12V Battery Monitor

The objective of this project is to monitor the state of the batteries used in our 12V UPS. This is because the UPS itself has no voltage monitoring and will simply keep trying to supply the connected equipment until the batteries are well and truly dead.

We are now using our Ethernet I/O board to monitor the battery voltage as it has analogue voltage inputs. These can only accept voltages up to 5Vdc though, so we are using a 5K potentiometer (the input resitance needs to be kept low for better accuracy) as a voltage divider and we have set this up to divide the input voltage by 4. To furthe rprotect the input we have also used a 4.3V zener diode across the analogue input, which clamps the input voltage below 5Vdc (there is a 0.7V forward voltage across teh diode). We have used the BZX79C (Maplin part no. QH05).

We took this approach because the raw battery voltage is already routed to this shelf in our Home Control System (HCS), to be used to power the alarms and other components.


It is pretty much essential that you have some view of the battery status or you will simply drain it flat and damage the battery.

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