Quicklynks Battery Monitor II
Monitor your car's 12-volt battery
I'd recommend this to everyone with a car!
See it on Amazon UK
See it on Amazon USA
I think my 12-volt battery is kaput so this device was recommended to me so I could log the voltage and see what was going on. So easy to set-up and use with a surprisingly good app!
These are affiliate links so I'll get a little money if you buy using them. Thank you!