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# how can i measure how many cicles (ie. 15 second cicles) a given dc motor can run on a battery? votage, current ? =O Answered

I have a rechargeable battery from a eletric screwdriver, and i want to run a dc motor with it. Also, i want the battery to be recharged by a photovoltaic cell. So:

+ the battery is 12v, 1.3Ah, Li-ion
+ the dc motor is still unknown, but i think we can use variables in the place of the data
+ the solar cell gives in 0.3Ah

What i plan to do is: the motor will run intermitently, being actioned by a switch. When actioned, it will run for 15 seconds and stop until the switch is pressed again. I need to know how many times the motor cand do this cicle until i have to charge the battery again and; how much time will it take to charge the whole battery with the maximum output (0.3Ah) from the solar cell.

help?

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Yes, need the motor's electrical properties.  If you can measure the current flowing to it from the battery, then you can calculate everything.  For instance, if the motor draws 1 amp, I suppose the battery will run it for 1.3 h.

Solar cells generally put out only 1.5 v, so I'm not sure how it would charge a 12v battery.  But 1.5v times 0.3 amps (the output of the solar cell) equals 0.45 watts.  The battery contains 12v times 1.3 A, or 15.6 W.  It would take about 35 hours (15.6W divided by 0.45W) for the photocell to charge the battery.  If it could (but I think you would need to up the voltage from the solar cell to 12 v).

I may have made a mistake in the calculations, but you see how one goes about figuring it out.

Ok, i got most part of it.... Let's say that i change my solar panel. I have two options: a higher voltage with lower current (5v, 80mA = 0.44W) and a much lower voltage with much higher current (0.4 volts, 6,5A = 2,6W). Which of them would be better?

I'm really not sure about the low voltage being able to "push" charge into the battery.

test the motor yourself!

hook a multimeter to the Li-ion (to monitor voltage)

then hook your motor to it, then count the time it takes for the motor to turn 15 times

You've got it wrong. I didn't mean turning 15 cicles. It was 15 second long cicles.
Based on cyrpageman i just have to measure the current through the motor and calculate the consumption for a 15 second period.

That thing will be spinning at 3600+ rpm. Who am i to count that 15 cicles time!

You need the calculations to take into account what the motor uses under load.

You need a rough idea of the motor's electrical properties, and to tell us what voltage the solar cell gives.

L