[Zebster]

Between long journeys, my car gets left for lengthy periods and I get regular 'low battery charge' messages. Occasionally using a cheap charger suitable for EFB/AGM batteries (my battery is an EFB type) keeps everything happy!

The positive lead goes to battery +, but the negative lead goes to the engine block (or body) so that the charge flows via the monitoring block on the vehicle negative battery connection to keep the charge state monitoring system properly updated.