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.
eBay
RMshebei 3P 100A Tuya WiFi Smart Bi-Directional Energy Power kWh Meter Leakage Over Under Voltage Protector Relay Switch Current
Aliexpress UK
Built-in LED indication (lights up when VSR starts). 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.
eBay
RMshebei tuya smart zigbee relay 220v wifi smart home switch circuit breaker 2P Automatic smart life 110v voltage protector 63A
Aliexpress UK
It simply connects between the two batteries. The relay is connected between the vehicles main battery (Positive) and the leisure battery (Positive). The most important part of the circuit are the fuses!
eBay
AC 110V 220V Timer Adjustable Disconnect Delay Controller 1-480Min Timer Power-ON Delay OFF Relay Switch 7A Voltage Output
Aliexpress UK
Simply connect the relay between your starter battery and leisure battery and the relay takes care of everything, making sure your starter battery is not drained for when you need to get going again.
eBay
KWS-303 Tuya WiFi Power Meter Smart Switch 8 in1 Energy Meter 85-290V AC Meter Timer Relay Voltage Current Electricity Meter
Aliexpress UK
The relay is connected between the vehicles main battery (Positive) and the leisure battery (Positive). It simply connects between the two batteries. The relay will suit all standard 12v conversions and in practice the 30 amp relay will be more than sufficient for charging a second battery from your vehicle battery.
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eWeLink Smart WiFi Power Energy Meter with CT Clamp AC 220V 110V Timer Relay Switch kWh Voltage Current Protector Remote Control
Aliexpress UK
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.
eBay
New Smart Dual Battery Isolator 12V 140A Waterproof Digital Voltage Sensitive Relay VSR Switch For Car RV Camper UTV Boat Marine
Aliexpress UK
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.
eBay
KWS-306 3 Phase TUYA WiFi Power Meter Over Under Voltage Protection Relay Switch Circuit Breaker Energy kWh Meter Smart Life
Aliexpress UK
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.
eBay
220V 230V Tuya WiFi Smart Circuit Breaker Leakage Over Under Voltage Protector Relay 2P 63A Power Energy kWh Meter Switch Timer
Aliexpress UK