Battery isolation switch for jump starting

Had a flat battery this morning, no biggie 2nd battery is there and jumper leads to hand. Not happy with the shorter than expected life from the optima yellow top though. Got me thinking as to whether a hard wired switch would be a better option in these cases. I have a single starter and the 2nd battery (in the engine bay) is charged via a ctek 250s. Question is are there any negatives with having say a cable (heavy) running positive to positive between the 2 batteries with an isolation switch similar to what you might see in a boat with dual batteries. I recall on the boat they suggest not switching between batteries while the ignition is on but I never had this mentioned in previous boats we have owned so I am not sure if there is a real issue of potential damage to anything. As always any ideas/recommendations are very much appreciated.

G'day Glen, I used the exact same method in the D21 with the battery in the tub with 10mm welding cable. The switch was a heavy duty knob that I got from a marine store. I had the ability to start from the rear battery and used it a few times when the lights were left on. There should be no problems with what you want to do as long as the cable is thick enough. I'd use the heavy cable for the earth too. The batteries should play well together mate.
Cheers

I had a switch at both ends as it was a long run in case anything went wrong. As there was no protection, you couldn't fit a circuit breaker or maxi fuse that would allow you to crank without tripping.

Thanks John will browse the local boat shops. As both batteries are earthed to the body/chassis do i need another earth and if so where to?

It's only a short cable run mate so it should be ok. I'm just a bit pedantic about earthing high currents though the body that's all.

As per John's advice.

What you CAN do is this: disconnect the DC-DC charger and simply connect the positive terminals to each other. Don't even bother with an expensive switch. The only time you're going to hook the two together like this is when you need the cranking power, so a heavy single jumper lead with the alligator clamp on each end will do fine. When not in use, wrap it around the battery and clamp the two clamps together.

There's no difference here from a normal jump start except you're only doing it with a battery, and not with a battery that has an alternator pumping juice on the other end of the cable.

The only thing to watch out for is the CCA rating of the second battery. It has to exceed 550CCA.

Thanks guys. 2nd battery is 660cca if i recall correctly and kicks it over just fine. I like switches as they tend to be more foolproof when others need to use them. What can i say, i was brought up with the Jetsons.
Do i need to disconnect the ctek Tony? Any risk of damage to it?

You're essentially going to be connecting the input and output of the C-Tek together then drawing a huge load off right in front of it.

Yes, disconnecting it isn't a bad idea.

A double-throw switch might not be a bad thing. These have three posts: "centre" which is either directed to the "on" post or the "off" post. Centre post to the aux battery, one post to the CTek and the other to the main battery. Can't make a mistake.

Relays could work too. I'd have to think up how to wire it up, and you'd want the aux battery supplying the juice for the relays (because we're assuming the cranker is dead).

Thanks again, sounds like a great option. I was concerned about someone leaving it switched the wrong way but your design solves that.