More in RV Solar Education...
Things to Consider
BEFORE purchasing an
RV Solar Battery Charging System
Adding a solar battery charging system to your RV can extend your stay while boondocking and give you a greater choice of destinations without hookups. However, the unique operating environment of RVs dictates an approach to system design that is somewhat different from other solar applications. Three of the more important things that deserve your consideration are listed below for quick reference and are expounded on in other sections of the educational pages.
- Batteries are exposed to widely varying temperature extremes which, if not taken into consideration, will lead to plate sulfation and/or excess water loss.
All lead-acid batteries need to be pushed to their gassing threshold in order for them to be fully charged. This threshold is reached at 14.1 to 14.4 volts for most batteries at 25'C (77'F) (note: some batteries want to be pushed to 14.6 to 14.8 volts). The problem with this is that the gassing threshold changes with the battery's temperature.
This threshold is reached at a higher voltage when the batteries are cold, and at a lower voltage when the batteries are hot. If you don't reach the gassing threshold, then your batteries will begin to sulfate. If you go beyond the gassing threshold, then your batteries will 'boil' off its water.
This dictates the need to include battery temperature compensation in the charge controller.
- Solar panels operate at higher than normal temperatures when laid flat on the roof of RVs, from which they suffer a voltage drop.
All solar panels are rated at Standard Test Conditions of 25'C (77'F) cell temperatures (NOT air temperatures). When these cell temperatures rise, there is a corresponding voltage drop. I've measured cell temperatures in excess of 65'C (150'F) which caused a voltage drop of almost 2 volts!
Think about this. If your car hood was the same color as solar cells (dark blue to black), and the sun is shining with an air temperature of 25'C (77'F), how hot do you think your car hood is going to be? Too hot to touch, right?
In a nutshell, make sure your solar panels are rated at about 17 volts or higher in order to allow for this voltage drop and still be able to fully charge your battery. Our GS100 solar panels operate at about 17.7 volts and our GO Series panels operate about 18.5 volts ! This assures you of overcoming any voltage drop that may occur.
- The use of inappropriate charge controllers will lead to unsatisfactory performance and possible battery damage.
Charge controller technology has come a long way in recent years. There is no need to settle for the old 'On-Off' (switching shunt) type of controllers that are notorious for 'boiling' off water and/or sulfating the plates of batteries (remember the first point made above?). There are controllers available that have 'tapering' charge strategies and incorporate temperature compensation.
Look for charge controllers that have tapering 'Pulse Width Modulated' (PWM) charging strategies with temperature compensation via an external sensor. Some of these will also have Maximum Power Point Tracking (MPPT) which gives a boost to the charging amperage. These new controllers will maintain your batteries at the highest state of charge with the least amount of water consumption.
Knowing the above points will help you become a more "Solar Savvy" shopper when considering the purchase of a solar battery charging system for your RV.