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Trickle-charging is a slow-charge that replaces the natural self-discharge of a battery in storage. The amount of self-discharge depends on the battery type and storage temperature. For example, a 12V Douglas AGM sealed lead-acid that is stored at room temperature will lose about 3% of its stored capacity per month.
At 50F (10C), the loss would be approximately 1.5% per month, and at 104F (40C) the loss would be around 10% per month.
For flooded lead-acid batteries, self-discharge losses might be as much as twice the losses of sealed batteries listed above.
A solar trickle-charger replaces the lost battery capacity so that the battery is always freshly-charged.
Tips for calculating what size solar panel you'll need to trickle-charge your battery:
1. To start, you'll need to know the capacity of the battery you want to trickle-charge (usually listed on the side of the battery in "Ah" or "mAh", where 1 Ah equals 1000 mAh. If you can't find it there, try looking up the battery model online).
2. Next, calculate the self-discharge losses. If you will be storing a sealed battery in the summer, use 10% per month. If you will be storing the battery in the winter, use 1.5%.
Then, divide by 30 to find the average Ah loss per day.
Example 1:
A 200 Ah battery stored in the summer:
200 Ah * 0.1/30 = 0.67 Ah per day= daily self-discharge loss of the battery
3. Next, determine the number of full sun hours to expect at your location, based on the time of year. If you're in the US, you can use the solar insolation maps provided by the National Renewable Energy Laboratory for march, june, september, or december to approximate this.
Note: 1 kWh/m2/day is the same as 1 full-sun hour per day.
4. Finally, divide the total capacity loss per day from Step 2 by the total number of full-sun hours per day from Step 3 to get the desired current of the panel.
Example 2:
The 200 Ah battery from Example 1 loses 0.67 Ah per day. In NH in June, the total full-sun hours per day will be 5.25 hours average. 0.67/5.25 = 0.13A.
For this case, a panel with a current output of at least 0.13A (130 mA) would be sufficient to maintain this battery in storage.
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