Measuring battery charging efficiency

I lied, it’s another solar post, sorry. I think I have another in the works too, but that one will be longer, and have maps!

Usually, I don’t get to see/use the full capacity of the two battery-connected panels on our roof. On perfect sunny days, the battery is usually out of bulk charging (it’s “full”, kind-of) before solar noon is reached, when the panels would be at full output. This is by design, as there needs to be extra capacity in the panels for cloudy days. Here’s what that usually looks like:

CurtailedAmps

By 9 am, the charge controller stops bulk charging, and reduces the amperage going in to the battery. Amps go back up periodically when the deep freeze compressor turns on, but most of the potential production is curtailed, so to speak.

A few days ago, the batteries had dropped to a pretty low state of charge after a few rainy days: slightly under the 50% mark. This is usually lower then I let it go, but won’t hurt unless it’s a regular occurrence. The next day was totally clear and cool – perfect for charging the batteries and trying to track the panels maximum output.

FullAmps

On this particular day, the battery stayed in bulk charging mode until about 1:30 pm. According to the specifications on my panels, under laboratory conditions, the maximum output for each panel should be just over 9 amps (so the output of two should be just over 18). At exactly 12:30 pm, the charge controller was pushing 17.49 amps into the batteries, which was the maximum that day. This is over 96% efficient! That seems like a pretty good result considering potential efficiency losses like the wiring run and the charge controller itself, which takes a percent or two.

So the panels, wiring and charge controller are all performing well (or can, when called upon). I thought this was the case, but hadn’t really tested it fully until now. Of course, getting the energy out the battery is a whole other story (a much more inefficient one). Exploring that will take some different measuring devices, and another post.