**Starter Battery** Blues?

[DCHitt]

Originally Posted by MountainBikeRoamer

Seems once you find/identify these devices, you can test for voltage loss across those directly as well.

It might be as simple as tightening the nut on the battery side of that starter relay. Be careful not to cause a short with your tool. Disconnect the neg side of the batteries first.

[arctictraveller]

Also inspect the connections between the cables and the lugs, where they are crimped. Often times, corrosion takes place between the wire and the lug, so the lug's connection to the post can be bright clean, but the wire inside the lug can be causing excessive resistance. Look for a green build up at the wire / lug junction.

[boywonder]

Mike: Keep in mind that voltage drop through a wire is heavily dependent on current flow, that's why we run big fat cables when lots of current and/or long runs are present. When the batteries are topped up and you have a mouse-fart's worth of current flowing through a fire-hosed sized battery cable there should be negligible voltage drop from the cable resistance.

Ohm's law V=IR

say your cable has .01 ohms resistance.....

@100 amps .01R(100A)=1V drop
@10 amps .01R(10A)=.1V drop
@1 amp .01(1A)=.01V drop

[MountainBikeRoamer]

Originally Posted by boywonder

Mike: Keep in mind that voltage drop through a wire is heavily dependent on current flow, that's why we run big fat cables when lots of current and/or long runs are present. When the batteries are topped up and you have a mouse-fart's worth of current flowing through a fire-hosed sized battery cable there should be negligible voltage drop from the cable resistance.

Ohm's law V=IR

say your cable has .01 ohms resistance.....

@100 amps .01R(100A)=1V drop
@10amps .01R(10A)=.1V drop
@1amp .01(1A)=.01V drop

Ahhhhh.....AHA.....
......so should I correctly interpret what you are saying....

....are you indeed saying that the .02v drop that is observed between the alternator and the starter battery is perhaps totally normal?

If I follow you....
The starter battery / ignition (and associated under-hood / cab electrical systems) are certainly drawing way more amps (when the van is running) than is the house battery, which is sitting largely idle and only sucking enough amps to stay topped off.

So....continuing to try to follow you here.....
....voltage drop / line loss in the lines to the low-amp-draw house battery would be almost negligible.....while the lines to the higher-amp-draw starter battery would see a far more noticeable voltage drop, simply because of the necessary higher amperage that's flowing through them ----

......and due to how voltage, resistance, and current all interact via Ohm's law......?

Apples to apples example:
If two sets of cables are equally sized, of equal internal resistance measure for a given length (and each run is cut to be an equal run length), and are both given an alternator input of 14.5 volts....and then you attach an electrical device at the end of one line that pulls only 0.1amp, while at the end of the other line you place an electrical device that demands a draw of 10 amps....you'll get 100 times more "line loss" or "voltage drop" in the line that's drawing 100 times more amperage.

So you could see 0.2v line loss in the branch that has 10 amps running, and then have only 0.002v line loss in the other branch. (And this second, tiny amount would be essentially invisible/un-measurable, at least with my voltmeter.)

So line loss of some degree is ALWAYS present. At least until we develop room-temperature superconducting wire.

This to me makes sense.
I might be totally misinterpreting what you are saying. But at this moment in time....it makes sense, haha....!

Finally ----
If I am reading you correctly.....
....would 0.2V of "line loss" from the alternator to the starter/chassis battery seem within the realm of normalcy/acceptability?

[MountainBikeRoamer]

Okay ---
I think I've answered my own question about the 0.2 volt line loss / voltage drop.

These two resources are pretty fascinating:

1) Electrical Amp Loads to run a car
Electrical Amp Loads

"Typical Current Loads for Automotive Systems, Lighting and Accessories:

Engine Idling (no lights or accessories on) - 35 to 50 amps. This will vary depending on the number of cylinders (more cylinders draw more power for the fuel injectors and coils), the type of fuel injectors (some draw higher amp loads than others), the type of ignition system (single coil or multi-coil), the amp draw of the PCM, and the fuel pump (the amp draw will be higher with higher pressure systems)."

2) Voltage Drop / Line Loss calculator
https://www.wholesalesolar.com/solar...n/voltage-drop

So I used this to analyze the 0.2v line loss/voltage drop observed between the isolator and the starter battery.

Inputting known values:
- what appears to be AWG 6 gauge wire
- 14.5 volts at the alternator output (there were a range of values for this number depending on the isolator being in or out of the equation, so I used a rounded number of 14.5 for this example)

And then using the following reasonable (rough?) estimates of the remaining values:
- line length (~ 5 feet between isolator output post and starter battery)
- starter/ignition circuit operating amperage (I used 40 amps, which I think is actually probably conservative)....

--> the line loss/voltage drop calculator yields the following calculated result.....



Predicted Voltage Drop:
0.2 Volts

.....MYSTERY SOLVED?