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 and I can recharge during the day.

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™.

HydroCaps replace the standard battery cap with a device that catalytically recombines the hydrogen and oxygen normally vented during charging. This does two things … the hydrogen and oxygen recombine into liquid water, which drips back into the battery dramatically reducing water use. Of course, for safety, you still need to allow for ventilation, since a small amount of hydrogen is vented during charging. They also significantly reduce the acid mist that accompanies venting and soils the top of the battery case, which can cause corrosion of the terminals and surface charge leakage. You need one for each cell (3 per battery), and they run $10-20 each depending on your battery style, but were well worth the cost in reduced maintenance. For anyone using lead-acid wet cell batteries, I highly recommend HydroCaps.
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.

9.4 kW-hr still gives us about 12 hours of backup if we only run critical loads. Those include smoke and security alarms, communications, ceiling fans in the bedrooms, and the microwave and fridge. If we turn off the fridge (and don’t open the door unless necessary) we can push that to 48 hours. If we get a sunny day during the outage, we can recharge the batteries and gain about another 7 hours.

In order to recycle the heavy (and expensive) 4/0 braided copper connecting cables, I needed to get some battery terminal adapters to fit the new AGMs. Solid connections are essential for applications like this, where currents in excess of 100 amps can sometimes flow. Connectors like those shown can be obtained at most auto supply stores. Cleaning both the lead and copper surfaces before mechanical assembly ensures good electrical contact.
Version 3.0
The version 2.0 batteries were doing fine until an extended power outage (combined with their 8 years age) finished them off. They would no longer hold a charge, as we found out during a subsequent power outage. Since we were upgrading the solar array at this time we added new batteries to the mix.

These 4 Lifeline GPL-8DL batteries are rated 255 Ah @ 48 volts. That provides 12.24 kWh of backup in a power outage, a 26% increase in capacity compared to version 2.0. With 15 hours backup for critical loads this gets us to the next day/night cycle, during which batteries can recharge (while being used). This assumes clear skies, of course.