Comparator circuit to detect working alternator

Question

I'm very new to electronics, but am trying to build a device that should be attached to a motorcycle (Arduino based sensor board) which should only draw power when the engine is running; specifically given the finicky nature of motorcycle batteries, and that my device will have an approximate two ampere draw.

I've been researching comparator circuits, but it seems as I want to avoid any draw on the battery when the engine isn't running.

I have self-imposed restrictions on specifically not interfacing with any of the bike's other circuitry, mostly because it's brand new, and I don't want to splice/hack into any wiring that might relate to the safety electronics, etc, etc; I can rely on a connection to the battery, and I'm hoping to detect 12v±1v when the motor isn't running, and closer to 14v when the alternator is doing it's job.

I heard a quip that I could use an opamp and a zener to somehow boost a 3v button cell to a suitable reference voltage to drive the comparator, that I might compare against 13v, for example.

The missing puzzle (in spite of lots of time spent with Google) is how can I use a rechargeable (I expect) button cell at 3v to compare against a 12-14(±1)v input.

Assuming that I detect the battery is currently charging from the alternator, I'll be stepping the voltage down to 5v using a LM2569, which is closer to the button cell's voltage, but doesn't help me to avoid drawing on the battery when the motor isn't running.

Answer 1

Use a zener diode to decrease the battery voltage so it comes in range of the inputs of your (rail to rail) opamp. Then compare that value with an (adjustable) reference voltage

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Simply use a relay exactly the same as a split charge system for an auxiliary battery on a vehicle.

Connect a "sensor" wire either to the sensor connection on the alternator (adding your ring crimp on top of the existing connection), or to the battery/alternator indicator light connection on the dash - which drives the coil of the relay. connect the contacts as you wish.

This gives zero current draw when inactive and doesn't require any other power source to function. Connection is by piggybacking a bolt on electrical connection so no modification to the existing wiring.

I have designed many things in the past where I have been bogged down in technical brilliance only to be blown out of the water by the most simple non-technical/low technical solution.

Remember - KISS - Keep It Simple Stupid!