Hi, I have two batteries in the car, a starter battery and an AGM battery. The latter for the supply of devices and as a start backup. I want to pair them now with a battery coupler for various reasons, such as Cyrix-ct 12/24 V 120 A and 230 A. To charge all this is no problem, but how does it look at the start? Suppose the starter battery weakens or is completely dead, then I can manually bridge the coupler and take the second battery to help. e Cyrix devices can handle 180A start current (120A version) and 500A (230A version). But how do I know how much current my starter actually needs? The starter has 1.1kW, which is equivalent to a current of ~92A. But is that really all? I have read a lot here in the forum and in others about it, because one number is thrown around the head up to 450A. The whole thing must work even with clear minus degrees, where the starter will have to do safely. The question is, Is the 120A version, which can be about twice the start currentStrength of what the starter should theoretically need? Or is the 230A version with 500A start current necessary? It is also a question of money, the large version costs twice as much, but also a question of power consumption and size. Thank you for helpful hints! I would like to not discuss at this point the sense / nonsense of such a construction.