Deep Cycle Battery Charging From Alternator

MEXICOWANDERER wrote:
A standard starter motor coupled to an engine will draw a battery down to 10.9 - 11.4 volts. About 4 - 5 times the amperage of a lawnmower starter coupled to nothing.

An RV starter armature coupled to nothing like the lawnmower armature in the video draws a fraction of what an armature does in real life.

He could have jammed a piece of wood into the starter drive gear to simulate a more realistic test.

Lock rotor's duration 1-2 seconds would have blown his numbers right out the window :) Touching the input lead would have yielded 90 - 125 volts not 25.

All I have ever measured is starters, windlasses, winches, fridge clutches etc. under load. This was the first time I bolted a starter to the bench unloaded. It was interesting to see the in-rush not change at all (in-rush happens before the engine has begun to turn) and the unloaded average current draw of 100A on a .8kW starter.

I've not noted transients any differently on boats while actually starting an engine and releasing the start button than I have on the test bench.. The brushes/motor do create noise on the scope, heck even a stable unloaded battery has noise, but I've not noted potentially damaging transients that approach that of the field collapse of a solenoid.

The transient duration can be captured on my o-scope but after talking with the folks at Fluke, Tektronix (my old scope was a Tek), Littelfuse (they make TVS diodes), Garmin, Raymarine etc.etc. they were not concerned with any duration shorter than what the Fluke 289 can capture transients at, which is 0.00025 seconds. I have successfully used my Fluke 289 to fight on behalf of customers for warranty denial too. I was never asked to test it with my scope once they knew I had been using the 289. Any time I come across a random death of electronics the first thing I do is test the boat for transients. It is rare that I seen in excess of 14-16V when the system is well wired but not extremely rare to poke into the low 20's when folks have haphazardly tapped a plotter or sounder or VHF etc. into the engine block for DC negative or engine circuit for DC positive.

I have only had a few boats where I had to install TVS diodes and these were very large solenoids on large windlasses causing the issue. Even then it was about 26V on a 12V system. Not a deal killer but worth throwing a TVS diode on..