If you are looking to charge devices or a Voltaic battery from a solar panel positioned far away from your device, you need to think about the gauge (thickness) of the wire you use to transfer the power. The issue is that the Voltage will drop based on three factors: 1) length of the wire, 2) thickness of the wire, and 3) amperage.
Why is this important?
Devices have minimum charging Voltages and they often charge better above a certain minimum threshold. Lets say the output of your battery is 5.2 Volts and your Galaxy Tab stops charging at 4.76 Volts. You need to design your cable so that you have no more than a 0.44 Volt drop or your tablet won’t charge.
How do I calculate the gauge I need?
1. Figure out the Voltage of your source and the minimum Voltage your load / device requires. Voltaic batteries typically output 5.2 to 5.3V for the broadest device compatibility, but you can measure yours with a multimeter to confirm.
3. Measure the maximum Amperage your device will draw from the source. You can look on the battery for Amperage, but better yet, measure the current flow using a multimeter.
4. Determine optimal and minimum current requirements for your device. This is a bit trickier. We use an adjustable Power Supply. If you don’t have one of these do research online to determine the device requirements. You can also ask us.
5. Measure the amount of cable needed between your source and load / device.
6. Calculate Voltage Drop Using the Resistance Coefficients
To calculate the Voltage drop at a given gauge, use V=IR and multiply: Coefficient x Max Current x Wire Length x 2. You need the “2” because the current is traveling down the length of the wire and back.
| Wire Gauge | R per Foot (Ohm) |
|---|---|
| 26 | 0.041 |
| 24 | 0.026 |
| 22 | .016 |
| 20 | .010 |
| 18 | .006 |
| 16 | .004 |
So if you have a 15 foot wire running from a V15 battery that outputs 5.3V, 1A and you used a 20 gauge wire, you would get a 0.3V drop or 5V output at the end of the wire. We encourage you to test, but that would be enough to charge the Galaxy Tab. If you can afford it, use a thicker wire for less Voltage drop.
Thanks Jeff!
How about some recommendations for the appropriate wire gauge when locating the solar panel away from the battery?
This will depend on the panel output and the optimal voltage for charging the V15 or V39 batteries.
Thicker is better may be a good rule of thumb – the less energy wasted heating up the wire the better, but the efficiency of the charging circuit at different input voltages may be a factor.
For the V15 and V39, you want to try and limit the Voltage drop to 0.5V. We found that the V15 and V39 charge fastest over their complete charge cycle in the 5.5 to 6 Volt range. If the Voltage drops 1A, it will still charge the V15 or V39, but you are wasting power and the charging towards the end of the charge cycle will be less efficient.