When your engine is switched off, the relay will isolate your leisure battery(s) from your main starter battery. So it does not drain your main starter battery and you will always be able to start your vehicle.
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ALLPOWERS Foldable and Portable Solar Panel 100 / 200W Solar Battery Charger，Outdoor Emergency
Aliexpress UK
Renogy 100/200/300Ah 12V LiFePO4 Lithium Leisure Battery or Battery Monitor. Optimize Battery Health. Simple and compact layout for easy installation. Alternator running detection safeguards the starter battery.
eBay
ALLPOWERS Solar Powerbank 154Wh 41600mAh External Battery Charger Powerstation 200W Portable
Aliexpress UK
Battery Types AGM, Gel, Flooded, Lithium-iron Phosphate, User. Optimize Battery Health. Alternator running detection safeguards the starter battery. Get an Answer within 24 Hours. Warranty information: 2-year material warranty.
eBay
iCharger X12 1100W 12s 30A / S6 1100W 6S 40A Smart Battery Balance Charger Discharger For LiPo Lilo
Aliexpress UK
Renogy 30W 50W 100W 175W 200W 320W 640W Solar Panel 12V Mono Off Grid RV Caravan. Compatible with Renogy 12.8V Lithium Batteries. Self-Consumption: 50mA Current Protection: 250V 10A Fuse built in. Protection: Overcurrent Protection (Fuse 250V 10A), Short Circuit Protection Certifications: CE, ETL, FCC, TUV.
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For Bosch Battery Charger 3A 18V 14.4V Li-ion Battery Charger For BAT609 BAT609G BAT618 BAT618G
Aliexpress UK
With IP65 level waterproof. Note:The20A is the new Anderson connector design, 40A does not have the Anderson connector design. Provides proper charging for AGM/GEL, Lead acid, Lithium and Calcium batteries.
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Standard Battery Charger Opus BT-C3100 V2.2 Digital Intelligent 4 Slots AA/AAA LCD Battery Charger
Aliexpress UK
The Current is NOT taken from the input battery and given to the output battery. Manual mode and regenerative braking mode allows you to override this. Regenerative braking (requiring ignition feed).
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SKYRC T200 AC/DC Lipo Battery Dual Balance Charger Discharger for Traxass RC Car Battery Charger
Aliexpress UK
Renogy 30W 50W 100W 175W 200W 320W 640W Solar Panel 12V Mono Off Grid RV Caravan. Bluetooth Monitoring Ready! Backup Battery Charging Mode : MPPT Charging Efficiency : Up to 97%. Maximum Charging Current : 50A (25A Alternator / 25A PV).
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Beytvyow Super Battery Charger for Dewalt 20V Battery 6-Ports Simultaneous Rapid Charger Station
Aliexpress UK
Renogy 30W 50W 100W 175W 200W 320W 640W Solar Panel 12V Mono Off Grid RV Caravan. Bluetooth Monitoring Ready! Backup Battery Charging Mode : MPPT Charging Efficiency : Up to 97%. Maximum Charging Current : 30A.
eBay
Lead-acid battery 12V20A electric vehicle battery 6-DZM-20 natural energy battery, lead-acid battery
Aliexpress UK