low voltage warning
 

I was just ejumicating myself on comparators and found this:

Motorcycle Low Voltage Warning, by RP.

good starting point anyway

...got a weak battery or something?

No, when you spend a lot of time with your engine off but still have battery drains, or have done an alternater cut-out or other similiar eco-mod, it can be good to have some feedback when it is time to think about how to get the alternator charging again.


For Engine off operation, it would ideally go off while there is sufficient power left to start the engine "reliably", especially at a stoplight with some lights on but the engine off, or when you have some long eoc coasts.

For alternator cut-out, it would probably also be good to know when the battery is getting marginal for a restart.

Other applications, use your imagination.

...most 12V batteries poop-out at about 9V at being able to crank & start.

...a simple analog voltmeter energized through a momentary-contact push button would get the job done.

Yes a plain old voltmeter (perhaps not enough drain there to worry about a pushbutton) can also be used in such a situation, but it won't sound an alarm. A comparator and a piezo can, so you don't have to keep a close eye on the volt gauge.

so you calibrate this for 9.5 volts and when it beeps you need to restart, or apply power to the alternator field, or take some other corrective action.