I think it's been said already, but to avoid any misunderstanding:
Those gadgets pour charge out of the on-board battery into your starting battery. It won't work if the dead battery you've hooked up to is very much larger than the on-board battery. They are designed to pump up a single
car's starting battery, not a huge bank of deep-cycle batteries on a boat.
That's one problem. The second problem may be that the amps are in any case not enough to start a diesel
The other sensible comment was that you really, really want a starting battery which is separated from your domestic batteries. There are about a thousand ways you can accidentally discharge your domestic batteries. You don't want to end up dead in the water
because someone accidentally left the light on overnight in your heads compartment, just to name one possible example.
You can separate your start battery from your domestic batteries with an echo charger. A far better method is to have an entirely separate alternator
. This can be achieved by adding a heavy-duty alternator to charge your domestic batteries, leaving the standard alternator to deal only with your starting battery.
I have separate alternators as part of the original spec of my boat. The main engine start battery lives next to the generator
start battery. The main engine start battery has not only a separate alternator charging
it, but also a separate battery charger. I can monitor
its state of health
from the nav table - it has a separate voltage meter.
I keep jumper cables
on board just in case something goes wrong. I can then jump it from the generator
But if you keep your engine start battery entirely isolated from any electrical equipment
other than your starter, you are very unlikely to have any trouble with it. It will never get more than 2% discharged and will always be fully charged. And that's exactly what you want, because starting your main engine can be a matter of life and death in some circumstances.