Booster packs, anyone?

[Vronsky]

My boat has one 100Ah semi traction battery, that also serves are a starter battery for my DF115A (Jeanneau claims this is fine: different topic)
I'm thinking of keeping a booster pack on board as back up, just in case the battery dies when out on the water and have to restart.

Does anyone carry one of these on board as well?
What capacity/power would I need for my 115A

BIG THANKS,
V.

[Hoosmatroos]

I have two batteries on board. When cold I switch both batteries parallel and start. When I'm on anchor then I switch it to the house hold battery. Most of the time not really necessary but always a spare to use. Look in the manual to see what battery you need minimal. A little bigger is always possible.