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Charging four 35AH 12V batteries from empty

Posted at age 30.

In preparation for the burn, it’s time to charge the batteries I use for my solar setup. I was surprised to find them registering only 5 volts on my multimeter; something in the solar regulator must have been discharging them. This at least gave me an opportunity to see how long they take to charge from empty and how much power that uses.

Equipment

My Burning Man solar setup uses:

For charging the batteries via vehicle or generator in emergency, or at home:

Other equipment you might need:

As an aside, I’m considering one of these inverter generators as a small and quiet backup, but have not pulled the trigger yet. The smaller one is apparently the same quietness, and I think should serve my needs based on the power draw data below:

Results

I plugged the 110V AC to 12V DC converter into an Aeon Labs DSC06106-ZWUS - Z-Wave Smart Energy Switch I use with my SmartThings setup so I could monitor the energy usage over time. I connected and started charging the four 35AH 12V batteries from empty. It began drawing 385W and slowly decreased.

The converter’s manual has a graph of its three stage charging.

PM3-12V-LK-Manual-2018-three-stage-charging.png

PM3-12V-LK-Manual-2018-three-stage-charging.png

I’m not sure it exactly matches my results (spreadsheet). Here are graphs of the instantaneous power draw in watts and cumulative energy usage in kilowatt-hours over both the first 10 hours and the first 24 hours. I think it was done before 10, but I wanted to see if the energy use would drop to zero. After 24 hours, I disconnected it.

Power to charge 12V 140AH battery 10h

Power to charge 12V 140AH battery 10h

Power to charge 12V 140AH battery 24h

Power to charge 12V 140AH battery 24h

The batteries then registered 13.1V, which is more than full according to the battery manufacturer’s table on the Amazon listing:

12.60V 100% Charged
12.50V 90% Charged
12.42V 80% Charged
12.32V 70% Charged
12.20V 60% Charged
12.06V 50% Charged
11.90V 40% Charged
11.75V 30% Charged
11.58V 20% Charged
11.31V 10% Charged
10.50V 0% Charged

I was pleased to see the reported cumulative energy was somewhere around 2000 kWh, which matched my assumption I have around 1700 watt-hours of battery power: 140AH * 12V = 1680WH (or perhaps more taking into account more than 12V; I am not totally sure how to think about that curve).

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