Charge controller - Different battery packs

Giel
Giel Registered Users Posts: 3
in Off Grid Solar & Battery Systems #1
Is it possible to charge 2 battery packs with different Ah ratings from a Outback Flexmax 80 charge controller?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Charge controller - Different battery packs

    Welcome to the forum.

    The answer is yes, but not at the same time.

    If you parallel two battery banks with the same Voltage but different Amp hour capacities they will charge at different rates for any given amount of current applied. This means one will reach full charge before the other, inevitably resulting in one being undercharged or one being overcharged or possibly both. How much so depends on the difference between the two and how the controller is set.

    However it is possible to use a battery switch to select one or the other for charging and, assuming they are the same type/Voltage spec, the controller will function according to the Voltage set points.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Charge controller - Different battery packs

    I will say that this is not an ideal setup--But if you already have the battery bank(s) and you are stuck with this configuration--It is possible it will work out OK for you.

    What are the two different banks you want to put together (battery type, voltage, AH, age, etc.).

    In theory, batteries that are the same chemistry and roughly the same age/condition, should behave the same way to the same voltages (charging/discharging). And should, to a degree, simply limit themselves to the current they are capable of handling.

    If the batteries are very different (flooded vs AGM vs Calcium low water usage type, etc.), the voltages are different enough that they may not charge/discharge evenly and damage one or the other (or even both) banks.

    Also, batteries are (typically) around +/- 20% (or better) to their rated capacity. So if the two battery AH capacities are far apart (i.e., 100 AH vs 10 AH), the smaller bank will probably not even be noticed in the overall capacity/operation of the bank... So, there, practially speaking, an argument that you really only want to parallel batteries that are of similar size/construction together.

    And lastly, besides the usual tools (DMM, hydrometer, etc.), I would suggest you get an inexpensive DC Current Clamp meter (really an AC/DC Current Clamp / DMM type meter). You can measure the charging/discharging current between parallels strings and make sure all is well--If there a problems (voltage, specific gravity, current differences), then you need to investigate before a small problem can take out multiple cells/batteries/strings.

    If you just want your system to work (and don't like puttering around your power system), then you should avoid mix and match battery banks--And, I would suggest, even avoid paralleling batteries (1-3 strings would be my recommended range, favoring 1-2 strings if possible/practicable).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #4
    Re: Charge controller - Different battery packs

    I ran into similar problems due to the fact that I didn't want to get a mega-expensve battery pack, so I have chosen to have several battery packs. The challenge became the different charge parameters and the remote temp sensors. I didn't have a good way to switch them automatically so I decided to have each battery pack have a separate charge controller. It works pretty well, but I won't move in for a few weeks, so I'll know more later. Battery switches are your friend and I switch the panels to differet controllers, but it would be super-cool if it could be automated to make the most of the panels output.

    Good luck and let us know how it goes.
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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  • Giel
    Giel Registered Users Posts: 3
    #5
    Re: Charge controller - Different battery packs

    Thank you for your reply. Your explanation is exactly what I expected , but being a bit new in this game I hoped that it might have been possible to use the extra batteries that I could get hold of.
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  • Giel
    Giel Registered Users Posts: 3
    #6
    Re: Charge controller - Different battery packs

    Thanks for the feedback.
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  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    #7
    Re: Charge controller - Different battery packs

    I have heard of people using another bank as the divertion load for when the first bank is full. I would think battery size would be less important as long as the batteries match the bank they are in. How to control the second bank in this situation eludes me though.
    gww
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: Charge controller - Different battery packs

    Lead Acid Batteries are "poor" diversion loads... The store (relatively) small amounts of power for their size/weight/costs (still better than most anything else at this time).

    Leave the batteries set for long periods of time at less than full charge--they sulfate and die. Pump energy in the battery bank after it is full--Electrolysis generates explosive hydrogen gas, uses up distilled water, overheats battery bank and dies (or catches fire and kills).

    Size your battery bank to your loads (and charging sources)--And install opportunity loads (water pumping to cistern) and true dump loads (resistive load banks). And monitor the battery bank/system performance for anything that does not "look right" and fix before things really go south.

    -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 ✭✭✭
    #9
    Re: Charge controller - Different battery packs

    I don't think he means diversion load per se, but rather having a back-up battery bank that the surplus solar power is diverted to once the primary bank is full. It is actually a common off-grid practice. The down side is that if the power stored in the 2nd bank is also used daily you don't really gain anything, so you may as well have one large bank instead. Putting that otherwise unrealized power to use is the key to improving efficiency, and that means using it for work. Hence the typical AUX programming to turn on things like water pumps and heaters once the batteries are full or nearly so. MidNite's "waste not" function is great for this.

    Either the thread is or I am beginning to ramble. Not sure which. :blush:
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  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    #10
    Re: Charge controller - Different battery packs

    I intend to use hot water heating as a oppertunity load. I was just thinking that if some one expanded there system and didn't want to mix new batts with old or something along those lines, they might be put in a situation where it made decent sence to have two banks. I am going to try to get by on 16 sams club batteries with 5600w solar and two 500 watt turbines. I hope to make up the battery shortfall with opertunity loads or lord forbid loss of production due to no place to put it. When I screw up I might be looking for new ways to fix my mistakes.
    Cheers
    gww
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