Battery Voltage Monitor with Relays

Projects General Guidance
1

Hi everyone

I have taken over a project which consists of a rail vehicle which has an onboard data acquisition system powered by an alternator when moving and a battery when still. The conditions under which the system operates is very rough and it has become a continuous maintainance project for me. The system consists of:
2 12V 65 Ah batteries
Alternator
Data acquisition system
Main battery voltage monitor
Backup excitation circuit
Main battery excitation circuit
Router Lockout circuit

The main battery powers the data acquisition system and the router when the vehicle is stationary. When the main battery drops below 11V, a latching relay disconnects the main battery from the router and data acquisition system. This brings the backup circuit into play, where every 26 minutes it sends a pulse to the alternator in an attempt to excite it should it be moving. If the vehicle is moving the alternator fires up and charges the batteries as well as provide a 13.8V to the terminals which allows the router and data acquisition to come alive again. Additionally the router sometimes jams when it can't find any network. The router is then rebooted by sending a pulse to reset a relay via the data acquisition system.

Instead of having a voltage monitor I would like to use an Arduino with a voltage divider to detect when the battery drops below 11V and then send a pule to the latching relay which will cut out the data acquisition and the router. Then when the battery level rises above 12.2 V another pulse can be sent to the relay to latch it the other way.

To excite the alternator via the backup circuit, a timer will be set such that every 30 minutes a 5V pulse opens a NO relay that connects the backup battery to the alternator for 5seconds.

This will cut out all the hardware logic and less maintenance will need to be done and the maintenance time will decrease. Is such a thing possible with an arduino Uno and if so how do we protect the relays and microprocessor as well as prevent chatter, in general.

I am new to the Arduino world and find it very interesting. Any help or guidance would be much appreciated.

2

Interesting - yes this should be doable - I presume you have a harsh electro-magnetic environment so careful design
is needed to protect the logic side of things. I'd suggest that a suitable precaution might be to route all control
signals from the microcontroller via relays so that its isolated from the existing system.

Measuring the battery voltage
is going to need some sort of direct connection though, I'd suggest using a 3-resistor divider circuit where you measure
the voltage across the middle resistor by taking the difference of two analog readings. This means than any mismatch
in ground voltages is going to be compensated for. Arrange for the divider to generate approx 1.5V and 3.5V at the
two mid points, and add protection components (diodes to the GND and 5V pins, use high-value resistors - best to
arrange that even 100V spikes can't cause damage the Arduino ...

Firstly you'll need to get the details of existing relays and what voltage/current they take to power them, I'd suggest
posting a circuit diagram here that's as complete as possible.

Where there are long cable runs there's scope for interference and noise spikes being picked up too, so things like the
battery voltage monitor will need low-pass filtering too to cut out any noise.

Relay chatter is dealt with by adding hysteresis margins in the code (and / or delays).