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Author Topic: Solar, Capacitors, Amp Hours and Discharge Time  (Read 1778 times)
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I plan to charge capacitors with a small solar panel during the day and then have them discharge to power an XBee/accelerometer for a few hours at night. I'm not entirely sure how to calculate the amount of time it will run after the sun goes down.

In my very limited knowledge I figured ohms law and RC calculations were in order here; but I'm not sure if I'm doing it right. The accelerometer draws 0.6 mA and the XBee module draws 50 mA. Using ohms law at 3.5v, it would seem that the resistance is 6,000 ohms and 70 ohms, respectively. (although, the way I'm going about this seems wrong). Assuming I'm using a cap at 0.5 F, it would seem that I would get 3,035 seconds, or ~50 minutes of run time.

Is this correct or am I doing something very silly?

Thank you!
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Here's how I'd think about it. You have a 0.5F cap charged to 3.5V, right? The capacitor discharge equation says that I=C*dV/dt where I is current (~50mA), C=0.5F, and dV is how much the voltage is going to change. Let's say you're willing to let the voltage discharge to 3.3V, then dV=3.5-3.3=0.2V.

From this you can solve for dt:

dt = C*dV/I = 0.5*0.2/0.05 = 2

So I'm afraid this will only last 2 seconds until it reaches 3.3V smiley-sad

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Here's how I'd think about it. You have a 0.5F cap charged to 3.5V, right? The capacitor discharge equation says that I=C*dV/dt where I is current (~50mA), C=0.5F, and dV is how much the voltage is going to change. Let's say you're willing to let the voltage discharge to 3.3V, then dV=3.5-3.3=0.2V.

From this you can solve for dt:

dt = C*dV/I = 0.5*0.2/0.05 = 2

So I'm afraid this will only last 2 seconds until it reaches 3.3V smiley-sad

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The Aussie Shield: breakout all 28 pins to quick-connect terminals


Thanks for the quick response! That makes more sense.  smiley So rechargeable batteries it is.
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What do you mean by "a small solar panel"

The solar generated power during daylight hours has to be greater than what you extract at night.  Say you want 75mA for 10 hours, then you are consuming 750mAH during the night.  Will this be a 24 hour load, if so then you are looking at 1800mAH/day.  Say your solar panel is seeing sunlight for 10 hours then it needs to be rated for 1800mAH, or 180mA constant recharge current.   However the sun doesn't shine constantly, so allowing for cloud, rain, fog and general overcast you might want to assume getting one third of the panel rating.  So you're  actually looking for a panel that can provide a recharge current of say 500mA.

jack
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