Thursday, September 14, 2017

State of Charge vs. Voltage

I get questions all the time regarding "is this voltage right?" Well, it depends.

Temperature. In the summer they run higher, in the winter, lower. Also, if the battery is heated from the charging process, it can be warmer than the air temperature. As you can see, there is a 0.6 volt range between summer and winter, depending on where the batteries are located. In the winter many electronics will cut of around 50% state of charge due to minimum voltage requirements. Thus, the usable power in the winter is much less. Fortunately, solar charges more efficiently, though for fewer hours and at a lower sun angle. No fans, but more light and heating appliance load.

Surface Charge. Just after charging the voltage can be a few tenths higher, sort of like how a lieden jar stores static, although the mechanism is different. Put a few amps on them for an hour before reading at no-load.

Under Load or While Charging. Both of these affect the voltage due to internal resistance effects. Although in theory these can be corrected for, in practice you will miss by a mile. Always test while resting.

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