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
ANENG B16 Voltage Tester Pen 12V-300V Non-Contact Induction Detector NCV Voltage Sensing Test Meter
Aliexpress UK
Dual Battery Isolators also feature surge protection and a L.E.D indicator designed to alert the user when the unit is engaged. - Suitable for most 12V charging systems- Surge protection. When your engine is switched off, the relay will isolate your leisure battery(s) from your main starter battery.
eBay
ANENG M114 600V DC/AC Voltage Multimeter Smart 4000 Counts Digital Resistance NCV Voltage Sensing
Aliexpress UK
Surge protection: built-in. The shell is made of premium ABS, anti-impact, strong and durable. Application: most 12V 4WD marine, truck, caravan, SUV ATV UTV, etc. A perfect fit under the hood to get close to your dual batteries.
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B16 Intelligent Sensing Digital Measuring Pen Non-Contact 30V-1000V Voltage Detector NCV Voltage
Aliexpress UK
It simply connects between the two batteries. The relay is connected between the vehicles main battery (Positive) and the leisure battery (Positive). Permanent live output (a live feed via relay from main battery + generally never used).
eBay
ANENG Digital Lntelligent Sensing Measuring Pen 2V-300V Voltage Detector Non-Contact NCV Voltage
Aliexpress UK
140A Max rated at 14 volts. Max Current Load: 140A. Relay Closes (Makes) at 13.3v & Opens (Breaks) at 12.8v. Three simple connections - Primary Battery, Auxiliary Battery & Earth. Continuously monitors the charging rate of the alternator to make sure your leisure battery is maintained in optimum condition.
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Waterproof Automotive Relay 4 Pin 5 Pin 30A Auto Relay With Fuse Coil Voltage DC 12V/24V Relay
Aliexpress UK
140A Max rated at 28 volts. Max Current Load: 140A. Relay Closes (Makes) at 26.6v & Opens (Breaks) at 25.6v. Three simple connections - Primary Battery, Auxiliary Battery & Earth. Continuously monitors the charging rate of the alternator to make sure your leisure battery is maintained in optimum condition.
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LANBOO J13A Micro voltage plug-in relay 5pin 8pin 12vdc 12A relay AC 220v DC 24V universal relay
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
TOMZN TOVPD3-63VA Din Rail 3 Phase Adjustable Over and Under Voltage Protector relay with current
Aliexpress UK
Suitable for most 12V charging systems- Surge protection. Dual Battery Isolators also feature surge protection and a L.E.D indicator designed to alert the user when the unit is engaged. When your engine is switched off, the relay will isolate your leisure battery(s) from your main starter battery.
eBay
DC 9V 12V 24V Voltage Detection Relay Module Circuit Monitoring Module Overvoltage Protection
Aliexpress UK