Parallel Charge Controllers off of same Solar Array?

NVsunnyNVsunny Registered Users Posts: 2
Hi, I have an application where there's a large solar array with its own charge controller and lead-acid battery bank; I also have a small Lithium-Ion battery bank of the same nominal voltage, but the actual voltage and desired charge voltages are obviously different. I'd like to tap into the DC bus of the existing solar array to run a separate charge controller for LI battery bank. Is there any reason that would cause a problem for the array or charge controllers? Thanks for any insight; I see a lot of threads about paralleling charge controllers to a single battery bank, but haven't found this question discussed.


  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Welcome to the forum NV.

    The answer is "it depends".

    If a simple pwm (pulse width modulation) controller, the answer is usually yes.

    If MPPT (maximum power point tracking), the answer is generally no.

    So, details about your system?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NVsunnyNVsunny Registered Users Posts: 2
    I'll have to get the model# of the existing charge controller, and see if it's an MPPT. For the new one, I still need to buy it, so I can choose MPPT, if the extra cost will make this feasible. But are you suggesting that if the existing one isn't MPPT, that something else drawing a load from the panels will confuse it?
    As for other details, the existing array is 3kw nominal 24VDC, and the new charge controller would be a 30A model, with expected loading well below that.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    An MPPT type charge controller "sweeps" or measures the Vmp and Imp of the solar array to figure out the optimum operating points. With several MPPT controllers on a single array--You have two controllers that are drawing varying current/sweeping the array--And they can make incorrect decisions on how to correctly "control" the array.

    There are specific controllers (one or several Schneider/Xantrex MPPT models--Only ones that I know of--but I am not in the business, so don't know much) that must have dedicated +/- connections to the solar array to correctly operate (they need to measure the current through the ground/return leg).

    With PWM controllers--They are simple "on/off" switches between the solar array and the battery bank(s)... So they can share a solar array.

    Be aware that the Array Voltage is basically "set" by the battery bank. If you have a well discharged Lead Acid battery bank, the charging voltage can range from ~12 volts (or less) to > 15.0 volts (12 volt battery bank). With LiFePO4 batteries, they have a much tighter range of working voltage.

    So--What may happen is the Lead Acid battery bank takes all the current until it gets up to the LiFePO4 bank charging voltage, then they share current for a while, but the lead acid will not fully/quickly charge until the Lithium bank is "full" (say your bank is 14.0 volts for lithium), then the PWM controller for the Lithium bank will "turn off" and let the array voltage rise again to finish the lead acid charging at 14.8 volts or so...

    The two banks should charge, but they may do it in an "uneven" manner. And that can be a problem for the lead acid batteries. LA batteries need as many charging hours as possible from solar--If they stop charging for X hours in the middle of the day until the Li batteries are full--You may not have enough hours of solar/sun to fully/completely recharge the LA battery bank.

    Anyway--I have not tried, or read of anyone trying to charge from single array to a LA + Lithium set of battery banks (with two PWM controllers)--But that is what a think could happen and you should watch for the issue.

    In summer, when you have lots of sun, may be a non-issue. Also, if you do not heavily discharge the LA battery bank, you do not need as many hours of sun per day to fully recharge (less than ~25% discharge as a rough example)..

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