That is a very good idea, I don’t really want to lose the possibility to use both batteries .
Car battery protection?
A simple switch is simple and cheap - but the downside is if you forget and leave them in parallel you will kill the battery again… especially if you have others using the vehicle. One time and you will wish you had automated it!
There are other more flexible options, including relay systems that have emergency start features…
Blue Sea Systems make really high quality stuff, Like this:
https://www.bluesea.com/products/7650/Add-A-Battery_Kit_-_120A
The ADD-A-BATTERY is as simple to operate as an ON-OFF switch, yet effectively manages the switching and charging of two batteries from a single charge source
The Add-A-Battery includes a Dual Circuit Plus™ Battery Switch and an Automatic Charging Relay. These components simplify switching and automate charging, so that all you have to do is turn the battery switch ON when you arrive and OFF when you leave.
Dual Circuit Plus™ Battery Switch
Simplifies switching
Isolates engine and house circuits
Combines batteries for emergency starting
And this - if you have a monster alternator:
https://www.bluesea.com/products/category/35/91/Automatic_Charging_Relays/ML-ACRs
500 Amp magnetic latching (bi-stable) relay automatically combines batteries during charging and isolates batteries when discharging and when starting engine.
Manual override knob provides an added level of safety allowing control with or without power and offering LOCKED OFF capablility for servicing
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That’s true. But no battery management system can predict the future, and making decisions on which battery to charge first is best done by a human in my opinion. But if there are other humans in the equation who are perhaps less reliable then an automatic system may be the way to go!
Sterling do some very good battery to battery chargers.
https://sterling-power.com/collections/battery-to-battery-chargers
The other option is a separate 240v inverter powering a battery charger. Might be a little inefficient, but on the positive side you have a 240v supply ‘for free’.
If you don’t go with the 100A switch, you can always use a jumper cable to start the vehicle from the leisure battery if you really need to.