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[on:]

I'm trying to run a train with a DC motor and AA batteries.  I want to wire small solar panels to the batteries to constantly trickle charge NiMH AA batteries and run the motor off of the batteries at the same time.  I'd like to use these solar panels because of their dimensions and price:http://www.siliconsolar.com/flexible-solar-panels-3v-051283-p-500993.html. If they won't cut it i'm willing to change them.

I believe if I wire 4 of the panels in series, I should get about 12V and 25mA.  Depending on the motor I choose, would that be enough to continuously charge AA batteries; and at the same time, run the motor off of the batteries without the batteries quickly dieing?  If so, how can I modify the circuit to prevent over charging (if that is an issue).  It it possible to also power an Arduino Uno from the same source?

My main problem is I don't know the scale i'm working with: how many AA batteries, RPM/Torque of the motor, size of the solar panels. The train will end up being about 10" long, 4" tall, and 2.5" wide (the only thing determining the length is using 4 solar panels from above).  I'm not sure how much it will weigh because a material hasn't been chosen.  I'm considering plastic or metal.  I'd like it to be as light as possible in the end.

Perhaps someone could shed some light on the subject and help me with powering the train?


Simple circuit diagram: http://img140.imageshack.us/i/indirectlypoweringdcmot.jpg/

[Reply #1 on:]

First Question, What size train?

[Reply #2 on:]

Second question. what proportion of the time do you intend the train to be running?

Using those panels, the answer will probably have to be very occasionally. Solar panels seem to under-deliver for me, but that may be because I live in the Northwest of England where it rains a lot.

Back to first question, do you have a multimeter, so you can check how many mA the train's motor draws?

[Reply #3 on:]

First Question, What size train?

The train will end up being about 10" long, 4" tall, and 2.5" wide as the post said.  The walls could be as much as a 1/4" thick for those dimensions.

Second question. what proportion of the time do you intend the train to be running?

Using those panels, the answer will probably have to be very occasionally. Solar panels seem to under-deliver for me, but that may be because I live in the Northwest of England where it rains a lot.

Back to first question, do you have a multimeter, so you can check how many mA the train's motor draws?

What do you mean by "very occasionally?" I'd like to run it as much as possible, but I know the solar panels might not supply what the motor is taking out of the battery (which is okay).

I do have access to a multimeter, but I haven't selected a motor.  I don't know what kind of RPM/Torque I need, nor what my power system can support.

[Reply #4 on:]

Basically you are facing an impossible system, you have a big heavy train and a very small surface onto which you wish to put solar panels. For a train this size, you are looking at about 2 amps of current at about 12 v to push it along at a moderate speed. What I would suggest, is using the rails as + and - supplies for the train, similar to the way most HO scale model trains work.

[Reply #5 on:]

If we crunch a few numbers, we can easily get an idea of how long the train can be run via the solar panels.

12V solar panels at 25mA = 300 mW of power.

12V motors drawing 2A = 24 watts of power (or 24000 mW).

At 100% efficiency (unrealistic, but just to get some ballpark numbers), for every minute the train runs, the solar panels will need more than 80 minutes to recharge.  If we factor in the inefficiencies of the system, you're looking at increasing that time by upwards of 50%.

[Reply #6 on:]

If we crunch a few numbers, we can easily get an idea of how long the train can be run via the solar panels.

12V solar panels at 25mA = 300 mW of power.

12V motors drawing 2A = 24 watts of power (or 24000 mW).

At 100% efficiency (unrealistic, but just to get some ballpark numbers), for every minute the train runs, the solar panels will need more than 80 minutes to recharge.  If we factor in the inefficiencies of the system, you're looking at increasing that time by upwards of 50%.

Hmm, I see the problem.  The train does not need to be completely dependent upon the solar panels.  If I use 4 AA NiMH batteries at 2100mAh in parallel with the solar panels, theoretically the solar panels should extending the life of the battery.  I'm heavily depending on the initial charge of the batteries.

[Reply #7 on:]

You're never going to get past the problem that the power capacity of the solar cell is really small compared to the batteries.

Ignoring inefficiency:
4AA batteries=4 x 1.5V x 2100 mAh= 12,600 mW
Solar cells = 12V x 25mA = 300 mW/hr

So it takes 40 hours of the solar cell -- at full sunshine -- to equal the capacity of the batteries. How much that contributes depends upon how much power your project consumes.

If your project drains the batteries in 40 hours then the solar cell can keep up. If your project drains the batteries in 24 minutes the solar adds less than a minute.