here's a link to Morningstar's recommendation for using one controller to charge multiple batteries:

    http://www.morningstarcorp.com/products/FAQ/Isobanks_one_controller.pdf

    Here's a link to Morningstar's recommendation for using one controller per battery from just one PV:

    http://www.morningstarcorp.com/products/FAQ/iso_banks.pdf

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #8
    Re: will this work?

    Thanks for the info I needed.
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