Replacing 1 House and 1 Starter with 2 House and a Lithium Ion Starter

[and4ew]

I have a traditional setup of one house and one starter battery. Both batteries need to be replaced. I am considering replacing them with two deep cycle and having one of those tiny Lithium Ion starter batteries as a backup. My logic is this:
- in identical conditions, two house batteries in parallel will last longer than one - I'm planning for discharge to be no more than 50%
- my engine is a Yanmar 2gm20f and the deep cycle batteries could each provide sufficient cranking amps to start it
- Lithium Ion emergency starter batteries are very small (the size of a couple of decks of cards) and cheap e.g. NOCO GB40, can be connected in a few seconds, and retain their charge for a long time without maintenance.

Anyone done this or have comments?

[glenn.225]

Done right, yes.


What type of deep cycle do you propose?
How are they to be charged?
How is Starter battery to be charged?


I you haven't, take a look at: https://marinehowto.com/1-2-both-bat...onsiderations/
And the other articles on his site, Rod has a ton of good info posted on his site.

Also:https://c34.org/bbs/index.php/topic,5977.0.html


Good luck with the project. I have to look at new batteries as well, mine are going on their 9th season.

[and4ew]

Hi Glenn,
I'm planning to put in lead acid group 27M. Charging would normally be from the 55A alternator. I will be adding solar when I move back closer to my boat (currently 1,000 miles inland). The Li Ion is so small I can take it home occasionally to make sure it's topped up.

My batteries arent as old as yours but they were left in the boat for two years without maintenance due to Covid travel restrictions.

Thanks for the links.