On 26/04/2021 07:10, williamwright wrote:
On 24/04/2021 01:06, Fredxx wrote:

So effectively a larger battery. That is one solution, but an expensive
one. And some will still leave it long enough for them to go flat.


In this case only one of these batteries would be flat. As soon as the
ignition switch is turned the two batteries are connected. I dread to
think the magnitude of current flow in the first few seconds.

Any flattened lead acid battery is going to be toast, or severely
damaged.


I don't know why but I can tell you that that just doesn't happen. I
don't know why. Maybe when the starter load is present there just isn't
a rush of current between batteries.

If the relay can reliably cope with the starter current in the coldest
latitude then I suspect with the odd occasion when one battery is flat,
creating an inrush, the relay's going to survive the event.