Electrodoc Pro - Calculating resistor needed to lower voltage

[ThermallyFrigid]

I have ElectroDoc Pro installed on my android device.

I cannot seem to find where I would calculate the resistor needed to drop voltage to the desired voltage given the following.....

1). I am using a small solar panel for input.  It is DC.  90mA, 12.3v in maximum sunlight

2). I will be charging 5 AA NiMh batteries (in series) so 1.54v x 5 = 7.7v for the pack is fully charged.

3). I assume 9v will be adequate to charge this pack.


I thought surely Electrodoc would have a function for calculation the resistor needed, but if it's there I just don't see it.

Also, I was thinking that maybe they made it 12.3v output to compensate for some determined likelihood of less than maximum sunshine?
Problem is, on sunny full Sun days this will overcharge the batteries quite a bit, no?

Thanks

[IanB]

NiMH batteries are charged by current, not voltage.

Assume your AA size NiMH cells have a capacity of 2000 mAh.

Assume the maximum current produced by the solar panel is 90 mA.

This produces a charge rate of 90/2000 = ~C/20.

You can charge NiMH cells at C/20 more or less indefinitely without harm.

Therefore you can connect the solar panel directly to the battery. You do not need a resistor.

[Siwastaja]

Your problem can't be solved with a resistor, so there is no way to calculate the resistor.

Slow charging of NiMH is possible with CC-CV, and if the panel is small enough that takes care of CC, so the problem will require a thing called voltage regulator. Pass or shunt type are both applicable.

At very low charging currents, the self-shunting of NiMH might be an acceptable compromise, i.e., there is no voltage limit, and the cell itself converts excess charge into heat without being damaged too much, too quickly. This is called "trickle charge". In such case, a series resistor limiting the maximum current could indeed work. For this, you need to calculate R = U/I where U = V_solar_cell_open_circuit - V_NiMH_max_charge, and I = max specified trickle charge current for the NiMH.

[IanB]

However, NiMH batteries are themselves voltage regulators. They will internally shunt any excess current if the applied voltage is greater than the voltage on their terminals.

The internal current shunting is perfectly safe for small enough currents and intermittent charging. It only becomes dubious for constant charging 24h/day, every day.

For a solar panel application (presumably for camping or some such activity) the charging will be intermittent and no external regulation is necessary.

Edit: A charge rate of C/10 or less is usually considered "small enough" for intermittent charging applications.

[mikerj]

Therefore you can connect the solar panel directly to the battery. You do not need a resistor.

A diode would likely be a useful addition.

[IanB]

A diode would likely be a useful addition.

Good point. Don't want the battery discharging itself when it's dark.