Solar Blocking Diodes
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Solar charger circuit question?
I have a 14V 3W solar panel, can i directly connect the terminals to a 7.2v (6x1.2v rechargeable NiMh) battery? between the solar panel and the battery i have put in a blocking diode. Would my circuit damage the batteries? i'm afraid to test it out and have it explode in my face.
Possible. You need to do some calculations.
1. You need to know the capacity in mA-hour of the battery.
2. Measure the short circuit current output of the solar panel with a multimeter, it will be somewhere around 200-400 mA.
To avoid damage, you should not charge the battery at a faster rate than C/10. So if this is a 500 mA-hour battery, the fastest charge rate should be 50 mA. If the solar panel is capable of more than that, you need a series resistor to limit the current to that value. It should take 15 hours to charge the battery under these conditions.
You can charge a a faster rate than C/10:
Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has some of its capacity left when the timer starts you would have a good chance of damaging the battery.
Series resistance calculation. The short circuit current test above will determine the internal resistance of the panel, as Ri = open circuit voltage / short circuit current.
If the charge rate is Ic (C/10) then R = [ (14–7.2–0.7) / Ic ] – Ri
(0.7 volt is for the diode)
For example, under the conditions above, if short circuit current is 300 mA, Ri = 14/0.3 = 47 ohms
Series R = (14–7.2–0.7) / 0.050 – 47
R = 75 ohms, 1 watt.
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