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I was looking at my manual for my 20hp Nissan/Hatsu 4s tiller it recommends a 70AH, 800 Marine Cranking Amps/650 Cold Cranking Amps battery. This sounds excessive for such a small motor and I see allot of you guys running the small AGM's like the Odyssey's that don't come anywhere close to these numbers.
Do they do ok? I will be using it for starting, lights and light electronics only. I'm looking to go small and light if I can.
Thanks
 

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send an email to Odyssey Batteries, I use the 925 for my 60 yammy 2 stroke, 650 cc sounds like a lot for that motor
 

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On the 4 stroke, a lot of times the higher required amp rating is because the computer demands the power to run the engine. One downfall to the more modern engine is that they need a constant battery source in order for their computers to run all the sensors, etc. not 100%, but I know a lot of motors do not generate enough juice at idle/low speed and actually feed off the battery. Of coarse once they get into the higher rpm range they generate an excess which in turn charges the battery.
My "real world"experiences with this have been on 150 optimaxs, new 150 merc 4 strokes, yamaha 225 four strokes, 60 hp merc efi 4 stroke, etc.
 

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On the 4 stroke, a lot of times the higher required amp rating is because the computer demands the power to run the engine.  One downfall to the more modern engine is that they need a constant battery source in order for their computers to run all the sensors, etc.  not 100%, but I know a lot of motors do not generate enough juice at idle/low speed and actually feed off the battery. Of coarse once they get into the higher rpm range they generate an excess which in turn charges the battery.
   My "real world"experiences with this have been on 150 optimaxs, new 150 merc 4 strokes, yamaha 225 four strokes, 60 hp merc efi 4 stroke, etc.   
Winner winner chicken dinner
 

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If its got carburetors then you can get away with no battery, if its fuel injected your gonna need a good battery
 
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