"tdacon" wrote in message
...
I know this isn't a metalworking topic, but you guys have a lot of
collective knowledge. I'm installing a refrigeration system in my
boat. It's a holding plate system, powered by 12V DC, turning a motor
that draws about 25 amps when it's running. It will probably run
about an hour or so a day, at unpredictable times. I'd like to be
able to measure how many hours in a day it runs, without having to
sit there and time it when it turns on and off. How would you go
about that?

Thanks,
Tom


This meter, shunt and a laptop can log the power:
http://www.amazon.com/dp/B000OPDFLM/..._26725410_item

http://www.amazon.com/Current-Resist...rent+shunt+75A
Any large enough shunt would do, that one reads directly at 1mV per
Amp.

You could also record the battery voltage with a second meter. I use a
Cardbus dual COM port card to read two more meters but AFAIK they work
on USB / COM adapters. Their separate log files can be combined in a
spreadsheet by aligning the time stamps if you start the second one
just after you see the first take a reading. The meter grounds are
optically isolated from the COM port and won't short-circuit the
battery through the laptop, but you should double-check that in case
of manufacturing flaws.

The laptop might cost you 2 to 3A for its Auto-Air adapter or 12-120V
inverter. The old one I use drops to around 10~12W between readings
with the screen and hard drive off. A Pentium II running Windows 2000
is good enough.

This will display the total Amp-hours, like a DC Kill-A-Watt:
http://www.amazon.com/dp/B00EYZS6R6/...332200_TE_item

It reads only (+) current source-to-load so don't hard wire it in. I
use 45A Andersons. The actual current resolution is 0.1A, not the
0.01A it displays. The shunt and DVM can measure and record both
charge and discharge. If you hard wire the shunt the safer place for
it is in the ground lead, so the dangling meter leads to it won't have
live +12V on them.

jsw