# Solar Panel strength for small radio

#### Stevemills04

Jan 13, 2014
3
I am new to electronics and need help. I have a mini radio that uses 2 AAA batteries to power it. The batteries are labeled Ni-Cd AAA 200mah 1.2 volt. My goal is to find a mini solar panel or 2 or 3 that can power the radio without batteries, but also charge the batteries when necessary. My question, how do I calculate or determine what strength solar panels I need to power the radio without batteries? I appreciate any help.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
You need to determine the peak power required by the radio. Knowing how long the batteries last will give you an estimate of the average demand.

From this you then need to consider the sky conditions (generally the worst expected) and the orientation of the panel (again the worst) to determine the minimum size panel you need.

#### Stevemills04

Jan 13, 2014
3
Thanks for replying Steve. So let's say the batteries last 5 hours and for the solar panels I would say at 20% sun power, like a cloudy day. The solar panels are portable so I could point then directly at the sun. Would is be a good enough example to figure a calculation? I'm just try to have a formula so I can calculate when I know the exact details.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
OK, for a 5 hour life, the average current is around 40mA. Let's assume the peak current required might be 80mA.

And let's assume that you're only getting 20% of the peak power from the cells. That means you want a panel rated for a peak current of about 400mA.

Now 400mA at (say) 6V is about 2.5W.

Added to this, you'll need some sort of regulator to limit the voltage to about 3V.

#### tryppyr

Oct 22, 2013
228
I'm wondering what advantage there would be to operating the radio without the rechargable batteries. I can understand wanting to run the radio while the batteries are discharged, but taking the batteries out of the circuit doesn't make much sense to me.

Generally, I would expect that the primary function of the solar panels would be to charge the batteries. The primary function of the batteries would be to drive the radio. When the batteries are discharged, the power to run the radio can come from the solar panels by way of the battery charging interface without removing the batteries from the radio.

- Greg

#### Ridgerunner

Jan 13, 2014
1
What is the model of the radio?

Can the radio charge the battery when plugged into a power source>

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
I'm wondering what advantage there would be to operating the radio without the rechargable batteries.

Not a lot.

The solar panels need to supply all the power (so peak power) not the average power.

Depending on the nature of the power draw, a capacitor may be sufficient.

Of course, you add the problem of overcharging the battery.

#### Stevemills04

Jan 13, 2014
3
I'm wondering what advantage there would be to operating the radio without the rechargable batteries. I can understand wanting to run the radio while the batteries are discharged, but taking the batteries out of the circuit doesn't make much sense to me.

Generally, I would expect that the primary function of the solar panels would be to charge the batteries. The primary function of the batteries would be to drive the radio. When the batteries are discharged, the power to run the radio can come from the solar panels by way of the battery charging interface without removing the batteries from the radio.

- Greg

Thanks again!

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