Dedicated Battery for Electric Start Outboard

[scrawley]

Looking for advice on a dedicated starter battery for my 9.8 Tohatsu. I currently use a single battery for the "house" and to start the outboard. I would like to buy and install a "smaller" dedicated starter battery. Perhaps an atv battery... Be nice to have smaller footprint and lighter weight. Any suggestions?
Steve

[NiceAft]

I didn’t read this to the end, so I hope it helps.
https://www.boatus.com/expert-advice/ex ... ne-battery

[Be Free]

You don't mention which version of the 9.8 you have but I can't imagine there is much difference in the battery recommendations between them. This is from the MFS9.8B manual: "Recommended battery: 12V 40Ah/5HR, 350 (Cold Cranking Amps (CCA), In case of cold whether: 12V 70Ah/5HR (650CCA))". The typos and odd use of parenthesis are in the original.

Translation: they recommend 350 CCA minimum unless you will be using it in cold weather in which case they recommend a 650 CCA battery.

Any 12V battery that meets or exceeds the minimum CCA will work. The form factor does not matter to the engine. Group 21 is the smallest "automotive" style battery I'm aware of. It's about 2" narrower than what you are probably using now (group 24).

Make sure the battery post style matches your battery cables (ATV batteries usually won't).

The alternator will probably work with FLA or AGM batteries. It may not work with LiFePO (lithium).

Don't forget to allow room for a battery box and tie-downs.

Don't forget to allow for any changes in your charging setup as well.

[wakataka]

There's a device called a combiner that makes this easy. This is the one I've been using for 3 years and it's been trouble free. https://www.yandina.com/c100Info.htm
It combines the batteries when charging, and routes the majority of the charging current to the most depleted battery (usually the house battery). The batteries are separated when discharging.

I've got my system wired with a "1,2,Both" selector switch so I bring both batteries into the circuit if needed. I think you can use the combiner without a selector switch if you never need to use both batteries at once. Here's a wiring diagram from the Yandina manual.

[scrawley]

Thanks all. I like the idea of the "combiner" for a future project.
I'm aware of the odd battery post styles on the smaller batteries and I'm searching for a suitable setup... possibly using an adapter of some sort.
I'll repost when I find the solution.
Steve

[Be Free]

Whatever you decide on in the way of an adapter, make sure it is tight. Even a 9.8 hp engine will draw a significant current while starting. Loose connections get hot. Not something you want on a boat.

[Russ]

Another simpler option might be to leave your single house battery and get a jumper pack in case it runs down to start the motor.

I have one of these onboard and in each of my cars. I once started my truck with it when the temp was 5F outside.