back to wire

effects of line loss:




Power loss over inadequately thick power wire (or too long of a run) can have a dramatic effect on delivery of
power, because the power losses obey a square law. The problem is further magnified when trying to charge
batteries over inadequate wire. Not only is there less power available, but the battery is accepting a lower
percentage of what could otherwise be supplied, which causes recharge time to snowball.

Lets examine trying to charge a battery that is 100 feet away from the power source, which for the sake of
argument will be 13.8 volts at unlimited current. We'll consider three different kinds of wire, and two
different charge states for the battery. For charge states, I'm going to establish some amount of current
the battery would draw if directly connected to the power supply, and use that to calculate the resistance
the battery is presenting to the supply. That resistance will then be used to calculate power draw and loss
with the various transmission wires. We'll compare two batteries, one that draws 10A and one that draws 25A.

Round-trip (100ft x 2) wire resistance will be:
#6 copper: 0.0790 ohms (0.0003950 ohms/ft)
#8 copper: 0.0995 ohms (0.0004975 ohms/ft)
#10 copper: 0.2128 ohms (0.001065 ohms/ft)
#10 CCA: 0.5403 ohms (0.027015 ohms/ft)


Lets start with the lightly discharged battery that would draw 10 amps if connected directly:
(battery resistance would be 1.38 ohms, and battery power would be 138 watts)

total resistance (battery+wire) will be:
#6 copper: 1.459 ohms
#8 copper: 1.4795 ohms
#10 copper: 1.5928 ohms
#10 CCA: 1.9203 ohms

current flow will be:
#6 copper: 9.5 amps
#8 copper: 9.3 amps
#10 copper: 8.7 amps
#10 CCA: 7.2 amps

battery power: (reduced current flow squared times battery resistance)
#6 copper: 123 watts
#8 copper: 120 watts
#10 copper: 104 watts
#10 CCA: 71 watts

percent charge speed, compared to a direct connection:
#6 copper: 89%
#8 copper: 87%
#10 copper: 75%
#10 CCA: 52%

Conclusion: #8 and #6 copper are pretty close, with #10 copper not that far behind. But the #10 CCA is
charging at half the speed of a direct connection.


Now lets connect a more deeply discharged battery that would draw 30 amps if connected directly:
(battery resistance would be 0.460 ohms, and battery power would be 414 watts)

total resistance (battery+wire):
#6 copper: 0.5390 ohms
#8 copper: 0.5595 ohms
#10 copper: 0.6728 ohms
#10 CCA: 1.0003 ohms

current flow will be:
#6 copper: 25.6 amps
#8 copper: 24.7 amps
#10 copper: 20.5 amps
#10 CCA: 13.8 amps

battery power:
#6 copper: 302 watts
#8 copper: 280 watts
#10 copper: 194 watts
#10 CCA: 88 watts

percent charge speed, compared to a direct connection:
#6 copper: 73%
#8 copper: 68%
#10 copper: 47%
#10 CCA: 21%

Conclusion: #8 and #6 copper are still pretty close, though #10 copper has fallen below 50% charge speed.
The #10 CCA has tanked, and is now charging at only 1/5 of what it would get with a direct connection.

You might think using the CCA results in a lot of wasted power, but it really doesn't because the resistance
the wire adds in causes a significant drop in current, which combines with the added resistance to have a
squared dropping effect on power drawn from the charging source. (at 25A, the CCA only radiates 135w
of heat) The bigger problem (for Field Day anyway) is how much slower the battery charges.

This doesn't mean it'll take five times as long to charge, because remember that the current will go down as
the battery gets more charged, which will cause the charge rate to improve over time. But I can't be bothered
to do the calculus to figure that out, so lets just call it 25% average. (over say the 70% of #8 copper)




last updated 11/30/2023 at 20:10:19