solar charge monitoring for robot!

Hello. I am building my first robot which is a solar rover and need some pointers for controlling the charging circuit. I have a 9.6V Nicad battery pack powering the arduino and a 12 volt panel. I know I need a blocking diode on the panel to stop power leaking out the panel. I also know how to split the voltage of either the panel or the battery so as to monitor it. What i can't figure out is how to use that information to protect my battery from over-charging. i thought maybe i could use a pin out to disconnect the panel when the battery is fully charged, but i don't know how to do this. I've looked through the forums and haven't found specific info on this. Thanks in advance for the advice!

Go to and look at their battery charging chips. Lot of smart chips that can control the charging for you and prevent damage. request a sample!

Let us know your progress. Sounds like an interesting project.

Here is some more info on the robot! His name is Colin the Security Robot (per HHG2G). As of now he avoids obstacles with the help of a servo-mounted ping sensor. The body was a H2 hummer (with treads for some reason?) rc car from stop and shop. As you can see in the video the solar panels have not been installed yet. When encountering an obstacle directly ahead of it, Colin lifts up the car roof in surprise. This roof is where the panels will be mounted with some photoresistors for solar tracking.

Youtube link

More specifics: Two full rotation servos drive the tracks. One standard parallax servo rotates the head. One rc servo from spektrum moves the roof. The spektrum servo seems to use more power than other brands and I had to power it separately using the original 4 AA battery compartment on the car. I plan on getting another parallax servo, or maybe futaba from sparkfun. Hopefully this will eliminate the need for separate battery power and simplify the solar charging setup.

The board is mounted inside an altoids tin and protected by another tin with a duct tape hinge for easy access.

The ping module is set in a mini breadboard and protected by a carved up deodorant stick cap covered in duct tape.

All questions and suggestions are welcome!

I want to add photos here, how do i do this? =P

You need to post your photos somewhere, like flickr, and put that link here inside
[i m g] your link [/ i m g] (without the extra spaces), the third button from the top left can pull up that code for you.

here are the pics!

If you want any more info or specific pictures i would be happy to oblige!

ok here is just the link to the photostream on flickr: williamanos | Flickr

Is there any reason that the circuit I have drawn in this photo would not work for managing my battery charging? The battery would be monitored by splitting the voltage and connecting to an analogue pin. When the voltage reading indicates that the battery is fully charged, an arduino pin would be made to write High to the base of the PNP transistor stopping the flow from the panel to the battery. Am I making sense in this diagram? This would appear to be a super simple way to manage charging. Thank you for the help!

Circuit: 2010-12-01-172320 | just a sketch i need help on. wondering … | Flickr

Yes, you'd need a voltage divider to monitor the charing voltage.
I'd suggest looking at some of the chargers, see what they are doing.
Most monitor the current or temperature or something to know when to stop. You have to put in higher than 9.6V to get the electrons in - am pretty sure 12V less one transistor drop is not going to be good for the battery.
They will send out samples too. for example for NiMH batteries
(people don't use NiCads anymore, do they?)

am pretty sure 12V less one transistor drop is not going to be good for the battery.

Sorry, I have no formal training in electronics (realized my passion too late in college). Are you saying that the solar voltage will drop to the level or below the level of the battery after going through the transistor?

It seems fairly straightforward to tape a thermester to the battery pack, or to divide the voltage and monitor it through the arduino. The actual cutting off of the solar panel from the battery is the tricky part for me right now.

You're right about the NiCads as far as charging circuits go. Oddly enough RC cars seem to be hanging on to NiCads. This pack i got at radioshack because it was the highest voltage pack they had. I saw an identical one at toysrus the other day as well!

I'm looking at the ICs now, trying not to be intimidated.

Also, update on development in general: I made the panel out of a few smaller modules and just realized I need to put a diode between each module to protect each one in the event of partial shade. I may end up switching to a thin film panel soon, as they apparently tolerate the shade much better.

Thank you for all the help!