BATTERY STORAGE

Version 1.0

Arizona enjoys an 85% sunshine ratio. But solar panels don't work at night, or (very well) on those 15% overcast days. We needed a power source that can supply energy on demand, even when the Sun is down. Eight Trojan L-16 deep cycle batteries provide this backup power for our system. Each is twice the size of a standard automobile battery, weighs close to 120 pounds, and holds about 2.5 kW-hr of electrical energy (about 20 kW-hr total). To put this in practical terms, the entire storage battery, consisting of eight L-16's, can power all the critical systems at Heimhenge for about three days ... longer if the Sun shines.

You can't store these batteries just anywhere. When fully charged, they contain an amount of energy equivalent to 2.5 liters of gasoline. This might sound dangerous at first, but for standard safety precautions the only requirement is a vented enclosure separate from the house. The greatest hazard comes from short circuits caused by metallic objects accidentally dropped across the terminals, a scenario that is easily avoided. You also need to keep them in a well-ventilated area, since they vent explosive hydrogen gas when charging.

The batteries came with a 2 year warranty and cost about $175 each. They require regular checks of their water levels and specific gravity, don't like to be discharged more than 80%, and prefer moderate temperatures. With proper care, good quality deep cycle batteries can last 5 years or more. One thing that helps to extend battery life, and reduce maintenance, is a nifty product called HydroCaps™ (image below).

Version 2.0

My first set of batteries lasted almost 7 years (above average) with the good care I gave them. But that involved a lot of maintenance. When the time came to replace them, I made two big changes. First, I switched from wet cells to Discover AGM (absorbed glass mat) batteries. They're about 2.5 times the cost of wet cells, but are essentially zero maintenance. These sealed batteries won't need any water or specific gravity checks over their expected 10-year lifetime. That's cool.

Second, I halved my backup capacity to 9.4 kW-hr. There's an unavoidable problem with running two parallel strings of batteries: the two strings will never have exactly the same resistance, so charging will be uneven, regardless of equalization efforts. This ultimately leads to battery failure. Also, I decided that I couldn't really justify maintaining 3 days of backup power, as the typical power outage in our area is resolved in less than a day 99% of the time. My initial design with 3 days backup was a knee-jerk reaction to the 4-day power outage we experienced back in 1994, the year before we installed the system.