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

We used a 6V 2W panel and a 10V 1.3W panel (discontinued), 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.

Configuration Open Circuit Volts Voltage under Load (V11 Battery) Amps Watts
6V 2W Panel 7V 3.6V .4A 1.44W
10V 1.3W Panel 12.4V 3.78V .19A .67W
6V 2W and 10V 1.3W Panels in Parallel 7.21V 3.78V .56A 2.12W

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.

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

Configuration Open Circuit Volts Charging Voltage Amps Watts
6V 2W Panel 7V 3.6V .4A 1.44W
Hand Crank Dynamo 13.5V 4.25V .56A 2.4W
6V 2W and Dynamo in Parallel NO diodes 7V max
*panel spins the motor crank backwards
4.3V
*when cranked, motor and panel work together
.58 2.5W
6V 2W and Dynamo in Parallel WITH diodes 13.5V
*cranking faster generates more voltage
4.5V .67A 3W

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.

14 Responses

  1. Mike

    Hi. Can you help? I have a solar input to a solar Controller then into a battery. In parallel I sometimes run a ac generator into a Ac to dc charger then into the battery so both inputs at similar source voltages into the battery. However over time the energy output of the PV has fallen so it is a lot less sensitive to lower light conditions than previously even when used alone. Question is could the cells be gradually degrading due to sometimes using the Generator in parellel (via the ac/dc charger) to top the battery up? No reverse diodes are used in the panels I think. Cheers Mike

    Reply
  2. Carlos Porras

    hello, great info.

    I have a home made panel , delivers around 13 volts, not sure about whatts, meter hasnt arrived yet.

    I purchased another one, Renogy 100watts and delivers 19.7 volts.

    The new panel working alone shows in the controller a maximum of 4.5amps. When I connected both in parallel i got maximum of 3.5 amps.

    What could be wrong , so they cannot work together?

    Reply
    • Ken Stone

      Carlos – Can you plug the 13V panel into the controller to get amps off that panel? Either way – it sounds like you are cooking your 13V panel with your Renogy panel. Diode both, but particularly the 13V panel. If you can restructure your homemade panel to a higher V, I would do so and diode both. You mentioned a charge controller. If you are trying to charge a 12V battery, 13V is not going to do much for you. You might be better off dropping the 13V panel entirely unless you can increase your V (at the cost of homemade panel amps…). If you are charging 6V battery, then just diode.

      Reply
  3. naren

    hello friends,

    i am thinking of project in which i want to merge two power source in a single lead acid battery one is the grid supply and other is the power from solar panel please any one can provide me the circuit diagram for this. actually i dont want to waste the solar power i am going to use 250 watt solar panel and a 12v 120ah battery for charging it i want to run 2 fans and 2 tubelight for 8 hrs in the absence of grid .

    actually the problem in india there is a massive power cut we use to get a power cut in very 2 hrs for 3 to 4 hrs so the battery need atleast 8hrs to charge i want to use both in the presence of grid i want both of them charge the batery and in the absence of grid solar panel should do its work . actually the planning is not to waste the solar power while grid is available.

    eg: it is like if a battery need 230v of supply to charge and if i am getting 120v from the solar so i want rest of 110v only from the grid, by which i can save the electricity bill based on this consideration can any body help me and what r the other equipment i should have while constructing this project

    Reply
  4. Michael Weidler

    The source of the confusion is that you have one too many parts. There is only the phone and the dock. The dock turns the phone into a laptop. There is no separate laptop.

    The phone charges up whenever it’s attached to the dock, so what needs to be charged is the dock.

    Reply
    • admin

      Ok, if you need to charge a 19V system, you really need to go with a combination that runs through our V60 – direct charging from solar frequently does not work well. So either 3 x 3.4 Watt Panels -> V60 -> Dock or 1 x 16 Watt Panel -> V60 -> Dock The reason why we were pushing on direct charging the phone is that phones are much simpler (weight, $, size) to charge than laptop-sized systems.

      Reply
      • Michael Weidler

        That V60 is a niffty solution. Unfortunately, it is out of my price range for the foreseeable future.

        Why doesn’t direct charging work well? What are the issues besides low voltage due to insufficient light?

      • admin

        Laptops are very picky and want to be plugged into a wall. If you don’t meet their current threshold, they ignore the charge. If the Voltage fluctuates, they will often shut down to protect themselves.

  5. Michael Weidler

    I am both weight and price conscious. I was thinking of maybe stringing together a few of your 10v panels. 4 of them would give me 20v and 2.6 watts for only $24. Not a fast charge, but I’m not in a hurry.

    It’s been over 30 years since I’ve messed with electronics projects,so I’m not sure exactly what I’m doing. So far, it sounds way too easy.

    The laptop should shut off the charge automatically when it’s full, just like it does with the wall charger. The electronics for that should be on board the the dock rather than in the charger. At least that’s the way I would have designed the thing.

    What am I missing?

    Reply
  6. Michael Weidler

    I am about to embark on an extended bicycle trek, and I am trying to figure out a way to recharge my laptop dock for the Atrix 4G smart phone. The transformer on the supplied wall charger states that the output is 19v at 1.58 amps.

    I am not in any great hurry to charge the dock. I will be riding for most of the daylight hours stopping only to eat and get on-line for a bit. What I am looking for is a way to make use of all those hours of riding time to recharge the dock. I am pulling a BOB trailer so space isn’t a problem. I am concerned about weight though.

    I have found all sorts of ways to produce 12v – including a bicycle light type tire contact generator – but no way to produce 19v. I’ve noticed that there are laptop chargers which convert 12v DC into either AC (into which I could plug my wall charger) or 19v DC – which I should be able to feed directly to the laptop. These are designed to be used in a car, which produces considerably more amps than I’m likely to generate on my bike by whatever means. Plus I quickly start to run into weight issues with many of these converters.

    Would you happen to have any suggestions for a solution?

    Reply
    • admin

      Hi Michael,

      We’re thinking that you don’t need to go through the laptop dock, you can charge the Atrix directly from a MicroUSB adapter at 5V.

      If that’s true, you can get away with something like 3.5 Watt Kit for even less weight.

      Let us know,
      Jeff

      Reply
      • Michael Weidler

        I don’t need to charge the phone. I need to charge the laptop dock. The phone automatically tops off when plugged into the laptop dock, so I’m not really concerned with charging the phone.

        At home I don’t even both with the wall charger for the phone. I simply plug the phone into the laptop dock and it charges from there.

        The original – and fall back – plan is to stop at the local library along the way and charge up while I surf. I eat at McDonalds a lot and they all have WIFI. Unfortunately, most of them (and other eating places) do not have accessible outlets, thus the desire to charge while riding and avoid the library stop.

      • admin

        Hi Michael, I’m still a bit confused about the role of the laptop dock vs. going directly into the laptop or phone. It might be easier to call to go over this. You can call the main number here 212 401 1192 and choose tech support.

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