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Advice needed - DIY off-grid solar electrics DC charging via AC from generator

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  • Advice needed - DIY off-grid solar electrics DC charging via AC from generator

    Hi all, new to this forum... I'm looking for some advice regarding an off-grid electrics setup i'm running. Currently i have a PV solar panel charging a battery bank through a charge controller. It's a 12vdc system. For 240vac i use a small (500W) inverter. For larger loads i use a diesel generator. The inverter is basic but very high quality sine wave, it doesn't do any charging nor does it automatically switch the load between the AC of the generator and its own AC (i do this manually). In order to recharge my batteries when there isnt enough sun (which is often!), i use a simple AC powered charger that isnt specifically designed for off-grid/solar applications. In other words it is not "intelligent" and doesnt have bulk, float or equalization settings - it's just a DC charger that gives decreasing current as the voltage of the batteries rises. In fact it is very prone to overcharging if left charging for too long and will keep putting current into the batteries even after they've reached 15.5vdc or more.

    My question is this: is it a good idea to feed the DC charging current from my AC-fed charger into my solar charge controller in order to allow the charge controller to regulate and/or limit the charge as appropriate? I did this for a period of time and it seemed not to cause any problems. However, my AC charger did eventually blow its recrifying diodes although i'm not sure if this was due to a separate issue with the AC power coming from my generator which at the time was an old one and may not have been giving proper AC voltage (it may have been low due to engine speed problems).

    The solar charge controller is a PWM type. The way i had it set up was the DC from the solar panel and the DC from the charger in parallel to the charge controller.

    If there is no difference between the DC from the charger and the DC from the solar panel, i don't see how there could be any problem as it amounts to the same thing as having two 12v PV panels in parellel. However, what i dont know (due to insufficient technical knowledge) is whether rectified DC is any different to the DC from a solar panel. Also, i wonder whether the current limiting and PWM that the solar charge controller does will affect or harm the AC-fed charger in some way?

    My setup is as follows:
    1 x 200W 12vdc PV panel
    60A tristar PWM solar charge controller
    2 x 6vdc Trojan t105 batteries wired in series to give 12vdc
    Mastervolt 500w pure sine wave inverter (12vdc in, 240vAC out)
    SDMO 6kva diesel generator (240vAC)
    Wolf Max 50 battery charger (AC powered 40A DC charger)
    Last edited by RionZion; 05-01-2016, 08:52 AM.

  • #2
    Update: following further research, i see that rectified DC is NOT the same as "pure" DC - it has "ripple" i.e. stray AC within it, which can be limited (or possibly removed entirely, i'm not sure yet) by use of capacitors/regulators. From memory of the last time i had a look inside my charger i dont think it has much in the way of smoothing, as i recall it was an extremely simple circuit consisting of the transformer, rectifying diodes and a voltmeter with a thick copper coil on the back of it (presumably this limits the current as the voltage of the batteries rises).

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    • #3
      No the problem is your AC charger was too large for your controller. In a Solar system the panel limits the current, not the controller. It does not take any special charger to charge a battery. For an Off-Grid System all you need is a CC/CV charger set to the appropriate voltage and current. You did say you have a 12 volt system, which means you need a charger that has 14.4 to 15 volts and supplies a C/8 to C/10 charge current. So if you have a 100 AH battery the charger needs to be 10 to 15 amps. It does not need to be an expensive 3 or 4 stage charger.

      Read these two threads:

      Bulk, Absorb, Float, Equalize
      How To Charge a Battery.

      There is no magic.
      MSEE, PE

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      • #4
        Originally posted by RionZion View Post
        From memory of the last time i had a look inside my charger i dont think it has much in the way of smoothing, as i recall it was an extremely simple circuit consisting of the transformer, rectifying diodes and a voltmeter with a thick copper coil on the back of it (presumably this limits the current as the voltage of the batteries rises).
        Battery chargers do not need a lot of filtering as the battery itself is a huge filter, a capacitor filter.
        MSEE, PE

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        • #5
          Originally posted by RionZion View Post
          My question is this: is it a good idea to feed the DC charging current from my AC-fed charger into my solar charge controller in order to allow the charge controller to regulate and/or limit the charge as appropriate?
          Generally no. That's not what controllers are designed to do. You might get away with it if currents/open circuit voltages line up OK. Not surprised you blew the charger; large transformer-based chargers don't like being slammed on and off by a PWM controller. You'll get very high flyback voltages and eventually blow your diodes or controller.

          If you really want to use such a charger, wire it so that a voltage detector (say the aux output from a good solar charge controller) shuts off the charger at the _input_ when the battery has reached its bulk-charge voltage. You will take care of most of the charging that way. You'll still need to finish the absorb phase, but that of course takes less power.

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