Hello,

I know many solar battery charging questions have been asked, but I wanted some clarification on a couple of subjects.

I have a solar panel with an open circuit voltage of 5.5V and a short circuit current of 40 mA and I would like to make a current limited trickle charger for two 700 mAh NiMH batteries in series. If solar panel short circuit voltage is under 0.1C, do I still need a current regulator (such as a LM317 or equivalent)? Also, 5.5V is a little high for my 2.4V battery pack. But the 5.5V is the open circuit voltage with no load. Will the solar panel voltage be pulled low by the charging batteries, and is this a safe enough configuration?

In conclusion, I've looked at some IV curves for solar cells and I'm a little confused about whether the current consumption drives the solar cell's voltage or whether the cell's voltage drives the current output. Should I think of the cell as a current source and match the amperage to my batteries or should I think of the cell as a voltage source and use a current regulator to charge my batteries?

 

Trickle charging NiMH batteries is not recommended. Here's an article from batteryuniversity.com.

 

You don't need current limiting, but you do need a blocking diode to prevent reverse current, if there isn't one in your plan already. It will cause a voltage drop of~0.5V (depending on the diode you use - look for a schottky).

To be certain, verify that your batteries can survive a trickle current of 40mA/700mAh=0.057C. That's really an upper limit, since the cell will not be able to hit 40mA against the batteries. Some manufacturers claim that 0.1C is OK, other sources say 0.05C is OK. One thing you might consider is just using batteries with a bit higher capacity.

A solar cell is essentially a current source.

 

J
Hello and welcome to the forum. Your question is a good one and is well written.

Simply put, a solar cell is a light controlled current source that has a maximum voltage output. The max voltage capability generally doesn't change much once a minimum amount of light strikes the cell. A solar cell connected directly (through a diode) to a battery is a good and simple way to charge it. Be cautious that there is nothing to turn off the current once the battery is fully charged.

You are right when you say, "current consumption drives the solar cell's voltage", this is exactly the case.

 

I would expect the maximum charging current from your 5.5V solar panel connected to your 2.4V battery pack would be no more than 20mA (especially with the added blocking diode in series).
That will likely not harm your batteries even if left connected indefinitely.