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ContinuousWave: Small Boat Electrical
Using Smaller Battery
|Author||Topic: Using Smaller Battery|
posted 06-17-2013 10:50 PM ET (US)
Is it possible to use a snow mobile battery as a starting battery for a 70-HP two-stroke-cycle outboard? At 8 x3 x 6-inches it would easily fit in the console I'm building for my 15-footer. I checked them out on the Internet, but none seem to list the cranking Amperes. I've seen small size batteries like this used in outboard race boats with electric starters.
posted 06-17-2013 11:55 PM ET (US)
A lot of us amateur sports car racers use Odyssey pc625's. They aren't the cheapest but they are 625 cca. They'll zing your motor with ease. But because they are small and light, they don't have the reserve capacity for minutes and minutes of cranking. I don't use the pc625 on my track car though. I use the pc1200. It's twice the size of the 625, but still smaller and lighter than a regular car battery. And with 1,200-CCA I can drive my car up a sloped driveway with cranking torque. The better to start a heat-soaked 7.0L v8 road race engine.
posted 06-18-2013 08:01 AM ET (US)
Borrow a clamp-on Ammeter. Measure the current used by your 70-HP outboard engine when starting. This will give you a basis to select a battery for starting.
posted 06-19-2013 01:09 PM ET (US)
Here is something I knew, but never considered a problem before. Marine batteries have wing nut screw studs to attach the cables and other wires to. Other batteries that I have seen do not. I think that is the only difference, although maybe not. I stopped at a battery distributor yesterday, and talk to the tech about a smaller battery for my boat. He told me that the lawn tractor and motorcycle type batterys only have about 400 cranking amps. He did have a smallish size car battery that put out 750 cranking amps and was 7" wide, 7" tall, and 9 inches long, but had auto type studs and you would have to use clamps. OK, that could be done, how about other wires running to the battery, such as wires from a fuse block? How would they attach? I guess I would have to build a custom box also.
posted 06-19-2013 03:41 PM ET (US)
Rich--it is not a good practice to attach more than one connector to a binding post, particularly in a boat where there is vibration that can loosen connections. Stacking up several ring terminals under one binding post is sometimes done, but you should avoid it, if you can. You can find some battery post connectors that have several multiple terminal posts on them.
Many outboard engine manufacturers specify very high current requirements for starting. I think their reasons are twofold:
--they want the engine to crank over rapidly to get good starting; I know that some Evinrude engines won't start unless the cranking hits a certain speed, about 300-RPM.
--they want to maintain a minimum voltage during cranking because the engine electronics won't operate if the voltage sags too much. This is a problem with some Mercury engines.
If your outboard is an older outboard, it probably won't have either of those concerns. As long as the battery can crank it over, it should be able to start. As long as you keep the battery near full charge--which should not be too hard--you can probably use a battery with only enough cranking Ampere rating to supply the actual current. For a 70-HP outboard, I think the cranking current might be less than 400-Amperes. I measured my automobile engine, a four-cylinder, and it only drew about 250-Amperes when cranking.
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