Ground Solar, Part V, The Batteries

We’ve been discussing our Hurricane Matthew ground solar installation (overview, combiner, charger and inverter), as well as fundamental solar panel principles and off-grid lead acid battery principles. This time, we will discuss the battery array used for this exercise, as well as the automotive DC safety breaker. Refer again to the photo of our inside components below:

As you can see in the photo, we used two Duracell 29HM marine batteries wired in series to form a 24 volt array. We paid about $95 each for these; we had batteries to exchange so we avoided the $18 Georgia core charge. The 20-amp-hour rate for these (see the battery principles article for the details of this rating and good battery management) is 105. If we were to treat the batteries gently, this would allow us to discharge at about five continuous amps (about 120 watts) for ten hours and still not exceed the 50% depth-of-discharge goal. More on this plan in a bit.

To provide protection against shorts and allow interruption of power if the charger firmware should fail, we also had a 150 amp circuit breaker wired between the charger and the battery array. This is the little black nugget dangling in space between the charger and the inverter. This circuit breaker can be tripped by hand by pushing the red button, and then reset with a lever. It is poorly sited in this photo; our plan was to mount it to the wall and then trip it with a broom handle from a safe distance. Clearly, this plan was never fully realized. Also, in a more permanent installation, a second circuit breaker should be used between the batteries and the inverter.

The DC breaker shown is an automotive Cooper Bussmann CB185-150, which can handle up to 42 volts, perfect for a 24 volt system with above 28 volt charging, and costs around $25. We’ve also evaluated some slightly less expensive alternatives, and they all seem fine. Several months ago, we paid a few dollars less for each than currently shown on Amazon ($19 and $16.50, respectively). We also have on hand a $10 model, but we haven’t evaluated it fully yet. Either way, thanks to automotive applications, this is one of the least expensive yet important system components. 150 amps is overkill for this application; a future article will optimize component selection for this kind of small power system.

All the wiring shown in this picture is 6 AWG THHN, with the exception of the two cables from the battery to the inverter which came with the inverter. We’ll drill into the wiring, connectors and tools more in a future article. Not shown are some USB chargers (similar to these from our sponsor) we temporarily tacked onto the 24 volt terminals, in addition to the 1000 mA USB charging available on the back of the MicroSolar inverter.

Back to the batteries, and how we abused them. Our original, pre-hurricane plan for these batteries was to support an energy-efficient freezer 24/7 in an emergency. Our freezer uses about 70-80 watts, which is somewhere around 12-15 hours of operation on these batteries (to a 50% discharge). But, when the hurricane hit, we had only recently stocked up on butter, eggs, milk and deli meat. It seemed a shame to let all that go to waste. So, after scratching out the math, and counting on the fact that the freezer, if left closed, only needs to be goosed a few hours a day, we decided to run the freezer and both refrigerators all day on solar while charging, and then run all three in the middle of the night as many hours as we could.

As mentioned above, gentle operation would use no more than about five amps, and our freezer load would be about 3 amps. However, the full freezer+refrigerator needed 350-400 watts, or about 14 to 16 amps, around three times the 20-amp-hour rating. As we saw in the battery principles article, this would give us five hours, at best, but in practice, we generally saw about four hours of operation at this level, to 50% depth of discharge. Fortunately, between daytime solar (about 8-10 hours a day given all the panels we threw at the problem) and four hours of battery at night, everything stayed nice and cool.

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