# How do I match a battery pack voltage to my solar panels?

We're building an autonomous boat which should be powered by solar panels. We want to put two 100W solar panels on it. Most panels I see are 18V and for redundancy we want to link them in parallel. That way when one solar panel breaks at sea, we still have the other one to finish the mission.

We're now looking into what kind of battery we are going to build for it. I read somewhere that the solar panel should have a 40% to 80% higher voltage than the battery. That means that a 12V battery pack should be logical. And in between the solar panels and the battery pack we'll put an MPPT charge controller.

My question is; does all this make sense? Is it true that the solar panel voltage should always be 40% to 80% higher than the battery pack? Or can I also use an 18V solar panel to power for example a 5S li-ion (nominal voltage of 18.5V and a max of 20.5V)?

All tips are welcome!

• The appropriate solar charge controller does the matching. There ARE boosting ones (for battery V > solar V), but rare and expensive last time I looked, unless you build your own.
– user16324
Commented Mar 8, 2021 at 20:24
• Just FYI if your solar panel is rated at 100W, you can usually look up the actual output voltage and current at that power rating for your panel. This will give you an idea of where the maximum power point voltage lies, which is much more useful than open circuit voltage. Better product is typically better documented. The magic word is datasheet.
– K H
Commented Mar 9, 2021 at 2:52

## 2 Answers

If the 18 V rating on the cells is the zero current voltage (open circuit voltage) it may not be high enough. Since the cells have some internal resistance, they will be at a lower voltage when there is a load. You may want to consider more than the batteries for the load since you may want to operate the boat and charge the batteries at the same time. If you need a boat that can function no matter what the solar profile, you need to make sure that you can get your batteries charged when conditions allow it, so you need to get as much current out of the solar cells as you can even when you batteries are near full. Some charge controllers have buck/boost converters so you don't have to worry about voltage matching. This allows you to pull as much current as possible, even if it means dropping the voltage on the panels down to 3 or 4 volts. If you are building your own, maybe you don't have this option. In this case, you need to open circuit potential to be as high as possible to get high currents at you battery voltage.

• So are you saying that as long as I have an MPPT charge controller (which does the voltage conversion) between the panels and the battery pack, it doesn't really matter if the battery pack has a higher voltage than the solar panels? Commented Mar 8, 2021 at 17:33
• Not necessarily. Some MPPT charge controllers need the PV to be higher than the battery. It usually goes this way. You can charge a 12V battery with 50V PV while keeping the PV voltage at the maximum power point. There are some boost MPPTs that can charge batteries at higher voltages than the PV but they don't seem to be the norm and you have to check to make sure this feature is on the charge controller you choose if you want to go that way. I'd say you should go with a PV of higher voltage than the battery and then use a boost converter to drive loads that need more voltage. Commented Mar 8, 2021 at 18:59

A solar panel is a constant-current source, not a constant-voltage source. The voltage indicated in the specifications are therefore only (more-or-less) the maximum and rather irrelevant. What you need is a charge controller that matches your battery voltage (12V in that case), the rest is regulated by that controller. For small size setups (such as yours) there are out-of-the-box combinations of solar charger and panels available. Some sets even include the battery.

However: What power rating does the engine of your boat have? 2x100W is not much if you're going to use it for anything but a toy engine. My 120W solar panel more or less keeps the battery charged on a clean day - but it powers only the electronics and the auto-pilot, but not the engine.

• "A solar panel is a constant-current source, not a constant-voltage source." It's worse than that -- it's constant current up to a point, and then the current starts dropping with increasing voltage. In general you need a battery controller with maximum power point tracking (MPPT). On the bright side, this has been a known thing for ages, so there should be plenty of controllers that can do that. Commented Mar 8, 2021 at 17:06
• @TimWescott - So you're saying that as long as I have an MPPT charge controller between the panels and the battery pack, I can even have a battery pack with a higher voltage than the solar panels? Commented Mar 8, 2021 at 17:29
• @PMF - The engine of the boat can run on about 20W. It's a very small and lightweight boat. It's basically a floating battery pack with solar panels and some electronics plus a compartment for one bottle of Whisky. The goal of the boat is to get us a bottle of Whisky from Scotland across the North Sea back to our home in Amsterdam. Commented Mar 8, 2021 at 17:37
• Oh, smuggling Whisky over the english channel... I fear you're going to have bigger problems than the solar panel, and these aren't the police either. Have you ever sailed that route (on a yacht or another small ship)? It's one of the most demanding sea ways of the world, with a lot of traffic and strong currents, and at times high waves, too. Your ship will need to be able to maintain around 6 knots, or it won't get anywhere. I'm not a specialist on miniatures (where some special effects may kick in), but due to the Hull Speed limit, that means the boat must be in the order of 7 meters long.
– PMF
Commented Mar 8, 2021 at 17:48
• Yes, the solar panel voltage is irrelevant. It will still generate some power also if the voltage drops below the battery voltage (i.e. due to clouds)
– PMF
Commented Mar 8, 2021 at 17:53