Is a current controller needed for a low Watt charge circuit?

Is a current controller needed for a low Watt charge circuit?

Here's the simple outline for the project, it's a solar charged outdoor light. I'm using an Arduino Nano, 9v rechargeable, a photoresistor, a few LEDs, and obviously a solar panel.

The panel is a 9v 1w panel, so only 111mA, the full sequence is only pulling 65-90mA of current depending on operation, only 40mA during the day when it's basically in idle.

The problem is the entire project costs about $12 in parts at the moment so I'm having a hard time justifying another $10-$15 for a current controller to keep form over charging the battery, especially since only about 60mA of current will actually be going into the battery.

What is the likelihood of damaging the battery if I just hook the solar cell and battery up in parallel to the Arduino?

The programming and hardware design is already done, only thing left is how to charge the battery, straight from the solar panel or through a regulated charge circuit? Obviously the charge circuit is the safest option, but is it really necessary?

Wait, your solar panel is rated 9V, 1W and you project draws 65-90mA all the time? Even if the solar panel would get perfect conditions halve of the day and supply 111mA, it would only do that during daylight, so 55mA on average. In practice, though, you will get far less current during the day. What is the project, can you work with deep sleep?

If you charge a NiMH with less than C/10, you should be fine. What kind of battery are you using? 9V blocks have much lower capacity. I think your numbers do not check out at all.

Yes the critical question is what TYPE of battery? They all behave differently when being charged.


Your best option would be to charge a supercapacitor. But remember that ratings for solar panels are MAXIMUM under ideal conditions. You'll never get that in the real world.

It is a fair point, I was already considering stringing a couple more panels together or finding a higher rated one. For the sake of this question let’s assume I’ll run a second 1w 9v panel in parallel so we can double the amount of juice to the battery. We can assume a max of 200mAh with an average of 100mAh with a draw of 40mA during charge time and a max of 90mA during run time. (after dark) Which would leave 60mA being used for charging the battery. (or realistically 74mah to the battery and 14 from because of parallel draw rates)

The battery is a simple 600mAh Li-ion. And I just realized that this battery will only last about 6hrs in the dark. And will not be effective for this project, I will need to run a minimum of 2 in parallel or switch to at least a high capacity 2S LiPo. Although switching to group of 3-4 RAM batteries will give me 1200-2500mAh which may be the way to go. Plus they are cheaper than LiPo and will probably last longer with an inconsistent charge rate. It would be nice if I could grab a LiCoO battery, but those are still pretty expensive last I checked and I am not trying to put a lot of money into this small project.

This project keeps getting more complex by the moment. Changing the battery means I’m going to have to redesign the electronics housing too.

I will have to also look into deep sleep, that’s not something I’ve tried before.

There is no such thing as a 9V Li-ion battery. And connecting any Li-ion/Lipo battery direct to a solar panel without using a proper charge circuit is a really bad idea.

Rechargeable alkalines are useless. You would be better off with LSD NiMH rechargeables but to get the capacity you want you'd need AAs and they would be quite large.


There are lots of great ideas in this excellent solar powered/supercap remote sensor project for Arduino.

@slipstick I do understand that the common Li-ion cell is 3.7v but this is what I was looking at using:

I also have a few of these powering smoke detectors and they do put out about 9v.
The RAMs have much higher power storage than NiMH although the NiMH has a better cycle rate. Both are about the same size, AA. Anyway, looking into other options.

@jremington I will take a look through that forum.

@skywatch Yeah, I’m looking toward super caps.

Thanks for all of the help everyone.

Supercapacitors are able to store orders of magnitude less current than a battery. If you need 9V only fraction of power stored will be available for use unless you use a boost converter. You should 1) use some simple circuit to limit maximum voltage and/or current on your battery. A Zener diode or a LDO voltage regulator are good simple possibilities. 2) try to reduce current usage as much as possible. Using sleep modes, remove everything unneeded from Arduino (use standalone ATMega maybe?) etc. 3) if you reduce the current needed a lot (tens of uA) consider switching to supercapacitor. Or primary battery and remove the solar panel.