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Solar battery charger

This small solar power supply charges a 6V sealed lead acid battery with constant voltage (and also limited current) while providing 5V output.

First of all, some words about solar cells. My experiments were based on 4 SOL4 solar cells from Velleman kits (rated 2V and 200mA). Solar cell ratings are given as separate figures - a floating output voltage and short ciruit current (at no output voltage, that is). With any kind of sun shining on it, you are not going to get both the rated voltage and rate current out of them. The actual figures will be somewhere less that. Quite a bit less of both, if you are not living in desert conditions. Here are some figures that I measured on a bright autumn day in Estonia, 4 cells were connected in series:

No load 6.63V
Cell outputpct of total
1.6224.4%
1.6424.7%
1.4321.6%
1.9429.3%
   
60ma 5.1V
Cell output
1.25
1.26
1.10
1.49

As you can see, not all cells are created the same, there is a 8% difference between the best and the worst! I first tried to use it as battery charger just using a diode in series with cells and found that the charging current in prett much a dissappointment, and I only get it for couple of hours a day when the sun is shining brightly. The circuit below is an outcome of my experiments, it provides about 4mA of charging current even under desk lamp. Any 8V output cell set will work just as well.

R14 is a current sense resistor for a switcher chip, but it also limits maximum charging current for a whole system. L2 is a cheap axial inductor rated at 1A. D6 is a clamping zener that turns excess energy into heat should the battery be fully charged and load too small to use it up.

The switcher output is connected directly to battery that provides backup power for dark times. This voltage does not go higher than 6.85V, so the the linear LDO regulator will provide about 73% worst case efficiency. In actual usage it will be better as the battery voltage will quickly drop to about 6V, at that point the efficiency will be about 83%.

The 34063A switcher typically starts working at about 1.8V input voltage, so if there is enough light to get about this much out of cells at about 20mA current, you are in business...

 

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