Amps Draw and Voltage Drop Question 2

MNRon wrote:
Unless I’m missing something, most of responses are missing the point of the issue (as the OP might have with how he’s thinking about this).

I believe the 0.2V drop is not IR from the 8A flowing through wiring to the furnace, but instead is internal voltage drop in the batteries (assuming normal battery connection wires). OP is not measuring 0.2V drop across the wire to the furnace, but sees that drop in battery voltage. Doesn’t sound like a big battery bank, or strong (low internal resistance) batteries like Lithium or Lifeline AGMs. There is a reason that SOC is measured with resting batteries and not under load, any battery will have some voltage drop when sourcing significant current.

To answer the OP question - no, a larger wire to the furnace will not help the situation. Any small IR change by beefing up this wire would have 2nd or 3rd order trivial change in motor draw, fan speed, etc. Better batteries and/or more capacity is the issue you’re dancing around.

OPs voltage measurement is highly dependent as to exactly where the voltage meter is connected.

If not directly connected to the battery post the measurement is taking into account the IR of the wires to the connection point of the voltage meter.

I would however concur, part of the voltage drop is most likely from the internal resistance of the battery bank which goes up as the batteries are depleted. That can't be changed other than adding extra battery capacity.

However, what one can do is minimize as much as possible the voltage drop to the high amperage loads like an big inverter (OP is using the inverter to run a toaster if I remember correctly, typical two slice toasters are 850W). That IS done by using as large as possible wire ga that will fit the inverter terminals and keep both the pos and neg wire runs as short as possible. With 12V items, even saving .1V drop under a heavy load can make a difference when dealing with depleted batteries.