I was excited earlier this week when my battery fully charged in a day, but it left me wondering how that charge had progressed. The solar panel is only leaning against the house for now, and it faces west. I wanted to watch the charging activity, but frustratingly have to spend the sunny part of the day away from home at work.

A full battery doesn’t need charging, and so I powered my laptop from the homebrewbattery for about 4 hours in the evening while researching how to log charging data from my PCM 60x charge controller. I estimated about 120-150 Wh of energy were used. In the morning I left a computer logging the PCM 60x every minute.

The battery was fully charged by 2pm! This is surprising since the solar panel points to the afternoon sun – and this is actually “visible” in the data because the 265 W panel only reached a maximum of 90 W before it scaled back due to the battery being at maximum voltage. By numerically integrating the shaded area under the charging power plot, I discovered that the battery took 135 Wh of energy to be charged. This is spot-on my estimate of how much I had flattened it!

The obvious conclusion from this successful test is that I need more cells to make my battery bigger. Back to the cell-testing regime for now…