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In this video, I'm going to show you how to determine how many solar panels you need to charge a 12V 100Ah lithium battery. To recharge this battery, you'll need 400W of solar panels.

I’ll break it down into three simple steps:
1. Finding out battery capacity
2. Looking up the amount of sun hours
3. Calculating how many solar panels you need

First, we need to convert our battery's capacity from volts and amp hours to watt hours. This measurement is crucial for the next steps and is independent of voltage, so it works for 24V and 48V batteries as well. For example, a 12V 100Ah lithium battery has a nominal voltage of 12.8V. Multiply this by the battery capacity and we get 1280Wh.

Next, we need to figure out how many sun hours you get, which depends on your location and the time of year. For example, in Houston, Texas, the lowest amount of sun hours is in December, with 3.5 sun hours per day. One sun hour equals 1000W/m². To find out the sun hours for your location, you can use pvwatts.nrel.gov

Now, let's put everything together. From step one, we know that a 12V 100Ah battery has a total capacity of 1280Wh. To recharge the battery in one day with 3.5 sun hours per day, you need 365W of solar panels. Considering the efficiency of the charge controller and the lithium battery, the combined efficiency factor is 90% or 1.11, so you need 400W of solar panels.

For our setup, we need 400 watts of solar panels, which can be two 200W panels or four 100W panels. We’ll wire the panels in series to simplify the wiring. To determine the charge controller's power requirement, we use the formula: 400W / 14.4V = 27A. A 30A charge controller will be sufficient.

If you have larger batteries, like a 20kWh system, follow the same calculation process. For example, 20,000Wh / 3.5 sun hours = 5714W, then multiplied by the efficiency factor or 1.11, you need 6400W of solar panels. For a 12V 100Ah lead-acid battery, which only has 50% usable capacity, you need 200W of solar panels considering the efficiency factor of 1.18.

If you’re using a battery charger instead of solar panels, a 12V 20A lithium battery charger delivers 240W. To charge a 12V 100Ah lithium battery with an energy capacity of 1280Wh, it will take approximately 5 hours and 20 minutes.

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