multiple battery - voltage divider harm?

bmet
bmet Solar Expert Posts: 630 ✭✭
in Solar Beginners Corner #1
In the instance of multiple batteries in series, is there any harm to tap off a single battery in the string to get either 6V or 12V for the purpose of running small devices? Would it be bad for the CC?

Now that I've purchased 24V panels, I still want to run my infrastructure (12V lights, 12V fans, 12V VCR). If I use series-connected batteries as a voltage divider, would that confuse the CC's charging?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: multiple battery - voltage divider harm?

    Yes, there is a problem with taping part of a battery bank. It won't confuse the charger, but it will unbalance the bank because part of it will be discharged more than the rest. The severity of the imbalance depends on the amount being drawn off the lower Voltage section and how long it is used. Equalization can bring them back into balance with the other cells, but only up to a point. EQ is in itself hard on batteries, so it is best not to do it more than necessary (some manufacturers prefer regular EQ however). Over time, even a small amount of current difference adds up.

    Ways around this include DC to DC converters capable of stepping down the Voltage and handling the current draw, separate lower Voltage systems recharged either from a separate array or the higher Voltage bank.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #3
    Re: multiple battery - voltage divider harm?

    Thanks for the information. I hadn't thought from the battery operation because I don't have that experience. It won't be long before I will have to invest in a bank, still looking at whether my first starter set should be 6V or 12V. Sam's carries the 'GC' and a Group 31 Marine at 12V. Other than voltage, do you think their is much difference between them?
    Cariboocoot wrote: »
    Yes, there is a problem with taping part of a battery bank. It won't confuse the charger, but it will unbalance the bank because part of it will be discharged more than the rest. The severity of the imbalance depends on the amount being drawn off the lower Voltage section and how long it is used. Equalization can bring them back into balance with the other cells, but only up to a point. EQ is in itself hard on batteries, so it is best not to do it more than necessary (some manufacturers prefer regular EQ however). Over time, even a small amount of current difference adds up.

    Ways around this include DC to DC converters capable of stepping down the Voltage and handling the current draw, separate lower Voltage systems recharged either from a separate array or the higher Voltage bank.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: multiple battery - voltage divider harm?
    bmet wrote: »
    Thanks for the information. I hadn't thought from the battery operation because I don't have that experience. It won't be long before I will have to invest in a bank, still looking at whether my first starter set should be 6V or 12V. Sam's carries the 'GC' and a Group 31 Marine at 12V. Other than voltage, do you think their is much difference between them?

    Yes. Actual "golf cart" batteries are real deep cycle. The Marine/RV are pretty much automotive batteries with thicker plates. I've known people who have used all three types: the auto and marine batteries end up in my van going to scrap inside of three years. Even the worst deep cycle install I've seen has managed five.

    The whole "6 or 12 Volt" issue is really a matter of how many Amp hours you need. The cases being the same size, the 6 will have more Amp hours than the 12. Roughly speaking, for the same amount of Amp hours requiring two 12's you'd also need two 6's. The difference? Parallel connection of the 12's is not as desirable from a current-sharing POV as serial connection of two 6's. This follows the bigger the battery bank gets, btw: it is better to have lower Voltage higher Amp hours batteries in series to come up with the total than to parallel many lower Amp hour/higher Voltage units. You probably already know that but I thought I'd reiterate in case someone who didn't reads this.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: multiple battery - voltage divider harm?

    I cannot find it right now, but somebody here posted a link to a power supply/charger that "balances" 24 volt battery banks (such as used on a bus/RV conversion) and allows 12 volts for accessories...

    It appears that it should be a good solution--Allows full rated 24 volt current and to use the full 24 volt bank to run 12 volt loads, plus the "12 volt" tap does have the full current (for surge loads) of 1/2 the 24 volt battery bank.

    NAWS sells some 12/24 volt converters:

    http://www.solar-electric.com/12to24or24to.html

    Solar Converters Inc (above) has application notes that talk about balancing 12 volt taps off 24 volt battery banks... Don't know how well they work--but also an option.

    Battery Equalizers/DC Autotransformers

    Of course, this does add an additional charge controller (plus losses).

    Is this of interest?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #6
    Re: multiple battery - voltage divider harm?

    you nailed it right on the head, whether Sams 'GC' batteries are actual golf cart type. Some members here have voiced their doubt.
    Cariboocoot wrote: »
    Yes. Actual "golf cart" batteries are real deep cycle. The Marine/RV are pretty much automotive batteries with thicker plates. I've known people who have used all three types: the auto and marine batteries end up in my van going to scrap inside of three years. Even the worst deep cycle install I've seen has managed five.

    The whole "6 or 12 Volt" issue is really a matter of how many Amp hours you need. The cases being the same size, the 6 will have more Amp hours than the 12. Roughly speaking, for the same amount of Amp hours requiring two 12's you'd also need two 6's. The difference? Parallel connection of the 12's is not as desirable from a current-sharing POV as serial connection of two 6's. This follows the bigger the battery bank gets, btw: it is better to have lower Voltage higher Amp hours batteries in series to come up with the total than to parallel many lower Amp hour/higher Voltage units. You probably already know that but I thought I'd reiterate in case someone who didn't reads this.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #7
    Re: multiple battery - voltage divider harm?

    Yes, Thank you for those links. The Samlex product appeals to me for two reasons, Price and Amps. I don't run much Current for my infrastructure, perhaps 5-6 amps at this point.
    BB. wrote: »

    Is this of interest?

    -Bill
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #8
    Re: multiple battery - voltage divider harm?

    I've heard good things about Vicor DC-DC converters, they can be found cheap used on Ebay. Not so cheap new.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: multiple battery - voltage divider harm?
    bmet wrote: »
    you nailed it right on the head, whether Sams 'GC' batteries are actual golf cart type. Some members here have voiced their doubt.

    Yes, and others have had good experience with them. What really 'hurts' is that the same thing can be said for Surrettes! :cry:

    I can't get the Sam's or Costco units here (tried a couple of different locations) so I don't know first-hand what they are. I'd suspect they're made right, but possible a tad 'cheaper'. On the other hand, it's easy to abuse batteries. As long as you don't pay too much for them you could give 'em a try not expecting 8 years lifespan. It'd be a shame to buy Trojans and cook them in two years' time. Yes; I've seen it happen.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #10
    Re: multiple battery - voltage divider harm?

    You are not kidding about Vicor modules being expensive... I was working on a telecomm product (large voice mail system)--And I could never justify the cost to the upper management (and we did not focus that much on costs ~25 years ago).

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