mike74820mike74820 Registered Users Posts: 44 ✭✭
edited February 2016 in Solar Beginners Corner #1
i know to small of wiring make you have voltage drop. my question does the to small wire make your battery work harder to push those volts and cause it to discharge faster?


  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Small wire means higher voltage drop--Which means more heat wasted in the wiring.

    However, the detailed answer is "it depends". Say you have a 120 Watt light bulb attached to a 12 volt battery with very heavy wiring:

    120 Watt / 12 volts = 10 amps
    V=I*R, R=V/I = 12 volts / 10 amps = 1.2 Ohms resistance of light (just keeping simple math)

    Now you use smaller AWG wiring and have a 2 volt drop in the wiring when passing 10 amps:

    R=2 volts / 10 amps = 0.2 Ohms (approx for wiring).

    The current flowing now with the new wiring resistance:

    V=I*R, I = V/R = 12 volts / (1.2 + 0.2 ohms) = 8.57 Amps

    So, in this case, the extra wiring resistance actually reduces the current flow in the system W.

    Watts = V*I = 12 volts * 8.57 amps = 103 Watts of power

    So longer battery life, but at the expense of less light from the bulb (and the bulb being more "yellow" in color).

    There are other types of loads--For example Constant Power loads like AC inverters... The inverter wants to send 120 VAC fixed voltage to the light, and will draw more or less battery power to keep the bulb voltage regulated.

    120 Watt light * 1/0.85 inverter eff = 141 Watts from battery. No wiring drop (very heavy copper wire):

    141 Watts from Battery / 12 volts = 11.75 amps from battery (12 volts, no wiring drop)
    141 Watts from Battery / (12-2 volt drop) = 14.1 amps from battery (2 volt wiring drop)

    So, in this case, you can see in this case, wiring drop actually causes the battery to supply more current and discharge faster.

    The above numbers are "made up"--but give you an idea of why load and wiring details matter.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike74820mike74820 Registered Users Posts: 44 ✭✭
    thanks i was wondering i noticed my wire run from my battery to my dc converter was about half a volt off. so i made a quick thicker wire run and now it just .2 difference now
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