Some years ago my partner and I went on holiday for three weeks on our boat. We have good leisure batteries and a decent inverter, so watching TV and running the fridge and freezer wasn't expected to be a problem while we were away. However, it soon became evident that if we wanted to stay in one place for a few days, the batteries would have to be charged by running the engine for at least a couple of hours daily. This became a real problem because it meant that we were tied to the boat while the batteries charged. It was then that we decided that solar power could be the answer.
We did a lot of research because we wanted to do the job right first time. The three major components required are the batteries, the solar charge controller and of course the solar panels themselves.
The batteries. Don't use cheap batteries or be tempted to use so-called starter / leisure batteries. You must use proper deep cycle batteries. We chose Trojan T105's. These batteries are commonly used on golf buggies. In that application the batteries are charged overnight and used next day around the golf course. Theses batteries have heavier internal plates than starter batteries and are a perfect choice to team up with the rest of your solar equipment.
The Solar controller. Choose very carefully. Your new Trojan batteries will last for years if properly looked after, so buy a decent controller. We bought a Victron MPPT 75/15 controller because it can be programmed to suit the charging regime that the Trojan's require. MPPT actually stands for 'maximum power point tracking'. It's an algorithm contained within the controller that maximises the output of the panels to aid fast charging. Cheaper units can fail and damage your batteries.
Solar Panels. We have three panels, each rated at 100 watts. Although nominally rated at 12 volts, the actual open circuit voltage of each panel is approximately 22 volts. We have wired them in series and in bright conditions they put about 66 volts into the controller. The controller will accept up to 75 volts and produce up to 15 amps. In brief, the controller changes the input voltage to AC current then converts it back to DC current at charging voltage. Any excess voltage is converted to extra amps.
When we first fitted the panels they were fitted flat to the roof. We realized that we were wasting power this way so made up a tilting arrangement to better align with the sun. This immediately increased the available amperage to the batteries.
