Electronic – Question about current output of a DC-DC stepdown converter

Your answers:

1. Yes, the current flows through both lengths of cable so you have to account for both.

2. Only voltage drops. Current is drawn through.

3. Basically I wouldn't go longer than a drop that would take it below the charge threshold of the batteries / charge circuit.

Basic ohms law is your friend:

For 1 amp of draw:

• 14AWG has a resistance of 2.525mΩ/ft

• So 200' has a resistance of 0.002525 * 200 = 0.505Ω

• V=IR

• Therefore V=1 × 0.505

• Which equals 0.505V

For the full 5 amps of draw:

• V=5 × 0.505

• = 2.525V

Which is obviously quite a drop.

This table has a nice list of the resistances of different cables. As you can see increasing the cross-sectional area (decreasing the AWG) decreases the resistance. Even just going from 14 to 12 AWG has a big difference (1.588mΩ/ft as opposed to 2.525mΩ/ft), and fatter cables are even better.

Connecting them in parallel may be a problem since the lower voltage cell will drain some current from the higher one. I don't know if this will damage the cell, but it doesn't seem healthy anyway.

Basically I can think about two options: a DC converter for each cell to equalize their outputs, or an electronic switch (like this one) to select just the higher source (and maybe redirect the lower one to a battery). Even then you'll need to somehow regulate the output (since both batteries and loads will probably have an operating voltage range to be met). But those things would be a bit of reinventing the wheel, since you can purchase a ready-made charge controller to take care of these things.