E-TEC Engine Starting Battery Specification Speculation

Electrical and electronic topics for small boats
alloyboy
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E-TEC Engine Starting Battery Specification Speculation

Postby alloyboy » Wed Jan 11, 2017 1:37 pm

Any opinions as to why the [engine cranking battery for an Evinrude E-TEC engine] MCA rating is as high as it is?

[Note: while not mentioned here, the MCA rating for an E-TEC V6 engine battery is recommended to be 1,000-MCA for use in temperatures below 32-degrees-F or extreme service, or 845-MCA for temperatures above 32-degrees-F or normal service--jimh]

A starter motor normally draws about 200-Amperes peak or less. The E-TEC apparently needs about 1000-Watts (83 Amperes at 12-Volts or 18-Amperes at 55-Volts) to operate itself on; so why the battery requirement being what it is?

Anyone care to speculate?

jimh
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Re: Normal Starting Current

Postby jimh » Wed Jan 11, 2017 6:24 pm

alloyboy wrote:A starter motor normally draws about 200 peak amperes or less.


That is a very interesting value for the "normal" "peak" draw of a starter motor. Where did you come up with it?

Did you measure it?

If so, can you explain the instrumentation you used and the technique? I would be interested in knowing where your data comes from.

I have measured the peak DC Amperes of the starter motor of a four-cylinder 2.5-liter automobile engine. About 350-Amperes peak current during cranking was typical. I measured with a FLUKE clamp-on DC Ammeter. This engine has a larger diameter flywheel than the typical outboard engine, so I suspect the gearing of the starter may make it easier to turn the flywheel.

jimh
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Re: Engine Cranking Battery

Postby jimh » Wed Jan 11, 2017 6:40 pm

alloyboy wrote:Any opinions as to why the [engine cranking battery for an Evinrude E-TEC engine] MCA rating is as high as it is?


I recommend you contact Evinrude to discover why they have established the ratings they have.

You can find the recommendation for Evinrude in their rigging guide. For the larger engines, 115 to 300-HP, it specifies a battery rated at 845-MCA for normal service and 1,000-MCA for service in weather below 32-degrees. In my case, I chose to conform to the higher recommendation.

Have you made a survey of other outboard engine brands to compare with Evinrude to see if there is much difference in the ratings of the battery? I know that the Brunswick outboard model called Mercury VERADO has quite specific requirements for its battery, including that it must be of a certain type (an AGM lead-acid). I don't recall the required MCA rating, but I am sure it is substantial since the VERADO cannot operate at all without a battery and is somewhat notorious for having problems running at low speeds if the attached battery is not up to proper voltage.

UPDATE: based on information from a respected dealer selling Mercury VERADO engines, the battery requirements for the VERADO are:

--must be AGM
--must have 1000-MCA rating or 800-CCA rating
--must have 180-Ampere-hour rating or meet the 135-RC 25 Capacity Rating

(Reserve capacity is the number of minutes a battery can maintain a useful voltage under a 25 ampere discharge; a rating of 135 suggest the battery must provide 135-minutes of reserve capacity.)

These battery requirements for cranking Amperes are quite similar to those for the E-TEC. I think they are really typical for all larger outboard engines.

The Evinrude outboard engine is unique among all the larger horsepower outboard engines of any brand in that it can be started with a pull rope and can run without a battery. It does not need a battery at all, other than to crank it over for electric starting. Evinrude engines have had, for many years, a minimum engine cranking speed, I think 300-RPM, before the engine will create spark and try to start.

jimh
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Re: Engine Cranking Battery

Postby jimh » Wed Jan 11, 2017 7:32 pm

alloyboy wrote:The [E-TEC] apparently needs about 1000 watts...to operate itself...


Have you measured the power consumption of the E-TEC? I would like to know the way you measured that power.

I think you are making an inference based on the notion the alternator can produce 1,700-Watts and dedicates about 725-Watts (or more) for battery charging. But you must take into account that the electrical load of the engine depends on the engine speed, and higher engine speeds will require higher electrical power, principally to run the injectors and fuel pumps at higher duty cycles. But all of that power is generated in the E-TEC itself, and has no influence on the Ampere rating of the battery. Once the E-TEC is started, it can run without a battery; this is evidence that the battery is not necessary to power the engine. I seriously doubt an E-TEC running at idle is consuming a kilowatt to operate its electrical system.

Note that 1-HP is about 750-Watts, so if a 300-HP E-TEC were to need 1,000-Watts to operate its electrical system when running at full power, the electrical system would be taking about 1.3-HP from the engine. That is less than half of one-percent of the engine power. It does not seem particularly unusual that the engine's own electrical load might impose a small burden on the engine to generate that electrical power from the mechanical motion of the engine.

jimh
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Re: Engine Cranking Battery

Postby jimh » Wed Jan 11, 2017 7:54 pm

ASIDE: a practical method to measure the DC current flow in a circuit, particularly when the DC current will be the range of hundreds of Amperes, is to use a Voltmeter to measure the voltage drop in a segment of the circuit whose resistance is know with fair accuracy. The method is simple; let's use a circuit with high current flowing through 10-feet of 4-AWG carrying power from the positive terminal of the battery to the starter solenoid of an engine:

--measure the voltage drop (E) in the wire segment during engine cranking;

--find the resistance-per-100-feet of 4-AWG from a table; (a good table is found at http://www.powerstream.com/Wire_Size.htm

--multiply the resistance value by the length divided by 1000 to find resistance (R) ;

--deduce the cranking current (I) from Ohm's Law: I = E/R

Here is the example problem worked out to find current:

--4-AWG has a resistance of 0.2485/1000-feet

--a 10-foot length of 4-AWG will have a resistance of 0.2485 x 10/1000 = 0.002485-Ohms

If we attach a Voltmeter between the two ends of the 4-AWG conductor with a peak-hold function, and crank over the engine, we might see a peak voltage drop across the conductor of, say, 0.666-Volts.

Now we compute the current from the two values:

E = 0.66-Volt
R = 0.002485-Ohms
I = E/R = .66/0.002485-Ohms = 266-Amperes

An alternative method is to measure with a clamp-on DC Ammeter. A FLUKE 374 is a typical choice; it sells for about $300.

Note that in the above example, there would also be another 10-feet of 4-AWG carrying the negative circuit back to the battery. There would also be the same voltage drop in that conductor, giving a total voltage drop of 1.33-Volts. That is more than ten-percent of the typical full-charge battery voltage of 12.7-Volts, so this sort of rigging results in quite a bit of power lost in the resistance of the conductors.

jimh
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Re: E-TEC Engine Starting Battery Specification Speculation

Postby jimh » Mon Jan 30, 2017 5:11 pm

Earlier in this discussion I directed the OP to seek an answer to his inquiry regarding the reasoning used by Evinrude in their battery specifications. I said:

I recommend you contact Evinrude to discover why they have established the ratings they have.

Insight into the reasoning used by Evinrude in creating their recommended starting battery specifications is explained by someone familiar with that process in a comment made in a discussion in another forum. Here is the explanation:

The battery specs that BRP provides for cranking batteries certainly have some buffer built into them. They need to do that because BRP does not provide a whole vehicle, just the outboard engine. They have no idea of the quality battery which will be installed or even whether the supplied battery cables will be used or the length of cables used. By quoting the specs they do, they are just forcing the purchaser into a battery that is more likely a higher quality than some and has ample reserve for rigging dependent items. Helping the consumer protect himself from himself makes for a better day of boating.


Cf.: http://www.etecownersgroup.com/post/show_single_post?pid=1294670668&postcount=16&forum=377900