# Automotive voltage drop

Discussion in 'General Electronics Chat' started by samjesse, Jan 15, 2011.

1. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
Hi
In an automotive vehicle, the Alternator is supplied a ground through its mountings to the engine block which is then grounded to the battery Negative by a cable. Another cable (positive) runs from the battery terminal to positive plug on the alternator.
Volt. Reading (a): alligator clip on the cable mounted on the Batt. (-) and the Alt. (+)
Volt. Reading (b): alligator clip on the cable mounted on the Batt. (+) and the Alt. Body.
Both reading is the same 12.6V but are not the same when cranking or running (charging).
What do I make off the difference between the readings?
Do I total the 2 reading, (a-b) or (b-1) for a total of voltage?

I want to calculate the voltage drop in the charging system but hooking up the alligator clips onto the cable connected on the battery posts does not take in account the connection between the battery post and the cable end. That is why I choose to do it as described first.

thx

Last edited: Jan 15, 2011
2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The difference you are seeing when a heavy load is placed is due to the voltage dropped in the resistance of the wires and connections.

The battery's internal resistance is under 1mΩ, while the internal resistance of the charging system/alternator is under 1Ω, the wiring (especially connectors) usually has more resistance than the battery and alternator combined.

3. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
Which components does each of the reading cover? i.e. volt. drop (a) belongs to which connections/components/cables? thx

4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
You can measure the voltage dropped across a wire by simply putting the voltmeter on each end.

Measure the voltage from the lead battery terminal to the clamp on connector by putting one voltmeter lead directly on the battery, and the second directly on the wire 1/2" away.

Do the same for ground, and other wires. The meter will show the voltage dropped across that length of wire. This will only work if you have a high input impedance DMM (Most all digital meters are high impedance).

If you find significant voltage drops on a connection, disconnect it and clean all connectors with a wire brush until they are perfectly clean, then bolt it back down. Adding some De-ox or other corrosion inhibitor is an extra step that can help.

5. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
The original setup (connection) was made and a scope reading both voltage drops in time. the test is done hands free once the scope cables are connected as per the original post.

I like to put up a math/function channel on the scope to give me the total v.drop. thus the question; a-b or a+b or b-a or...etc