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Open Roads Forum  >  Tech Issues

 > Amps Draw and Voltage Drop Question 2

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BFL13

Victoria, BC

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Posted: 10/16/21 09:26am Link  |  Quote  |  Print  |  Notify Moderator

Question 2 -17 Oct
--------

There are no dumb questions--until now!

If the furnace draws 8 amps and there is a 0.2v drop when it is on, and you change the positive wire to the furnace to a fatter wire, will the furnace draw fewer amps, or will it still draw 8 amps but have less voltage drop at the battery bank?

* This post was edited 10/17/21 09:57am by BFL13 *


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Gdetrailer

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Posted: 10/16/21 09:37am Link  |  Quote  |  Print  |  Notify Moderator

Will draw slightly MORE current due to less voltage drop, results in motor spinning a bit faster.

RV furnace motor is a DC motor, DC for practical intents can be considered like a resistor since you are not dealing with impedance/inductance like you do with AC devices.

If you want the furnace motor to draw less current, you would need to REDUCE the voltage going to the motor. However, that results in less RPMs for the fan which results in less air flow which results in possibility of overheating the furnace and/or creating a fire hazard..

wolfe10

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Posted: 10/16/21 09:45am Link  |  Quote  |  Print  |  Notify Moderator

But, another way to look at it is how many amps are drawn from the battery.

If smaller gauge wire, instead of the vast majority of the amps turning the motor some are turned into HEAT by the resistance in the wire.

So, the answer really depends on how you phrase the question.

Bottom line is you want to maximize the amps taken out of the battery that are turned into RPM by the motor, not wasted in the wire.


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theoldwizard1

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Posted: 10/16/21 09:52am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

If the furnace draws 8 amps and there is a 0.2v drop when it is on ...

To properly measure voltage drop you need measure between the connection at the motor and the positive post on the battery.

BFL13

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Posted: 10/16/21 10:09am Link  |  Quote  |  Print  |  Notify Moderator

Thanks. So just live with it as is I guess. The 0.2v drop doesn't matter except when the batts are low and running an inverter load gets the voltage near the low voltage alarm. Then the furnace comes on and pushes it over the edge. Have to shut the furnace off until the inverter job is done.

The 8 amps doesn't matter except when the generator is on to charge the batteries. 75 amps charging drops to 67 amps every time the furnace comes on, which adds to gen running time. Same idea, you can shut off the furnace, but then pay the price after the batts are charged in more furnace time eating the AH you just put in to reheat the RV.

I suppose the LFP guys like their furnace fans spinning faster due to the higher voltages they run at.

I measure voltage drop by seeing it drop on the Trimetric monitor as soon as the furnace comes on. Right after the furnace shuts off, the voltage comes back up part way and then up more only slowly.

Gdetrailer

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Posted: 10/16/21 10:29am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

Thanks. So just live with it as is I guess. The 0.2v drop doesn't matter except when the batts are low and running an inverter load gets the voltage near the low voltage alarm. Then the furnace comes on and pushes it over the edge. Have to shut the furnace off until the inverter job is done.

The 8 amps doesn't matter except when the generator is on to charge the batteries. 75 amps charging drops to 67 amps every time the furnace comes on, which adds to gen running time. Same idea, you can shut off the furnace, but then pay the price after the batts are charged in more furnace time eating the AH you just put in to reheat the RV.

I suppose the LFP guys like their furnace fans spinning faster due to the higher voltages they run at.

I measure voltage drop by seeing it drop on the Trimetric monitor as soon as the furnace comes on. Right after the furnace shuts off, the voltage comes back up part way and then up more only slowly.


Couple of issues at play.

Your monitoring the voltage at your Trimetric connection, that voltage is not the same everywhere depending on wire ga and current draw of the loads combined.

Your inverter when loaded heavy presents a very high current draw and depending on how much wire resistance and load the voltage and current from your power sources may not distribute evenly.

Depending on your current battery bank setup, the wire gauges used and the loads your batteries and converter may not be robust enough to handle all the loads which sends you inverter into low voltage alarm or shut down.

To minimize the inverter low voltage alarm or shut down, one should use the largest wire ga possible that will connect to your inverter terminals and keep the runs (negative and positive combined) as short as possible (reduces the voltage drop the inverter sees).

Has nothing to do with your furnace wiring and more to do with the wiring to the inverter and size of battery bank for the loads presented.

Measure the voltage at the battery terminals and at the inverter terminals with your heavy 120V load, you want to see very little voltage drop between battery and inverter..

You might also wish to check the voltage drop between the converter and batteries, often factory wiring is just good enough to make it charge but upsizing the wire could deliver a bit more voltage and charge current to the batteries..

BFL13

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Posted: 10/16/21 10:57am Link  |  Quote  |  Print  |  Notify Moderator

My standard for acceptable voltage drop is that in the morning when the batts are down to 12.1 (50%) and will be recharged later that morning, I can still make toast using the inverter.

I have fat enough wiring to do that. No need to use fatter wire than now. If the batts do get lower for some reason before I can recharge them, I have to juggle things.

Toast comes first! The furnace can just wait a few minutes if a choice has to be made. [emoticon]

Itinerant1

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Posted: 10/16/21 11:08am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:


I suppose the LFP guys like their furnace fans spinning faster due to the higher voltages they run at.


Not only furnace but all 12v items running at peak performance and using the inverter/ 120v appliances is icing on the cake even at a low SOC. [emoticon]


12v 500ah (5,120Wh usable), 20 cells_ 4s5p (GBS LFMP battery system). 8 CTI 160 watt panels (1,280 watts)2s4p,Panels mounted flat. Magnum PT100 SCC, Magnum 3012 hybrid inverter, ME-ARC 50. Installed 4/2016 been on 24/7/365, daily 35-45% DOD 1,800+ cycles.

Gdetrailer

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Posted: 10/16/21 11:13am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:



Toast comes first! The furnace can just wait a few minutes if a choice has to be made. [emoticon]


[image]

CAMP STOVE TOASTER for $7

A low cost/low tech solution to a expensive high tech problem?

[emoticon]

Fisherman

Angus, Ontario, Canada

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Posted: 10/16/21 03:52pm Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer wrote:

BFL13 wrote:



Toast comes first! The furnace can just wait a few minutes if a choice has to be made. [emoticon]


[image]

CAMP STOVE TOASTER for $7

A low cost/low tech solution to a expensive high tech problem?

[emoticon]


Still have mine and it's never broken down or needed TLC.

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