Aboard my sailboat I have one 130W PV panel and three Rolls 4-D batteries, 185 AH each. I may add another 130W PV panel. What size amp MPPT charge controller should I use?
Generally speaking charge controllers are rated by their maximum output amperage. Let's assume you want to size for two KC130 (2 x 130 watts), then that's 260 Watts total. I'm going to assume you have a 12 Volt system. So to figure out the maximum amperage output we divide the power by the voltage (from the formula Power = Volts x Amps). So here's what we'd get:
260 Watts / 12 Volts = 21.7 Amps
In reality there's a number of effects that could actually cause your panels to produce more power than their rating - like reflected sunlight off of the water, cold temperatures, battery voltage is very low (10.5V instead of the 12V assumed above), etc. So for those reasons we generally boost up that calculation by 25%:
21.7 Amps x 125% = 27.1 Amps
So you'd need an MPPT charge controller that can handle 27 Amps or more.
Here are some of the MPPT controllers that Alt-E offers in that range:
Blue Sky Energy's Solar Boost 3024i:
http://store.altenergystore.com/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Blue-Sky-Solar-Charge-Controllers-MPPT/Blue-Sky-Energy-SOLAR-BOOST-3024i-30A1224V-wo-Display/p240/ BZ Product's MMPT500:
http://store.altenergystore.com/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/BZ-Products-Charge-Controllers-MPPT/Bz-Products-MPPT500-40A122448V-Charge-Controller/p295/One of the neat things about both of these controllers is that you could connect your 2nd panel up in series with 1st one for 24V and the controllers will automatically downconvert very effeciently. Wiring them in 24V will reduce the amount of wire runs you'll need to do. Handy trick.
