Voltage variation

Thinking more about Jay's undervoltage issue...

I can't see why he can't do all the testing with the engine off. That's
LOTS safer. Yes, he'll drain the battery slightly but we're talking only
a few minutes here.

He'll need a DVM. In my experience, even really cheap [as in $5 at Harbor
Fright] DVMs seem to be surprizingly close. (But you can always test it.
Ask your avionics shop to compare to their lab meter at 13v and see.)


Then make a simple schematic:

JPI

1-----------2breaker3---bus4----[master contactor]5-------6Battery+

and measure two ways; from each point to a known clean ground, and
from one to another, such as 2-3, 3-5 etc.

Each technique has its own advantages and disadvantages. "To ground"
will be hard to see small drops; 'across' is twice as many good
connections to make in tight places.

But what you are looking for is: How much does the voltage drop from the
battery along the way? With no/little load; the answer should be 0, no
drop, even if there are bad connections. Under load, the voltage drop
will be more visible.

So the first thing needed is this: What is the voltage at 6, the battery
post? And 1, where the other device connects? If the JPI (and the master
relay) are the only thing on, and it's more than one volt of drop; I'd look
deeper.

If the battery is much less than 13.8v, with the master off; I'd wonder how
it cranks the plane.

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