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 Arrival Three Phase Photovoltatic Circuit Breaker WIFI 100A 380V AC Power Current Energy Meter KWh Voltage Relay Protector
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
3 Phase WiFi Smart Circuit Breaker Bidirectional Energy Power kWh Meter Over Under Voltage Protector Relay Switch Remote Control
Aliexpress UK
20A Max continuous current. Automatically monitors the tow vehicle battery. Dual input and output, for charging the caravan battery and powering the fridge. A professional automatic split charging relay.
eBay
Tuya WiFi Smart Energy Meter Power kWh Meter Earth Leakage Over Under Voltage Protector Time Relay Device Switch Circuit Breaker
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
TOMZN 63A 1P+N WIFI Smart Switch Energy Meter Kwh Monitoring Timer Relay MCB TUYA Smartlife Voltage current Protection
Aliexpress UK
(via a fuse!). and link to the leisure battery. Fully epoxy sealed unit & stainless steel fittings for years of trouble free service. Bi-Directional, zero loss Split Charging. Only 3 Connections to make.
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Tuya WiFi Smart Energy Meter Power kWh Metering Earth Leakage Over Under Voltage Protector Timer Relay Device Switch Breaker
Aliexpress UK
The relay will switch on at 13.3volts and switch off at 12.65volts and is indicated on with a red LED.
eBay
New Household Digital Pid Temperature Controller Thermostat For Heating Cooling Instrument Thermocontrol Relay/Voltage Output
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
Backlights LCD Display Din Rail Three Phase AC 380V 80A Voltage Surge Protector Relay with Limit Current Protection
Aliexpress UK
【EASY TO INSTALL&USE】 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
Tuya Smart WiFi Energy Meter Power kWh Meter Earth Leakage Over Under Voltage Protector Time Relay Device Switch Circuit Breaker
Aliexpress UK