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 A3010 Pen Type Multimeter 4000 Counts Smart Dual Use NCV Voltage Sensing Backlight Digital
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
M18 8mm sensing DC 5V inductive proximity sensor switch NPN NO LJ18A3-8-Z/BX-5V work voltage 5VDC
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 B16 Lntelligent Sensing Digital Measuring Pen Non-Contact 12V-300V Voltage Detector NCV
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
AC12V-300V B16 Intelligent Sensing Digital Measuring Pen Highlight Light Aneng AC Voltage Detection
Aliexpress UK
The relay cuts in at 13.3V and breaks (cuts out) at 12.8V, allowing split charging of a second battery bank without discharging the primary or start battery. Maintains battery isolation when not charging, so starter cannot be drained by leisure use and vice-versa.
eBay
T-Mux DVR45-10KVA Relay Control 10KVA 10KW Adjustable Ac Automatic Voltage Stabilizer Regulator
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
Tuya WiFi Smart Power Meter Earth Leakage Protector Switch Wattmeter Current Voltage Monitor Relay
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
Dual Display 40A 63A 230V Din Rail Adjustable digital Over Under Voltage Relay Surge 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
Relay Switch Controller DC 6~80V Voltage Detection Charging Discharge Monitor with Case Digital
Aliexpress UK