# Circuit for charging RC car battery with solar

For my high school engineering course I have to make a project that involves solar panels. I have chosen to convert an RC car to recharge the battery while driving with solar panels, because I realize it wouldn't be possible to fully power the car with solar. Now I need help on the circuit. The car has a 6 volt 700 mAh Nickel Cadmium battery, meaning it is five 1.2V cells in series. This battery of course powers the DC motor - of which I do not know the power consumption etc. Now my question is how exactly I should make the circuit to charge the battery while in use. I have already purchased three 2V, 240mA solar panels, which I planned to place in series to meet the voltage of the battery and thus be able to fully charge it.

My plan is to place wiring from the negative terminal of the battery to the negative terminal of the solar cell array (the 3 in series), and from the positive terminal to the positive terminal, this would be parallel. Would I need to place a diode in between either of the poles? And would this diode mean I need a greater voltage than the 6 volt of the battery? Also, is this a dangerous thing to do, should I use charge controller such as http://www.maximintegrated.com/datasheet/index.mvp/id/1666.

Also, if I do this, would I be charging the battery while it is being discharged, or does the current from the solar panels go straight to the DC motor, with additional current being pulled from the battery?

## 1 Answer

As you have described it, your solar cell configuration will not be able to charge the battery. To fully charge a "6 volt" NiCd battery you need a charging voltage that is somewhat higher than that...something like 8.4 volts.

Charging a 700mAh battery at 240mA could be dangerous, it's hard to know for sure without having any specifications for the battery. To make sure the charging is safe you would want to limit the current to about 1/10 of the battery's capacity, or 70mA in this case. Of course, that means it would take at least 10 hours to charge the battery and probably more like 20 hours.

Charging at higher rates requires pretty careful monitoring of voltage, current, and/or battery temperature. This is turning into a very broad topic so I suggest you do some more research and come back with specific questions.

• Thank you for replying. If i managed to get a voltage of 8.4V, could i then just place a resistor of 120 ohm and achieve a current of 70mA? And another question, would the way i described the circuit work? Placing my solar array in parallel to the DC motor and the battery, with a diode to block the current from going back to my solar panels when it isn't in direct sunlight? Commented Jan 12, 2014 at 22:52
• I don't know enough about solar cells to tell you if a diode is needed. As for the resistor value, you could assume that you have a supply of 8.4 V and the minimum battery voltage is 6.0 V. That makes the voltage across the resistor be 2.4 V and according to Mr. Ohm you need a resistor value of about 34 ohms. Note that the charging current will slowly decrease as the battery voltage increases so it will take a very long time to fully charge the battery. Commented Jan 13, 2014 at 0:16