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I've just stuck a red flashing LED across the input to the power distribution box I'm building, after the circuit breaker. It proves the battery is connected the right way round and the circuit breaker has not tripped. These LEDs are rated 3-10v and 60mA at 25% duty cycle.
I've stuck a 1k1 resistor in series and with the 12.5V from a lead acid battery it appears to use about 3mA. That gives 12.5/0.003 ~4k2 total.
1100/4200*12.5 is 3.2V across the resistor and (4200-1100)/4200*12.5 or 9.3V across the LED.
Maybe I could use 2k1 which would make it 2.3mA and 7.5V across the LED.
Is my math right?
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