Just fitted this little bit of kit to my 88". You fit it directly to the + & - terminals on your battery.
Download the Battery-Guard to your phone (iOS or android) from Google or the AppStore. Open the App and it will pair with the sender you have attached to the battery. It will show you the condition of the charge in your battery.
It displays the voltage as an analogue readout and digitally, you can select 6V, 12V or 24V the display can be personalised to you vehicle (eg. reg No., name etc) you can even put in a pic the your vehicle. the app will read and store the records for up to 5 batteries. Should your battery charge fall to the point of needing a re-charge, an alert will be sent to your phone. The app also keeps a record in graph form so you can see the performance of your battery, over time. The connection is via Bluetooth and has a range of about 5 metres
So, what's the catch? Well I can't see one, yet. the unit costs £35 (available from the usual suspects) dead easy to use, you can even see the state of charge when the engine is running, so you can check that the alternator is doing it's job