The 12v / 24v 120A Cyrix CT battery combiner from Victron Energy is a feature-packed intelligent split charge solution for charging your leisure battery from your alternator and ensuring you never accidentally flatten your starter battery. More commonly known as a voltage sensitive (or sensing) split charge relay, this unit will automatically connect your starter and leisure batteries when a charging voltage is sensed and will automatically disconnect them when it senses the voltage drop away when the engine is turned off. This isolates the batteries when they are not charging so your starter battery cannot be drained by your domestic circuits.
This relay features voltage trending software to overcome short-term fluctuations in voltage levels and prevent unwanted disconnection (e.g. when first connected to a leisure battery with very low charge and/or of a very high capacity, the resultant voltage drop across the starter battery can be significant and can trigger a false disconnect in some other voltage sensing relays - sometimes referred to as 'chatter' as the relay quickly opens and closes repeatedly).
Another useful function is the emergency start assist. If the starter battery has become flattened and you wish to use the leisure battery to start the vehicle, you can manually close the relay using a momentary switch wired to terminal 85 (relay remains closed for 30 seconds).
PLEASE NOTE: These relays are not suitable for vehicles with 'smart' or 'intelligent' alternators. As a guide this applies to vehicles having engines that comply with Euro5 emissions standards and onward, often also incorporating regenerative braking (if in doubt please consult your vehicle manufacturer to confirm). If your vehicle has a smart alternator then you will need to use a battery-to-battery charger instead of a VSR.
Features

12V or 24V operation (auto detection)
Voltage trending software
Dual sensing (also allows leisure to starter battery charging, e.g. when using a battery charger)
Over-current protection
Over-temperature protection (automatic re-engage once cooled)
Emergency start assist
Virtually zero voltage drop and low standby current draw
No alternator connection required
Sealed for use in harsh environments (marine use, vehicle engine bays etc.)