500w 12v panels charging 2 battery

KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
I have 500 W at 12 V solar array. I want to be able to charge two separate battery banks using two separate charge controllers from the one PV system. Both battery banks are 12 V. One battery bank is my main bank and the other is a very small one that runs the motor for my sun tracker for my panels. I don't want to have to have a separate small panel to charge the small battery, I just want to tie in to my main system and then to the second charge controller connected to the smaller battery. Is there any problem with this? One charge controller is 30amp which can handle all 500 W. The other charge controller is only 15Amp for the smaller battery. I am assuming the smaller charge controller will cut the amps at 15 and still allow the other to remain at 30 Thx!


  • BB.BB. Super Moderators, Administrators Posts: 33,318 admin
    • First, both baVstems must share a common ground bus.

    Second, both controllers must be PWM (pulse width modulation). MPPT controllers should not share a common array (they will usually MPPT maximum power point tracking correctly/optimally).
    • 500 watts / 17.5 volts typical Vmp = ~28.6 amps Imp
    Connecting that through a PWM controller rated for 15 amps--Not ideal. PWM controllers cannot safely/effectively modulate/limit current through though them. Many can shut down if over heated (possibly over current). But they will not limit to 15 amps rated current (MPPT controllers will do that nicely all day long).

    If you have small capacity 12 volt battery and (usually) do not discharge it very deeply, the battery itself may limit to less than 15 amps. I.e. an 15 AH battery will probably take ~25% maximum current flow or (15a*0.25=) ~3.75 amps typical maximum charging current.

    If you have a larger "small battery" and/or deeply cycled, then the 15 amp controller may be on the small side. If it is cheap and you have ti, put a 20 amp fuse on the battery charge output let it run and see what happens.

    Some of this is the question of how much you want to "hand hold" the system(s). A well designed solar power system is pretty much setup and forget with monthly battery maintenance. Usually, people don't go out and check charging current after 2-4 days of stormy weather and then sun to check that the charging current is "OK".

    You could do a quick modification to your small charge controller... Say you want 15 amps max. Put a power resistor in series the 500 watt solar array:
    • V=I*R; R= V/I = (17.5 volts - 12.0 volts low batt voltage) / 15 amps = 0.37 Ohms

    Or you can put in 67 feet of 14 AWG wire in series with the solar panel for ~0.37 ohms.

    That will limit the charging current pretty effectively without too much issue (assuming you are not after "optimum" solar charging for the small battery).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KNLsolarKNLsolar Registered Users Posts: 18 ✭✭
    That makes sense... my issue is I have my house wired with the 12 V running the lights. So when my solar tracker motor turns on to adjust the lights flicker since it's all running off the same system. Can you think of a different solution to this problem other than having my SOLAR tracker run off a different battery bank?
  • BB.BB. Super Moderators, Administrators Posts: 33,318 admin
    Check your 12 volt wiring from the batteries to the common12 volt bus. Light bus wiring means higher voltage drop when the motors cycle.

    Run motor wiring to the bus (plus fuse or breaker). Don't share your lighting wiring with the motor wiring/current. That will make flickering worse.

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