Combining Non-Matched Power Sources

There are times when you may find that you want to expand the power producing capabilities of your solar setup. This post will cover the basics of how to determine if you can combine power sources, and how to do it.

General Guidelines for Combining Power Sources

• Voltages of power sources should be similar. i.e. Combining 6 and 10 volt panels is probably OK. Combining 6 and 14 volts is a poor match
• Panel voltages should be higher than the charging/load/battery voltage. A panel voltage that is lower than the battery voltage isn’t going to charge the battery. We find that panel Voltages 1.5x the Voltage of the battery typically results in fast, efficient charging.

Once you’ve checked that your solar panels fit the criteria above, you can wire them in parallel. To do this, simply connect the positive connections together, and then connect the negative connections together. Our circuit boxes have a switch that allows for parallel connection of multiple panels.

Lets look at how combining different panels worked out for us in the lab.

The setup
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We used a 6V 2W panel and a 10V 1.3W panel, connected in parallel using a modified 3-panel circuit box. The device we charged was a completely drained V11 battery pack. The table below shows the open circuit voltage of both panels independently, then combined in parallel, and the performance of each configuration charging the V11.

This means that it is ok to combine panels of different Voltages together as long as they are roughly similar and they don’t fall outside of the specifications of the device you’re charging. The Voltage of the combined panels drop to whatever the load requires to charge and the POWER output of both panels on their own is roughly additive.

Combining solar panels with other power sources

In the case above, with two panels of similar voltage, we see that the power is approximately additive. In a situation where you might combine solar with energy generated from a dynamo or wind generator, you will need to use diodes to prevent the solar from driving the dynamo as a motor and the dynamo’s energy from being dissipated through the solar panel.

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Clik here to view.Image may be NSFW.
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We tested the output voltage of our dynamo from a previous post and combine it with a 6V 2W panel to charge a V11. We want to show the difference between connecting the two sources in parallel with and without the diodes

If we combine the max power output from the panel of 1.44W with the 2.4W from the hand cranked dynamo, we expect to see 3.84W going into the battery. Where’s the missing 0.84W? In the case where the panel and dynamo are connected in parallel without the diodes, some of the extra power from the dynamo is actually absorbed by the solar panel. In the case where we have diodes in series with the parallel power sources, the diodes are responsible for most of the missing 0.84W. The reason is that each of the diodes requires voltage to operate, at least .7V, and this drop comes at a cost in terms in power. The amount of energy lost in using the diodes is the current multiplied by the voltage drop of the diode. If we want to minimize this loss, selecting Schottky diodes would be the way to go, as they only “cost” .5V to operate.

To summarize our findings, it is possible to combine multiple power sources in parallel. If the power sources are similar in Voltage and type (i.e. both solar panels), then it is straightforward and you increase the total power produced. If the sources are different in type (i.e. a solar panel and a dynamo), the system will likely increase total power generated, but you need to include diodes in your design and that will cause some power loss.