I'm in the process of designing a small solar regulator / charger (no more than 5A) for sealed lead acid batteries. I have put a working prototype together and all seems to be working.

However, after Googling solar charger circuit diagrams, I have found that a lot of chargers seem to short the solar output to ground via a dummy load resistor using a FET or transistor rather than just just disconnecting the solar panels using a FET or transistor.

In my prototype controller I am just disconnecting the positive of the panel using a P channel FET and it seems to be working fine.

From Googling and searching this forum extensively, I can't seem to find any reasons why one should short the solar panels to ground using a dummy load rather than just disconnecting the positive of the panels.

I am using an AVR micro controller to control the charge process using PWM. However, a lot of the circuits I saw using a dummy load used an analog method instead of a PWM micro controller. Is using an analog method maybe a reason that one needs to use a dummy load to stop charging but with a PWM method disconnecting the charge input is fine?

I cant seem to find any reason why one must use a dummy load to stop charging from the panels... It seems it won't damage the panel being disconnected or anything like that. Also, in my thinking I would say there would be more strain and possible damage on the system (FET and dummy resistor) if the output of the panel is shorted while not charging vs just being disconnected.

Does anyone have any feedback on what method I should use (dummy load or disconnect) in this controller I'm working on, and why?

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    \$\begingroup\$ I use a water heater as a dummy load, this way I can put some of the energy to good use. But in the end you don't need one. \$\endgroup\$ – PlasmaHH Mar 24 '15 at 10:16
  • \$\begingroup\$ @PlasmaHH - Thanks for that! Yes it would make sense to use that "unused" energy for something but in this case its a relatively small amount of current and in this application there is nothing we could use that energy for! Thanks for your feed back! \$\endgroup\$ – Tristan Mar 24 '15 at 11:08
  • \$\begingroup\$ For very small panels a shunt controller can make it easier to regulate battery voltage - current is progressively shunted as the battery approaches full charge to maintain float voltage. This does not hurt a PV panel at all but as you say, has a thermal impact on the load. As power goes up ( and 5A at 12V = 60W+ is significant) it is easier to open circuit the panel. As long as Imax is within battery spec PWM without filtering is probably OK. If Imax is > battery spec you may need to limit current flow and you will again get dissipation in the switch. \$\endgroup\$ – Russell McMahon Mar 24 '15 at 11:48
  • \$\begingroup\$ The ideal method is a buck regulator that provides only what is needed at reasonably high efficiency. Ones that are capable of 5A out at 12V are cheap enough (eg on ebay) as to be worth considering. \$\endgroup\$ – Russell McMahon Mar 24 '15 at 11:48


"If you use a windmill that does not have its own charge control, or use hydroelectric generating, a diversion control must be used, since those generators can be damaged if their charge is not absorbed in the batteries at all times".

Solar panels will sit happily with no load, while other kinds of generator will not.

  • \$\begingroup\$ Hi @pjc50, Thanks for your feedback, this does make sense. My guess is that any motor based charge source (such as wind and hydro) can and probably will spin at self destructive RPMs if their load is not connected to something. Just strange that many of the online circuit diagrams for solar chargers "dump" their charge when not needed like google.com/… \$\endgroup\$ – Tristan Mar 24 '15 at 12:08

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