It does this by taking a small current from the leisure battery. Battery guard cut-in:Starter battery below 11.5V. Battery guard cut-out:Leisure battery at 11.7V. Suitable for battery types:All types of lead-acid battery inc. wet, AGM, Gel.
Ablemail AMT12-2-PRO 12V Battery Maintainer - Suitable for Lithium Batteries
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Bosch 12V System Inspection Camera GIC 120 C (12V battery + charger) in l boxx