Tiny solar battery project

[jbernal]

I have automatic chicken coop doors that run on 2 AA batteries, and I have been thinking about putting in some NIMH batteries and a tiny solar cell to keep them charged. I’ve already got some little 3v solar panels from another project. Since the stakes are so low, can I just put the solar cell parallel on the battery circuit without any diodes or resistors? Or does it need a little something to keep it from eating itself? TIA

 

[meetyg]

When the cells don't get any sun (at night or heavy clouds) they will actually consume power from your batteries.
I would at least add a diode between the cells and the batteries.
But then you might need to consider connecting two cells in series, because there is a voltage drop of around 0.6v when using a diode.
You mentioned that the cells are 3v, is that open circuit voltage (Voc) or max power point voltage (Vmp)?

 

[Risky Rob]

I might use a single 18650 li-ion cell.

 

[meetyg]

I might use a single 18650 li-ion cell.

I wouldn't do that without some protection circuit.
The Li-ion could get overcharged (we don't know if the 3v stated it open circuit or not), or dump energy back to the solar panel as I mentioned.

Li-ion are not very forgiving if you don't treat them right. Thermal runaway, smoke, fire... that kind of stuff.

 

[DThames]

I wouldn't do that without some protection circuit.
The Li-ion could get overcharged (we don't know if the 3v stated it open circuit or not), or dump energy back to the solar panel as I mentioned.

Li-ion are not very forgiving if you don't treat them right. Thermal runaway, smoke, fire... that kind of stuff.

put the battery in parallel with a zener shunt circuit so that if the voltage gets above X, the zener will bypass the panel output.

 

[jbernal]

When the cells don't get any sun (at night or heavy clouds) they will actually consume power from your batteries.
I would at least add a diode between the cells and the batteries.
But then you might need to consider connecting two cells in series, because there is a voltage drop of around 0.6v when using a diode.
You mentioned that the cells are 3v, is that open circuit voltage (Voc) or max power point voltage (Vmp)?

That's open circuit. 2 AA batteries. Solar panel puts out 3V @ 120mA not sure there is a chance of overload here, my main concern is not having a parasitic load from the cells.

 

[Warpspeed]

I would follow DThames advice, fit a zener across the whole thing to prevent overcharging damage.

I would worry less about parasitic load, if it does tend to drain down in cloudy weather, just add a larger (or more) solar panels, and/or a larger battery.
You will do better installing a slightly larger system overall, if that proves necessary, rather than trying to squeeze the last one percent out of something that is just under powered.