I agree with Richard in that MINUS 160mV is probably not excessive
Not quite what I said. BUT I missed the 110amps comment first time (Am I going blind too
or was that an edit
To give any answer then we'd Need to know :
- if the engine is running or not
- if the 110amps is alternator output or actual load (so precisely where the amp clamp is)
- what's battery voltage (charge)? (affect alternator output)
- What fault symptoms are you trying to diagnose, might help understand why you did this test (engine to battery negative), which is a very valid test, but not for an "engine not running" scenario!
If it's purely user created load and engine is not running, why measure voltdrop (VD) to engine, should be chassis?
110amps on the earth lead of the battery, giving 160mv drop, I think is high, but a weedy cable and it might be acceptable. What is the normal load the cable is supposed to handle, that would tell you if someone has swapped to an inferior cable ?
Yes "Engine Running", then we're looking at alternator output, so the volt drop will be reversed, from say a cranking capture. VD goes both ways, depending on what's consuming or generating!
I could create 40 or more scenario's from the description, so it's pointless to guess.
Setting up a scope I'd do this:
ChA Battery + and earth to - as our base line for all VoltDrop
ChB Amp clamp to BATTERY earth lead (will show what the battery is doing) (clamping all leads to Positive will show the NETT replenishment of the battery - even if the Alternator lead is shared)
- Test 2 - Move to Alternator Lead (if a separate cable on the battery, there would be good)
- Test 3 - Move to Earth Strap of Engine (each in turn if more than 1)
ChC Chassis (near the earth strap but not on it)
ChD Engine Block
The Total Volt drop for an Alfa 147 during cranking (so engine strap and weedy battery lead, with identified need for a clean of all terminals/bolts) at 110amps was 180mv negative, but only 120mv Positive (hence clear need for clean and recheck), negative reduced to about 100mv, could have benefited from new engine earth strap, but wife's car, so we'll monitor!
If this was just the battery lead then definitely would clean terminals / connections and if no improvement replace the lead, but I'm fussy about volt drop!
What you can calculate from amps and Volt drop is RESISTANCE (Ohm's law) as a maths channel, that will tell you a huge amount, rather than guess as to good/bad...
That's all a bit excessive though, this can be done on a DMM far quicker, but not as much fun I agree!!
I don't have a graph/chart/table of Amps to volt drop, but this topic sparks the ideas so I might create one ... just so our tech's can have a guide as to good, questionable, excessive per range of amp loads.
While my proposed "table" will be "IMHO", I'll consider sharing it and getting comments from you guys, as the more robust it is the better. Don't hold your breath, but any upfront ideas welcome. See a draft at the end of the message (it's off the top of my head, so don't shoot, but please comment!) ... cable length/diameter clearly are a factor, as are the number of joints ... otherwise we could just use a default wire resistance table ...
In fact no draft, I'll think some more, but any comments on the idea welcome! While some might say it's pointless, this thread illustrates the broad range of interpretation possible, and need for advice/comparison.
HTH a little bit?!