My Work: Living on Solar Power - Part 9

March 2001

Ever since we moved in, solar friends and colleagues had urged us to buy an amp-hour meter to measure the state of charge of our batteries, like the fuel gauge on your car. I always resisted, preferring to judge the batteries by their voltage and saving the cost of the meter, but that was inaccurate because of the many variables that affect voltage. Plus, we were tired of running down to the basement and looking at the inverter readout every time we wanted information about the state of our power system.

The meter has several names, depending on who manufactures it, but it provides digital readouts of battery voltage, instantaneous amperage gain or draw, amp-hours left in the batteries, and hours the batteries will last at the average usage over the last 30 minutes. It was very convenient to push four buttons in the kitchen and accurately read the state of our system. It was also much easier to yell at the kids, “Come here, look at how much power you’re using.”

By this time, our batteries were growing weaker. Some bad cells were drawing the others down, so we jumped out the worst in each string to bring our system down to 10.8V. It didn’t seem to affect the operation, but the batteries held their charge longer without having to compensate for the bad apples in the strings.

September 2001

This fall, I read an article on the Mr. Solar website (www.mrsolar.com) about a nifty box called a Maximum Power Point Tracking (MPPT) Photovoltaic Charge Controller that claimed to increase the power from our solar array by 30% (www.rvpowerproducts.com).

When solar modules charge a battery, they typically charge at the existing battery voltage, say 12V. But solar modules can produce 17V, and that means you aren’t using the module to its full capacity. The Solar Boost draws current from the module at 17V, then feeds it to the battery at just above the existing battery voltage. Going from 17V on the input side to 12V on the output side increases the current that charges the batteries if the watts remain the same. I called a few people and they said the technology was sound, so I ordered one and had it installed. It worked beautifully. It did increase the power production from our 12 modules by 30% — for the cost of one module. It also acted as a charge controller, and was an outstanding addition to our system.