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Old 03-09-2019, 10:02 AM   #1
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battery for dinghy out board

I'm trying to make a little more room in the inflatable and would like to downsize the starting battery. I currently use an identical battery to the one I use to start my Onan 7.5MDJE. It's 900 miles away and I can't remember if it's a 24, 27 or larger. The motor is an 18hp Tohatsu w/elec. start. The manual calls for a group 24 which seems like overkill. I was wondering why one couldn't use a motorcycle battery or one like the one in my home gas genset. I'm looking at ones that have between 240-275 CCA. What do you all use and any other thoughts?

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Old 03-09-2019, 10:13 AM   #2
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150 hp motorcycles are started by 15 AH batteries. If you want to go really small and light, use an LFP designed for motorcycle starting.
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Old 03-09-2019, 10:27 AM   #3
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I got a small AGM "sports power" battery for our outboard. Deka ETX30, I think. The outboard is a Suzuki DF15A (15-hp, same basic platform as their current 9.9 and 20-hp models).

Started with the outboard manual, found minimum cranking amp recommendation, went from there. AGM to mitigate self-discharge over winters (I could periodically recharge, but have found that not necessary with AGMs).

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Old 03-09-2019, 11:06 AM   #4
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Check the charging volts with engine running at say 2000rpm, and with battery fully topped up prior to starting. See what voltage plateaus at. Some of these little motors have very crude voltage regulation (volts go over 14.5, maybe a good bit over) and a bigger batt can tolerate that better than a little one.
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Old 03-09-2019, 11:19 AM   #5
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Take a look at this Odyssey AGM battery. We used them in my job for mobile power needs. Small, dense and powerful.

roughly 10"x6"x4", 20 lbs with a 400 CCA rating.
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Old 03-10-2019, 10:23 AM   #6
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I bought a U1 size Lawn Tractor battery from Walmart for $20 4 years ago. Put it in a small battery box (available on Amazon) and its been starting our Tohatsu 9.8 great. Granted it is a standard Lead Acid battery, so it can't be tipped on its side, but if you aren't going to do that then its all you need.

https://www.walmart.com/ip/EverStart...U1R-7/49988409

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Old 03-10-2019, 11:39 AM   #7
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I'd pitch the starter motor and battery. save 60 lbs or so. But, that's just me.

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Old 03-10-2019, 12:05 PM   #8
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I use an Odyssey PC625. Great battery. Small, light, and stands up to years of abuse it gets on the floor of my dinghy.
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