Modern Battery for Old Motor?

[dbuffington]

Hi Folks!

I'm reconfiguring the rear of my 1957 Whirlwind to hold a second fuel tank, and I'd like to replace the lead-acid beast back there with a smaller, safer, more modern battery. (Probably an Odyssey, which I've used with great success in motorcycles.)

The primary use for the battery is starting the motor -- a 1955 Mercury Mark 55E (40 hp, roughly 40 cubic inches).

My old Mercury manual specifies a 70 amp hour battery, but that sounds like WAY more than necessary. (For example, I use a 16 AH Odyssey to start 60+ cubic inch motorcycle engine.)

Am I running into a difference between 1950s and modern ratings? Or is there something I'm missing?

Thanks!
Dave

 

[Silvertip]

Re: Modern Battery for Old Motor?

Take it out of the bike and try it on the boat. Me thinks you will change it back to a good old school marine starting battery. But no! Electrons still behave the same way they did in the 50's. I was around then so I know this to be true. The difference between your bike and the boat is if the bike doesn't start you can thumb a ride. If the boat doesn't start because you have a whimpy battery, you either swim, paddle, drift, or call for a tow. Even airplanes use relatively small batteries. But one does not generally stop the engine and restart in-flight. If the engine doesn't start on the ground that's not a big deal. A cycle also has a very small electrical load whereas a boat typically has far more devices to power. Power Tilt & Trim is just one of them.

 

[dbuffington]

Re: Modern Battery for Old Motor?

Oh, I fully understand the desirability of reserve capacity. Let me put the question another way: What amp hour rating is needed to start the motor?

 

[Silvertip]

Re: Modern Battery for Old Motor?

You have an older Mercury that is known for needing a bit of crank time. You said Mercury recommends a 70 AHr battery so it would be fairly apparent that you need a 70 AHr battery. Mercury built the thing so they should know. Can you get by with less. Possibly! Can anyone guarantee you trouble free starts -- I doubt it. Outboards have a magneto ignition system and need to spin quicly to fire off. A whimpy battery won't do that. You seem determined to do what you want to do so why debate this. Try your bike battery and go from there.

 

[jhebert]

Re: Modern Battery for Old Motor?

Most likely the Odyssey battery is a lead-acid battery. Almost all batteries used with marine engine starting are lead-acid batteries. The Odyssey likely uses absorbent glass mat construction or AGM, and is a valve-regulated lead-acid battery, or VRLA

I don't recommend using an VRLA AGM battery with an older outboard motor due to the poor voltage regulation of the battery charging circuits often found in those motors. An AGM battery should not be charged to more than 14.5-volts. Older outboard motors often try to charge a battery to 16-volts. If you keep pushing high voltage on an AGM battery there is a good chance you will force venting from the battery as the electrolyte is turned into gas. Once the gas escapes it cannot be replaced. This will shorten the battery life.

A better choice for use with older outboard motors with poorly regulated charging systems is a flooded-cell lead-acid battery where you can easily add water to the electrolyte. With high charging voltage, you may have to add water frequently.