Hi Carl,

When you say you would prefer to monitor the battery current, I'm not
sure what that means. How will that tell you when the battery is low?
I would be concerned about the ability to measure currents less than 1
Amp accurately under the hood. I think I have seen currents of 10s of
mA flowing between different parts of the car chassis due to
thermocouple effects after things get hot. In other words, I've seen
current flowing into the negative battery terminal from one lead and
then OUT of the same battery terminal on another lead. Unless you can
measure the total current of all of the leads going to one terminal of
the battery, you may see spurious ground current readings because of
small to moderate currents caused by differences in temperature in the
car.

Sean


On Sun, Jul 22, 2012 at 2:10 PM, Carl Denk <cdenk@windstream.net> wrote:
> Recently the wife had a dead battery with our 2002 Mercury Cougar. There
> was no illuminated warning light. Checking the cars official service
> manual, I find the following paragraph:
>
> " If there is an open circuit in the regulator (built in) or generator
> rotor circuits, the warning lamp will not illuminate"
>
> Per the owner's manual, supposedly the charging system warning is
> supposed to warn of low voltage, but wasn't on with engine running (jump
> start) and 10 volt battery voltage.
>
> In this case the fault was worn rotor slip rings in the generator
> (that's what they call it, it's an alternator).
>
> I'm thinking of adding a low voltage warning LED. Space is extremely
> limited, and yet keep the installation neat looking and visible to the
> driver, but there is an unused module between the traction control and
> rear defroster switches that an LED could be mounted. Otherwise I would
> prefer a 3 digit display of voltage.
>
> To control the LED thinking of a 12F PIC, need one ADC to monitor the
> battery (system) voltage, and one output to control the LED, including a
> flashing mode. The Pic would preferably have a built oscillator, then
> there would be voltage regulator, a couple of caps, and a few resistors.
> I still am through the hole, and not into surface mount. I do have a
> PICKIT 2 for programming, and usually use 18F13220 for my projects.
> Haven't tried to look yet for the exact PIC yet, but will after I settle
> on some of the issues here.
>
> I would have preferred monitoring the amperage to/from the battery, but
> that cable goes directly from the battery to the starter in an
> inaccessible area, and then to the vehicle system. Adding a shunt in
> that area is not feasible, though a hall effect or other device is
> possible, then if the battery is discharging for more than a short
> period,  set the warning.
>
> Thanking ahead for any comments. :)
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