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
1PC Voltage Sensitive Relay Dual Battery System Digital Display VSR 12V 140A Split Charge For Camper RV Yacht Smart Isolator
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
1P+N WIFI Smart Energy Meter Timer Switch Relay TUYA Over Under Voltage Protector Current Limit Power PV Meter Leakage Setting
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 6~80V Voltage Detection Charging Discharge Monitor Relay Switch Controller with Case
Aliexpress UK
This ensures thats secondary bank can never drain the main bank. Direct alternative to the Durite 140A VSR Complete with E Mark approval. Not suitable for vehicles fitted with smart alternators Product Code: RY80.
eBay
High Voltage Detector ETCR‑1860C 50V‑6.6KV High Voltage Detector with Alarm Wrist Type High Voltage Alarm High Voltage Alarm
Aliexpress UK
The relay will switch on at 13.3volts and switch off at 12.65 volts and is indicated on with a red LED.
eBay
12V 200A Universal Battery Switch Relay Start 4 Pin Dual Battery Isolator Relay Current Start Car Relay
Aliexpress UK
160A Max rated. Max Current Load: 160A. Suitable for 12v Split Charging systems. Continuously monitors the charging rate of the alternator to make sure your leisure battery is maintained in optimum condition.
eBay
LCTC SSR Solid State Relay Single Phase Din Rail Mounted Slim Relay With Heat Sink 10A 25A 40A 50A 60A 80A Industrial Relay
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
ATDV-YC On Off Timer Relay On Off Timer Switch On Off Twin Timer Relay Knob Control Time Switch ATDV-YC 6S-60M Twin Timer 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
24V Excavator Battery Relay 08088‑30000 Electrode Relay Replacement for Komatsu PC200 PC60 PC120 PC300 PC400 Electrode Relay
Aliexpress UK