We’ve been seeing a number of different solar panel setups while on the road (most of them probably fairly small compared to the typical US RV). Ours has worked well for us, but I just recently helped troubleshoot someone else’s setup, and there are some lessons that may help improve on common problems.
The setup was using a Zampsolar 60W portable panel (similar to this one), hooked up to a 90Ah battery. Given the small size of the panel, it isn’t the main charging source, but is primarily geared to slow down the discharge of the battery while the car is standing for a few days (essentially it keeps an Engel fridge running). The battery is usually charged from the car’s alternator with an isolator setup, similar to our own setup (using a BlueSea ACR).
The Zampsolar panel is nice and simple in that it comes as a complete portable package. It includes the panel, charge controller and cable. The charge controller is fixed on the back of the panel. While this is simple, there are problems with this setup that make it almost useless from my perspective (without alterations). The length of the cable (>20 ft) and the small gauge (16AWG) cause significant voltage drop. For example, if the charge controller on the panel puts out 13.2V, the voltage that gets to the battery would be 12.5V, a loss of about 5%. While this doesn’t sound like much, it significantly affects how / whether the battery gets charged. This problem gets amplified by the fact that the charge controller sits far away from the battery (it sits on the panel itself). The charge controller regulates the voltage from the solar panel (typically 13-20V) down to a level that charges the battery (typically 13-14V). For the charge controller to work well, you want to reduce the voltage drop between charge controller and battery as much as possible. This can be done in 2 ways – 1) minimize the length of the cable; 2) use a heavier cable gauge.
So how did we change the setup to work better? Since the cable is fairly long, you want to use a heavier gauge. You can use an online calculator to find out which one would work. In our case, we used about 20ft of 10AWG cable. Secondly, we removed the charge controller from the panel and put it next to the battery, with less than 2 ft of cable in between them. This improves the setup in that a higher voltage runs on the longer part of the cable. Higher voltage has lower voltage drop – this is why long-distance transmission lines run at very high voltages. The solar panel’s 13-20V output flows for 18ft to the charge controller, and then only gets regulated down close to the battery, with minimal voltage drop between charge controller and battery.This way, the highest voltage runs for the longest distance. The voltage drop is now almost negligible, less than 0.5%.
So buying a new cable and changing the location of the charge controller was a fairly cheap way of improving on this existing setup. There are limitations to how well everything will work together, i.e. the small 60W panel will never be sufficient to keep the battery charged for most people’s energy usage (fridge, lights, computer and cell phone charging). Solutions to this would be a larger panel or a larger battery to extend times between having to run the car alternator. We have only a slightly larger battery (100Ah) but our panel is 135W, and this works really well for our purposes. If anything, I would want more battery capacity for the rare occasions that we’re parked in the shade for a more than 3 days and charging our computer a lot. This has happened once in the last 8 months and our battery level went down to 50% – still not a problem.