Two separate PV sources feeding one battery.

Options
I have a 75W PV panel that I'd like to add to my array.

My 20A controller is maxed out but I have an unused 10 contrtoller.

I intend to run a separate feed from the PV panel to the 10A controller. This will put two controllers in parallel. Sort of.

Has anyone heard of problems with mppt controllers wired this way?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    Options
    Re: Two separate PV sources feeding one battery.

    Can't think of any issues off hand... Just wire them to the same battery bank, but treat each controller/panel circuit as separate devices (don't share leads, grounds, signal connections, etc.).

    You may wish, after the battery bank is fully charged--run each controller separately for a little while to make sure that the voltage setpoints are correct and everything is working (few hours or day or two)... Might be hard to see issues with both controllers running at the same time.

    However, a question, are you sure that they (especially the 10 amp) controller are MPPT types? From what little I have read, I would worry that a true MPPT controller will burn more power operating than it will "save" if the panels are much less than 200-400 watts. A simple non-MPPT controller will be more efficient for the smaller panel arrays.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Options
    Re: Two separate PV sources feeding one battery.

    bb,
    that holds true for the mx60, but my sb50 operates with less power and there are other mppt controllers that are made for smaller power levels like the sb2000 that are mppt.
  • System2
    System2 Posts: 6,290 admin
    Options
    Re: Two separate PV sources feeding one battery.

    Actually my 10A controller is not mppt. Nor does it equalize.

    I'm wondering if there could be a problem when the 20A mppt controller goes into equalization.

    If the voltage outputs of the two controllers are different the battery will pull them down, until it's charged. Then what?

    The voltage differential should not be high enough to cause reverse current flow through any diodes but I don't know what kind of components may be in the output circuit of the mppt controller.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    Options
    Re: Two separate PV sources feeding one battery.

    Don't worry about reverse current flow... Since the battery always has power, and the solar panels don't, every solar charge controller has some sort of reverse current protection built in.

    Once the battery voltage (and depending on charge mode/controller design, time) has hit the point where a controller decides no more charge is needed, that controller will simple stop charge current flow and monitor the battery voltage/temp until it drops to the point that it decides to turn on again.

    Each controller will make its own decision independently of the other (assuming good wiring practices--i.e., low voltage drops).

    Your bigger issue is to make sure that each controller behaves correctly... For example the 20 amp controller may have a remote battery temperature compensator and more accurately change its charging voltage according to battery temperature (float charge voltage should drop as temperature increases).

    Whereas the 10 amp controller may not--and could overcharge the batteries on a warm day or if the batteries are hot from heavy charging or discharging currents. Probably not a real problem unless you have a small battery bank and/or some other installation issues (like controller in cool room and batteries in warm room or other issues).

    Just make sure that both controllers are set to the recommended voltages for your batteries, and either use a remote temperature sensor and/or are close to the batteries themselves (approximately the same temperature to use internal compensation) and have at it. Put the two controllers in parallel and watch your battery water level, temperature, and voltages.

    When you want to equalize, just start the cycle on the 20 amp controller--other than the fact the 10 amp controller will probably not supply any charging current during equalization, you should not even notice anything different with your system other than another X amps of charge current during bulk charging.

    Unless you have a very small battery bank where adding 50 watts (4 amps of current) will just be too much charging current, I can't think of any way that you will get into trouble unless something is broken.

    From other threads I have read here, ideally you will want your charge current (going into the batteries, excluding steady state loads) at around 5% to 10% of total bank Amp*Hour rating (3%-13%+ will work too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Options
    Re: Two separate PV sources feeding one battery.

    Thanx Bill.

    I hate expensive surprises.