Years ago, I mean years and years ago, a common mistake in purchasing a marine battery (for a boat under 25’) was to chose a deep cycle battery because it was thought that it would hold a charge longer when out on the water.
The problem is that deep cycle batteries are engineered to deliver a steady flow of electricity over a long period of time. Supplying things like lights, radio or an electric frig that draw low constant amounts of electricity. They tend to not deliver the gross amount of amperage all at once compared to a standard ‘Marine Starting’ battery although technology has improved this issue.
The ‘Starting Battery’ is engineered to deliver lots of amperage all at once rather than a little at a time over a longer period. They could also be damaged if drawn down to zero power as opposed to a deep cycle that can usually handle that issue.
The ‘Dual Purpose’ battery in the video is a perfect example of what technology has provided. It can deliver a constant draw of power and yet maintain enough juice to crank over and start an engine. However if one uses too much of that battery there might not be enough left to start the boat.
My solution is to have 2 Dual Purpose batteries on my pontoon boat with a switch to select either battery or both. I always run on battery ‘1’ or ‘2’ alternating between boating days. That way I know both batteries are good because each one gets a chance to start the boat on its own. Plus each one gets charged separately fully on ‘its’ day.
In the past I’ve had 2 batteries charging at once only to find a bad battery took most of the charge leaving the good battery not being charged properly. Running each one separately provides constant proof that it is in good condition.