Battery mix

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cards385
cards385 Registered Users Posts: 21 ✭
in Solar Beginners Corner #1
Is it a problem if I hook up a DEKA DP24 (dual purpose) and a DEKA DC24 (Deep Cycle) batteries together?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #2
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    Re: Battery mix

    First warning is that mixing battery types/lots/ages/brands is not the best thing. Batteries need to be "matched" so they discharge and charge "in balance". If you have one "strong" battery and the others are "weak"--the strong one will do the most cycling and probably fail the quickest.

    From the Deka Data Sheet (PDF), the DC24 has a ~15-20% higher AH capacity vs the DP. So--if these batteries were connected in series, I would not discharge a string that had DC+DP in series to less than 40% State of Charge of the DC AH rating, or less than 20% SoC of the smaller DP AH rating (typically, over discharging batteries and causing "cell voltage reversal" is pretty much going to kill a battery).

    If they are in parallel, they appear to need the same charging voltage (nothing called out as different). Assuming you can pop the caps on the batteries, you might check with a hydrometer ever once in a while to see if they discharge evenly or not (if one type discharges more--then the other battery type is not really adding to your string capacity very much).

    Note that the DC battery has "Low Antimony" plates which has several effects. Antimony vs Calcium does the following to Lead Acid Batteries:
    Lead Calcium Batteries
    Lead acid batteries with electrodes modified by the addition of Calcium providing the following advantages:
    • More resistant to corrosion, overcharging, gassing, water usage, and self-discharge, all of which shorten battery life.
    • Larger electrolyte reserve area above the plates.
    • Higher Cold Cranking Amp ratings.
    • Little or No maintenance.
    Lead Antimony Batteries
    Lead acid batteries with electrodes modified by the addition of Antimony providing the following advantages:
    • Improved mechanical strength of electrodes - important for EV and deep discharge applications
    • Reduced internal heat and water loss.
    • Longer service life than Calcium batteries.
    • Easier to recharge when completely discharged.
    • Lower cost.
    Lead Antimony batteries have a higher self discharge rate of 2% to 10% per week compared with the 1% to 5% per month for Lead Calcium batteries.

    So, you will probably see differing water usage and aging between the battery types.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: Battery mix

    The problem with mixing different batteries is that their performance curves are different. Whether it's a question of age or type or capacity, they won't charge/discharge the same. Too much difference and the 'weaker' battery will have significant detrimental effects on the 'stronger' battery - dragging it down to the lower level, so to speak. Conversely, the charging difference will cause the battery needing less Amps to heat up more, possibly over charge, boil off water, burn up plates. It's a matter of how far apart the performance curve; bigger difference, bigger problems. There are even minute differences between 'identical' batteries made in the same 'batch'.

    The two you mention are close enough that there shouldn't be any significant problem for light usage, unless they are different in age (older = greater sulphation and therefor less capacity). Just keep an eye on them for any significant changes like large Voltage differentiation between them, et cetera.

    (Somebody's bound to start nit-picking and say I'm wrong. :p )
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  • cards385
    cards385 Registered Users Posts: 21 ✭
    #4
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    Re: Battery mix

    Thanks, you guys always have good advise.
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  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
    #5
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    Re: Battery mix

    re "Too much difference and the 'weaker' battery will have significant detrimental effects on the 'stronger' battery - dragging it down to the lower level, so to speak."

    With lead acid batteries, the issue is voltage differences in both charge and discharge. As age or size do not influence battery voltage, there is no "dragging down" by a weak battery. As BB noted, the issue is simply exercise as the share of the load (or charge) is proportional to the strength of the battery.

    Where you do need to be careful is when there are voltage differences. The Calcium vs Antimony doping doesn't seem to do much in use but the self discharge differences could be important for long periods of non use. Mixing AGM's with wet cells could also be something to be careful about depending upon the battery. Mixing entirely different chemistries would definitely be a concern.

    But consider that many RV's parallel their house and chassis batteries when the engine is on. That works.
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  • System2
    System2 Posts: 6,290 admin
    #6
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    Re: Battery mix

    Remote Cabin--Dc Off Grid System

    WE just installed a 100W Solar Panel for an off grid system at a remote cabin. Currently we have two 6V golf cart deep cycle batteries in the system, which are connected in Series. We have an opportunity to acquire a few 12V Gel Cel batteries very cheaply. Can we safely connect them in parallel with the 6V lead acid batteries, and leave them connected, and charging while we are gone for extended periods of time. Or, should we disconnect the Gel Cells when we are not there and just charge the Lead acid batteries during periods of extedned absence?
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #7
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    Re: Battery mix

    100 watts of panel with such a large battery bank, there is little danger of over charging the bank with extended absences.

    As for mixing the different chemistries :http://www.batteryfaq.org/
    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    I have a hunch that one set will be a bit under charged through out depending on how you set the controller.

    Also, a 100 watt panel is not going to be much of a charge source if you are ever going to load the batteries to power your cabin. The general rule of thumb is that charging capacity should be between 5-15% of battery capacity. The 2 golf cart batteries whould have a ah capacity of ~225 ah, needing ~12-36 amps to charge properly on a regular basis. (Something on the order of 2-300 watts minimum) Your hundred watts might put out ~5 amps on a perfect day.

    Good luck, welcome to the forum,

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Battery mix

    Gel's and AGM's have a much lower 'self-discharge rate' than flooded cells. If they're fully-charged when you leave, chances are they will be still good when you get back.

    That said, mixing types of batteries is probably the worst thing you can do. The charge specifications are much different for gel's as opposed to FLA's. Too much Voltage and the gel's will burst. You never equalize gels either for the same reason.

    Also, I don't see you getting a decent charge out of a 100 Watt panel even for a set of 6V 'golf cart' batteries. Your maximum charge current probably isn't over 7 Amps. Considering that a golf cart battery is usually 200 Amp/hrs, you're not even making the 5% minimum for those batteries (10+ Amps).
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