Detect the state On/Off a DC 12V single coil latch relay

Question

A LiFePo4 charging/discharging circuit (12 V, 200 Ah, 100 A max) is protected by a single coil latching relay with manual override. This relay is controlled by an ATMega328 (through a FET with flyback diode) but does not have state contacts. For safety and operational reasons, I would like to detect the open/close state of this relay and feed information back to the micro controller.

I have been so far considering measuring the differential voltage accross the contactor (<1 milliohm). This is somewhat simple but may fail if charging voltage equals the LiFePO4 battery (quite unlikely as CV/CC chargers will drive to ~14.4 V in absence of load). Contactor would be declared closed if voltage difference is less than 100 mV for some amount of time (~250 ms).

I am not certain of the type of differential amplifier and protections to use in these conditions:

• differential voltage range will be within -20 V/+20 V
• 50 V transients (+/-50 V) may exist during switching and load dumps

I would be very grateful if anyone could comment and criticize the approach and maybe advice on the kind of differential amplifier to select.

Best regards,

Edit:

• Manual override : the relay has an external push button to On/Off manually
• Schematic

Answer 1

It's not clear why you don't just monitor the coil voltage. It seems as though you do not trust the relay. 100 mV across a contact seems high.

CV/CC chargers will drive to ~14.4V in absence of load.

That seems to me to be your solution. Use a comparator set to 14 V to monitor the PSU voltage. If it's over 14 V then you've an open circuit. If it's less then you're connected.