Low drain battery monitor

Other Hardware General Electronics
1

I am building a system to monitor a remote facility. Normally, the remote system will be powered by a 12 v. battery attached to a charger. If the mains power fails, the battery voltage will descend, of course, and I am designing in a voltage sensor to shut a relay off at about 11.5 v. Unfortunately, once the relay shuts off, the monitoring system (which is powered beyond the relay) is disabled, so if the main power comes back on, and the battery charges back up, I have no means to know it or switch back on.

If I design a monitoring system before the relay, then it is running off what is left of battery power, so in an extended outage, it may drain the battery to a damaging level.

So my question is, does anybody know of a very low drain (like a few microamps) battery monitor (or circuit) which could be set to monitor the battery voltage and turn it back on when a preset level is reached? Thanks in advance for any recommendations.

2

Check out jeelib.h
You could easily write the sketch to switch to a low power mode when the supply drops below mains voltage, conserving battery.

I've never used it myself, but will when I have the need.

3

Your monitoring system only needs to check the voltage ever few minutes, perhaps every 5 minutes. Then you could place it before the relay.

Paul

4

Or use a micro-power Op-Amp, like an LT1006, it consumes only 340 micro-amps ...
LT1006

Use circuit similar to below (swap out the LED, add in your relay & diode)

Circuit consumes less than 1 milliamp when the relay is off.

Also, add one more resistor for Positive Feedback Hysteresis to stop relay chatter.

5

No it doesn't, the '741 alone consumes more than that, upto 2.8mA worst case part...

Use a comparator to compare, not an opamp, you'll find comparators that can draw a few microamps,
but even the humble 339 performs way better than 1mA.

Use voltage divider resistors of a few megohms and add 1nF (plastic film) caps to prevent noise
pickup. And you must have hysteresis in such a circuit!

6

Righto on the hysteresis: I can see the relay turned into a buzzer.

7

MarkT:
No it doesn't, the '741 alone consumes more than that, upto 2.8mA worst case part...

Use a comparator to compare, not an opamp, you'll find comparators that can draw a few microamps,
but even the humble 339 performs way better than 1mA.

Use voltage divider resistors of a few megohms and add 1nF (plastic film) caps to prevent noise
pickup. And you must have hysteresis in such a circuit!

Yes it does.
You failed to read the line that clearly stated:
Use a micro-power Op-Amp, like an LT1006, it consumes only 340 micro-amps.