Battery requirements

[Bowline]

I'm looking to fit a battery capable of starting my 9.8 tohatsu 4 stroke, and also powering my depth sounder. The recommendations in the tohatsu manual seem excessive (40 to 70 ah) and I'm not sure I want to pay the weight penalty. Does anyone know why such a large battery is recommended to start a tiny engine? Does the manual only quote for a different version with power tilt or something?

I would like to fit a yuasa u1 (30ah x 270 cc, sealed and maintenance free) which at less than 8kg is perfect. The battery is designed for ride on movers, ATVs etc, so should easily start a 200cc motor.

My motorbike (GSR 750) runs fine with a 10ah battery, am I missing something!?

[sparerib]

I would think the smaller battery would be fine, as you say it's only a 200cc motor, i used to have a Tiny battery on my ride on mower, can't think of the exact spec but the mower was 13hp !

[Paul]

My thoughts are that the OB manufacturer will over spec the required battery size to cover their arse and allow for repeated cranking over of the engine during failing to start conditions.
For example my inflation pump a Bravo states a 20ah draw but I operate it on a 7ah battery and it's no different than using my trucks 100ah battery.
I suspect a 20ah would be enough to do the job ........ you could always use your motorbike battery as a quick test to see how it spins over

[Bowline]

Thanks for your replies. I used a ride on with a Briggs and Stratton once and the battery on that was nowhere near that size. I have noticed since my post is the charging rate is pretty high at 6 amp. Like you say Paul if the manufacturer has allowed for failed starts, they must also have allowed for a fast charge rate. When I get a dry day I'll give it a go on the bike and see what happens.