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  Batteries and Cranking Amps

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Author Topic:   Batteries and Cranking Amps
Yiddil posted 04-19-2008 09:54 AM ET (US)   Profile for Yiddil   Send Email to Yiddil  
I know [the Mercury Verado outboard motors] need higher cranking amps--I heard 1000. Do the standard outboard motors two-cycle outboard motors need [1,000 cranking amperes]? Is [use of a battery that can supply 1,000-amperes] [better] [than using some unspecified battery] or not? Can't rmember what my classic 115 Mercury says you should use; I don't have the book where I am right now. I thought I would ask the brain trust.

To recap, do E-TEC motors, Mercury two-cycle motors including the 115 classic 2004, need 1,000 cranking amp batteries? What battery originally came with them? What is the best [battery] to replace [the battery they originally came with] with?

My [battery] is fine for now, just wondering out in the open, as there are so many batteries to choose from.

Thanks, Henry

glen e posted 04-19-2008 10:25 AM ET (US)     Profile for glen e  Send Email to glen e     
E-TEC motors are the king in this areaa, needing only like 650 or thereabouts. They are very elelctrically independent. Virtually every other motor out there needs about 800 MCA's except Mercury motors which need 1000-MCA. Optimax due to the computers and compressor, and Verado due to their ancillary systems

I'm sure some of the above is a bit off and I'm sure many will tell me so very soon here, but the above is a good approximate answer.

Yiddil posted 04-19-2008 11:50 AM ET (US)     Profile for Yiddil  Send Email to Yiddil     
Thanks Glen, appreciate the quick response. Does [your specification of virtually all other motors out there] include the classic two-cycle Mercury 115 and 125 as well? For some reason I thought they were [specified with a battery with] less [rating].
jimh posted 04-19-2008 01:25 PM ET (US)     Profile for jimh  Send Email to jimh     
None of these motors draw anywhere near 600, 800, or 1,000 amperes when cranking, but the batteries that the manufacturers recommend for them need to have that rating. The parameter that worries the outboard manufacturers is the voltage sag during starting. The battery rating for cranking amperes allows for a voltage sag during cranking. Pardon me but I don't have the precise value handy, but the battery rating permits the voltage from the battery to sag while delivering the rated current. That means that a battery with a rating of 1,000-MCA can deliver that much current, but its terminal voltage is allowed to sag somewhat. It might drop to 10-volts during cranking.

The outboard motor manufacturers have built some motors which will not be able to start properly if during the cranking the battery voltage sags very much at all. It is because of the electronics and electrical devices in those motors. They like to operate at a certain voltage, and not lower. Those motors need the battery voltage to remain high while cranking, to stay close to 12-volts. The motor manufacturers have found that if they specify a battery with a very high MCA rating, they'll get a battery which maintains its terminal voltage better when cranking. But, again, during cranking those motors are not pulling 600, 800, or 1,000-amperes. The motors are probably pulling 100-amperes. By using a battery with a very high rating for cranking amperes, they get a battery that delivers the 100 to 200-amperes they actually need with very little voltage sag.

The other concern with the cranking battery is how much charging current the motor can generate at idle speeds. On motors with a lot of electrical accessories associated with them, when the motor is running at idle speed it is barely able to generate enough electrical power to keep itself running, and there is not much (if any) extra electrical power to re-charge the battery. This means that if you run the motor at idle most of the time, and your pattern of use involves a lot of starting, the battery will become undercharged.

When the engine speed increases, all motors can generate a lot of charging current. However, the amount of charging current a battery can absorb depends on its design. A motor that has 50-amperes of charging current available when it is running at high speed is great, but if the battery cannot absorb that much charging current, it is not going to make use of it.

What I have described above is more or less the situation with the Verado. They need a stiff battery to start. At low speeds they may not have a lot of excess charging capacity. So Mercury is recommending a really large cranking current battery and also an AGM battery. With an AGM battery you get the ability to absorb high charging current. This means that a short run at high speed with a Verado can re-charge an AGM in a hurry.

An E-TEC really does not need a battery to power its electronics during starting. It will self-power. It just needs the battery to crank over the starting motor. Once the E-TEC is running it has good charger current output at low speeds, and it can immediately begin to recharge the cranking battery, even at idle speeds. So you do not have to use an AGM battery to permit 50-ampere charging to occur.

jimh posted 04-19-2008 05:02 PM ET (US)     Profile for jimh  Send Email to jimh     
When a battery is rated to deliver a certain number of amperes under either the MCA or the CCA rating, that current is measured under the condition that the battery maintain at least 1.2-volts per cell. Thus for a typical six cell battery that means the output voltage can sag to 7.2-volts during the time the battery is delivering the rated current.

Cf.: http://en.wikipedia.org/wiki/Car_battery#Terms_and_ratings


cooper1958nc posted 04-19-2008 05:23 PM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     
1000 amps at 12 volts = 12kw = 16 hp. That much power would REALLY spin the engine, and would actually propel the boat if the neutral safety switch was not stopping that.

However, a typical battery might store 80 amp-hrs of charge, which at 1000 amps delivered, would last 4.8 minutes.

AWG 4-O wire is only good to about 350 amps.

So its not likely that there are 1000 amps flowing through starters.

davej14 posted 04-22-2008 01:53 PM ET (US)     Profile for davej14  Send Email to davej14     
My maintenance and Warranty manual covers the 75-125 HP Merc ELPTO (2-stroke carb'd) models. The battery specification for all four models is 525 MCA.
contender posted 04-22-2008 10:49 PM ET (US)     Profile for contender  Send Email to contender     
Hook up two batteries on a off-on switch and quit playing around...good luck

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