I can see what you are saying-some power is converted to heat or light or rotary motion, so does it need to return to the source?
Not sure what to say here about that. I do not want to give false info, nor try to explain things I don’t understand, but I know that the battery ground cable is always as big or bigger than the positive cable. That indicates to me that each side of the load carries the same amount of amps, somehow.
Yes, I think you will get a lower voltage reading on the ground side of a filament as compared to the powered side.
When I am trying to describe voltage drop, the actual voltage reading is really not that important. What we are doing is comparing the voltage reading of a theoretically perfect connection with the actual reading, under load. The number seen on the meter reflects how much is MISSING due to an imperfect path, whether it’s on the powered side or the ground side.
Just for your own entertainment, set your meter to register at least 2 volts, 12 volts will work fine as long as the resolution is hundredths or more, as yours seems to be. We are talking volts here, not ohms, OK?
Hook up a parallel path on some steady load, like a bulb. (The digital meter doesn’t work so well on something like starter draw or a turn signal, because the meter does not find the correct number and display it before the number changes. The meter continually hunts for the right reading.)
I have a scar on my chin from my jaw hitting the floor the first time I saw how simple and valuable this voltage drop reading is.
If the meter reads something like O.21 you have a pretty good connection. More than a volt, or a volt and a half, indicates you can make some improvements.
I discovered this working on snowplow headlights on our Highway Department plow trucks. Mine was a tri-axle, 12 foot main plow, 10 foot wing and a sander on the back. When the plow was off, the regular headlights were pretty good, but when the plow was mounted, we switched to auxiliary headlights mounted up higher than the hood, so they would shine over the moldboard. These lights were so dim you could hardly see at night, especially in heavy snow.
When voltage drop was measured, first on the power side, then on the ground side, we were missing about 3 volts total. That makes a nominal 12 volt headlight run at 9 volts!
The problem was power from the OEM headlight switch was intercepted near the OEM headlight and routed back to the cab, into a selector switch, then back out another wire to the aux lights. Too light of a wire gauge, long distances, several connections, an extra switch, high-low beam connections, etc. degraded the voltage a volt or more.
The bigger culprit was the ground path.
I remember preparing a bigger, better ground wire, from the headlights ground terminal to a bare spot on the frame. This was in a pretty dark, dingy building. Since I did not remove the existing ground wire, the plow lights would shine on the far wall at what we had considered normal brightness. When I touched the new ground wire to the bare spot on the frame, it was like night and day difference. I could make the lights go from dim to bright by touching and removing the new ground wire from the frame. Every driver out of our shop wanted a better ground wire.
Since it was much more complicated to improve the power supply side, I never did do anything more there, but I would have liked to hook up a relay, powered (and fused) from the alternator output terminal, the output going directly to the headlights, then use the OEM wire to merely trigger the relay. That would bypass ALL the wires, switches, connections, etc. being used and replace them with one heavy, short wire from the alternator, thru the relay, then on to the sealed beams. These relays, called ice cube relays, can be had for $3-5.
Never did. Retired now, so it does not matter.
