I have a 30 watt solar panel and I'm looking to add a 80 watt solar panel to help power an 85AH battery. I know I need to connect them in parallel (positive to positive, negative to negative) but I wonder if the difference between the wattage of the two panels would cause a problem?

  • \$\begingroup\$ There's very little information here to make any kind of sound judgement. But if these are indeed panels (strings of solar cells), then the voltage of the larger panel will probably be a lot higher than that of the smaller panel. Connecting them in parallel may back-bias the smaller panel and damage it. \$\endgroup\$
    – Dave
    Jul 8 '16 at 1:18
  • If you have blocking diodes in each panel you will do no harm to the panels by paralleling them.

I assume that the panels are both of the same nominal voltage - ie made for use with the battery voltage you are using. If a 12 V battery they are probably 36 x PV cell panels with a nominal voltage of about 18v. This allows them to be loaded to about 80% of their rated no or low load voltage and still do all the things that a lead aci battery may need done to it.

IF they have the same Maximum Power point voltage (Vmpp) then they will parallel up well.

If Vmpp of one panel is lower than that of the other the low Vmpp panel will take most of the load until the load exceeds the panel capacity and then the other one will join in. No harm will be done but loading is not even.

This imbalance occurs because the low Vmpp panel will produce a greater percentage of its output at a given voltage.

If you REALLY want balance (and there is no real reason to have it) then place a resistor of value about R = 0.5/Imax in series with each panel where Imax is the maximum rated current of the panel concerned. These resistors will drop (by design) half a volt at full power which should be enough to help them track each other. Power rating of resistors = > 0.5 x Imax.

Example. 85 Watt panel at 12v probably has Imax ~~ 85/15 = 5.7 A.
30 Watt panel Imax probably ~~ 30/15 = 2A.

*Iuse 15V as a guess at likely Vmpp

Resistor for 85 Watt panel = V/I = 0.5/5.7A = 0.18 ohms.
This resistor is so low that just including extra wiring length in the high wattage panel may be enough.
Power dissipation = Vr x Ir = 0.5 x 5.7 = 2.9 Watts. A 5 Watt resistor would notionally suffice.


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