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Author Topic:   Smaller Battery For Starting Smaller Engine
skred posted 04-23-2009 12:18 PM ET (US)   Profile for skred   Send Email to skred  
I've been speculating on battery requirements for my Boston Whaler SPORT 13 with 1997 Evinrude 30-HP. Since I have no other electrical requirements, save occasional navigation lighting, wouldn't a smaller battery, say a motorcycle battery, suffice for starting duties? If they are used to start those big Harley motorcycles (at significantly more hp and displacement), wouldn't that be adequate for a 30-HP two-cycle outboard?

The main idea here is to eliminate a large percentage of the battery size and weight being carried.

Dick E posted 04-23-2009 08:26 PM ET (US)     Profile for Dick E  Send Email to Dick E     
Check your manual for your outboard requirement of cold cranking amps (CCA). Get a battery that has least that rating for CCA.
jimh posted 04-24-2009 12:22 AM ET (US)     Profile for jimh  Send Email to jimh     
If you can pull-start your outboard motor, then using a more compact starting battery shouldn't be a problem--when it is a problem you can pull start.

Let's say the starter motor need 300-amperes to crank over the outboard motor. If you only have a battery that is rated for 300-amperes, it should be able to crank the motor, as long as:

--it is 100-percent charged

--the temperature is not too low

--the electrical system does not have one milli-ohm of extra resistance

--the engine doesn't have any extra resistance to moving

What I am saying is simple: as the battery size decreases, so does your safety margin. But, since you can always play your Ace in the hole--manual pull starting--you should be able to live closer to the edge. Thus you will be able to use a smaller battery. Instead of some 1,100-MCA brute, get something a bit more diminutive. Also, if you go to a premium battery in a VRLA AGM construction, you can get some additional capacity in smaller size cases than you may find in conventional flooded cell batteries.

Fishmore posted 04-24-2009 12:28 PM ET (US)     Profile for Fishmore    
With my 92 Yamaha 40hp motor 2cyl 2-stroke, I used a Sea Volt Deep Cycle AGM 32, 335 MCA and mounted it under the seat on it's side. It worked fine in that application. Because it was an AGM mounting it on it's side was not a problem and it kept it out of the way.
Dick E posted 04-24-2009 05:04 PM ET (US)     Profile for Dick E  Send Email to Dick E     
I know you have an Evinrude, but I believe the starting requirements for a 2 stroke Mercury outboard similar size should be similar.
Mercury says 465 MCA or 350 CCA for operation above 32 Deg F.for 30 hp 2 stroke.
Looking on the Internet motorcycle batteries have a CCA of 200 to about 300 CCA so I think you will be a bit low. You need a margin of safety I would stay with a group 24. php
skred posted 04-24-2009 11:36 PM ET (US)     Profile for skred  Send Email to skred     
Thanks for the replies... I may experiment with a Harley-sized battery, If it doesn't work out, I'll be carrying the group 24 battery as reserve.
davej14 posted 04-29-2009 04:40 PM ET (US)     Profile for davej14  Send Email to davej14     
I like Skred's original question. Why do marine engines specify such a large starting battery vs a higher HP motorcycle? It doesn't seem to be logical.
jimh posted 04-30-2009 09:39 AM ET (US)     Profile for jimh  Send Email to jimh     
Some modern outboard motors specify that their starting battery be rating at fantastically high amounts of cranking amperes because during engine starting the motors are very big consumers of electrical energy.

Let us look at a Mercury OptiMax motor, for example, as a motor that requires a huge starting battery. I believe the recommended battery is something like 1,000-MCA rating.

During starting the battery will be supplying all the electrical power for:

--the electrical starting motor
--the electrical fuel pump
--the electrical fuel injectors
--the electrical air injectors
--the engine spark ignition system
--the engine control system using microprocessor

The engine control system using microprocessors can be sensitive to variations in the supply voltage. So can other elements of the electrical system. When the engine is cranking, the battery voltage must not sag below a minimum voltage, or many of the electrical devices connected to it will not operate properly.

On some modern motors, once the motor starts the alternator in the motor can take over for some of the load. However, with some motors, there is not enough electrical energy generated at low engine speeds to even met the demand, and the battery may still be needed to stabilize the voltage and supply some of the current.

However, not all motors are like this. Many modern outboard motors can be run without any battery at all and can be pull started. These motors generate all the electrical energy they need to start and run themselves.

The main consumer of the electrical current in starting is the starting motor, but often a large battery is needed to prevent too much voltage sag. Older motors often will start with very low battery voltage, as long as they can be cranked over. Newer motors will often fail to start if the battery voltage sags.

davej14 posted 04-30-2009 01:24 PM ET (US)     Profile for davej14  Send Email to davej14     
Skred is only looking at starting a 30 HP 2 stroke. As he points out, this is much smaller than many motorcycle engines that would have the same ancillary equipment. I'll bet the motorcycle battery will work just fine.
Ridge Runner posted 04-30-2009 01:57 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
I wouldn't use a motorcycle battery. The boat battery is subject to a lot more vibration and bouncing. If you want a smaller battery I would suggest using a PWC battery, it will be constructed for marine use and should be able to take a lot of abuse. Most of the PWC batteries are 300 - 350CCA, and they start those "big" 120hp PWC engines.
TampaTom posted 05-04-2009 10:40 AM ET (US)     Profile for TampaTom  Send Email to TampaTom     
I bought Odyssey battery at Battery Plus (might be the PC 625). Paid about $100. Put it under the console of our 15' Sport and ran 4 gauge cables back to the 70 Johnson. Starts strong no problems in the last 9 months.

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