Solar Panel with Battery

I created a robotic boat that runs on a 12 volt 12 AH lead acid battery. I figured if this is sitting in the water with the sun beaming down on it, it would only make sense to use some of that energy. The boat has 12 volt motors and a 5 volt switching regulator for the Arduino, Xbee, and sensors. The battery is activated with a switch. What I am hoping for is to have a solar panel power the 5 volt electronics through the voltage regulator even if the battery is off or on. The idea here is so that if I leave this boat floating in a pool or lake for days that the solar panel will power the 5 volt stuff to transmit back sensor data. Of course there will be times I want to turn on the battery and drive it around too (Is drive the right word…? haha) So I need at least 200 mA on the 5 volt side and the regulator is at least 80% efficient. So my questions are can I connect a 12 volt solar panel in parallel with the battery system with out any other electronics? and can anyone recommended a solar panel that would be fitting for this project and be under $50?

Thank you for your time!

Why not just use an 18V solar panel, and charge the 12V battery? Three of these would do it, and you could use two sets of three in parallel for a faster charge.

Would I need to connect them to a charger circuit of some kind?

Depends on battery type, battery size and solar panel size. Lead/acid batteries are more tolerant to abuse. Others are not. You always need a "backflow" (schottky) diode between panel and battery, to prevent the panel discharging the battery during night time.

There are also special 12volt panels that you lay on the dash of a car and plug into the cigaret lighter, to keep your car battery charged when you're not using it for some time. Maybe that fits your project better. Leo..

Lead acid batteries are very forgiving, and the current provided by those low power solar panels would probably qualify as "trickle charge". In that case you need only a diode to prevent the panels from draining the battery at night. See

For complete safety, or for use with a bigger panel, a simple float charge voltage regulator set at 13.8 to 14.2 volts can be purchased, or made from about four parts, as shown here. You need a diode between that circuit and the solar panel.

A better circuit that actively limits the charge current is shown here.

Great collection of solar charger circuits and more here.

Would I need to connect them to a charger circuit of some kind?

Yes. I bought a commercial electric gate opener some time back. It came with a solar panel and a sealed lead-acid battery. It produced just enough energy to open the gates 2 or three times a day in the winter but in the summer the battery exploded due to overcharging. The manufacturer supplied me with a new battery and a voltage regulator under guarantee.

So, let’s examine your requirements:

You say you need 200 mA at 5 V or 1 W. To be safe lets assume 2 W to take into account inefficiency in the regulator, the charging circuit, and the battery itself. So, during one day, you need 48 Wh of energy from the solar panel.

You don’t say where you are but here in the South of France we get an average of 4 kWh/day of solar radiation per square metre, more in summer and perhaps only half that on a dull winter’s day.

Solar panels have efficiencies of 10 - 20 %. If you buy a low cost one it’s likely to be nearer to the 10 % figure. So, if we assume you need it to work in the winter and you live in a similar part of the world to this, you will need about 1/4 square meter of panel assuming that it is pointed to the sun all the time. If it is laid flat you will need much more.

How big is your boat?


Would definitely recommend a good charging circuit - lead-acid need care just like any other secondary battery, over-charging, under-voltage, over-current are always issues.

You would need to back off to the correct trickle charge rate when the battery's full or risk gassing and losing electrolyte, over-pressure and venting. This depends on the type of battery, if you have a datasheet for it then it should have such details.

You also need to ensure hydrogen cannot build up to an explosive mix, so if the battery is openly vented you need to run a tube from the vent outlet to the outside air. Even if its sealed it could vent if abused, so good airflow round the battery would be needed to disperse hydrogen and oxygen gasses.

What kind of motors? Ordinary DC motors generate sparks on the commutator so you have a good ignition source in that case!

MarkT: Would definitely recommend a good charging circuit - lead-acid need care just like any other secondary battery, over-charging, under-voltage, over-current are always issues.

Yes. Too high an initial charging current is unlikely with a solar panel. What is needed is to control the maximum voltage. For continuous charging (float charge) the voltage should be regulated to 13.65 V for a 12 V battery, ideally reducing the voltage by 4 mV/C above 25 C.

For best results I would use a buck converter between the solar cell and the battery set to 13.6 V for a 12 V battery or 6.8 V for a 6 V battery and then another buck converter to reduce that voltage to your required 5 V.