The battery current sensor is a serviceable component that is connected to the battery cables. The battery current sensor is a three-wire hall effect current sensor. The battery current sensor monitors the battery current. It directly inputs to the BCM. It creates a 5V pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100%. Normal duty cycle is between 5 to 95%. Between 0-5% and 95-100% are for diagnostic purposes.
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Another:
**broken link removed**
I can't find any decent information about the sensor pinout. It's "probably" +5 from the BCM, ground and an open collector output.
Something like
PWM to DC (design):
http://www.egr.msu.edu/classes/ece480/capstone/spring15/group10/Application Notes/Kyle.pdf
can hopefully convert the output to something useable for your design.
You need something really rugged.
Then there is these
http://www.pololu.com/category/118/current-sensors guys, but they are likely not going to be reliab;e/
You can look at either the output of the alternator or the input/output of the battery.
Again, ideas.
Maybe a buck-boost converter, an ACS sensor and a undervoltage lockout looking at just alternator output. You can also create a charging indicator too or a real ammeter feed.
I'd hate to have to add a 2.5V to 15V buck boost regulator for %V out to your design.
http://www.analog.com/en/products/ltc3111.html#product-overview
Shutdown current is 1uA, so you could be OK there too.
Sorry, been pre-occupied with stuff like GPS stolen when car was in the shop and my credit card was used in Nigeria.