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Old 04-03-2012, 16:52   #1
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Yanmar 2GM20F Starting Amps

I have a YANMAR 2GM20F in a 1987 Hunter 28.5. I am moving the location of my batteries and I want to make sure I use the proper battery cable size to avoid an unacceptable voltage drop. I heard that a good rule of thumb is to figure 2 amp per cubic inch and I figure this to be a little less than 80 amps. Does anyone know for sure how many amps are needed to start this engine?

Thanks
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Old 04-03-2012, 21:41   #2
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Re: YANMAR 2GM20F STARTING AMPS

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Originally Posted by JSG View Post
I have a YANMAR 2GM20F in a 1987 Hunter 28.5. I am moving the location of my batteries and I want to make sure I use the proper battery cable size to avoid an unacceptable voltage drop. I heard that a good rule of thumb is to figure 2 amp per cubic inch and I figure this to be a little less than 80 amps. Does anyone know for sure how many amps are needed to start this engine?

Thanks
Don't have my manual handy but I seem to recall they are fitted with a 1KW starter motor so 80 amps sounds right. ???

Please make sure the terminals are properly crimped and kept clean, I see more voltage dropped across terminals than I see in the wire!
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Old 05-03-2012, 02:54   #3
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Re: YANMAR 2GM20F STARTING AMPS

I would size for the nominal of the starter. If err, then on the oversize side.

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Old 05-03-2012, 03:12   #4
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Re: YANMAR 2GM20F STARTING AMPS

Checked manual and yes, the starter motor is a 1KW unit (nominal) so 80 amps is spot on
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Old 05-03-2012, 05:26   #5
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Re: YANMAR 2GM20F STARTING AMPS

Greetings and welcome aboard the CF, JSG.
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Old 05-03-2012, 11:48   #6
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Re: YANMAR 2GM20F STARTING AMPS

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Checked manual and yes, the starter motor is a 1KW unit (nominal) so 80 amps is spot on
Agreed. I would use even thicker cables as the 1kW may at times be drawn at around 10 Volts thus I would size the wires at 150%.

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Old 05-03-2012, 14:31   #7
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Re: YANMAR 2GM20F STARTING AMPS

Although a #6 AWG cable is rated for a maximum current 102 A (in engine spaces), to minimise voltage drop, you might consider using a #4 (or larger) if the distance between battery & starter exceeds about 5'-6", or #2 AWG if the distance is between 7' & 13'.

See ➥ Wire Size Chart.1 - Read /w "Ohm's Law & You" Cruisers & Sailing Photo Gallery
And ➥ Wire Size Chart.2 - Read /w Wire Size Chart.1 & "Ohm's Law & You" Cruisers & Sailing Photo Gallery
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Old 05-03-2012, 15:52   #8
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Re: YANMAR 2GM20F STARTING AMPS

I have an Ancor clamp on DC ammeter which has a full scale setting of 200 amps. I tried it on a Yanmar 3gm engine (one more cylinder than yours) and it pegged the meter. It might very well have dropped to less than 200 amps after the starter got up to speed after a few seconds but I didn't try it for that long.

So be very careful of the 1 KW starter designation. It can draw a lot more than that for a few seconds. I would size my cable for a 10% voltage drop at 200 amps and I would't go anything smaller than #2.

David
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Old 05-03-2012, 16:37   #9
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Re: YANMAR 2GM20F STARTING AMPS

Yes david is getting closer. a startet free spinning will draw about 80 amps. but under a load it will pull 230 to 280 amps and yes to gord 2 gauge if over 5 or 6 ft run would be wise . by the way how far are you moving your battery from the starter? (not to say a starter with problems wont pull more amps)
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Old 05-03-2012, 17:11   #10
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Re: YANMAR 2GM20F STARTING AMPS

I have a 3GM30, the 1GM, 2GM and 3GM all have the same starter, but obviously will load the starter differently. I have been thinking about fusing the starter battery, found conflicting information on the web, so I measured the voltage drop of a measured length of the starter wire and calculated 220 amps after the inrush settled out. I had the person turning the key also hold out the fuel stop for the duration of the measurement.

The service manual for the GM series of engines has a set of performance curves for the starter. At the maximum output of 1100 watts the graph shows the terminal voltage to be 9 volts and the current to be about 225 amps. This suggests to me that the input power of around 2000 watts for 1100 watts of output gets an efficiency about 50%. Considering that you're not supposed to crank the starter for very long so you don't burn it up, a low efficiency doesn't seem improbable. I found one website calculating starter currents in general suggested a 50% efficiency.

Using the linear rule of thumb from my data point, and that my engine is 1.5x the displacement of the 2GM, that means 150 amps might be what the 2GM draws.

Another data point showing at least 200 amps of current draw for a 27hp(same as 3GM30):
From post #13 of this thread I found:
Engine Starting Video / Real World Amp Load Data

1. I tried to measure the starting current on a 27 hp Yanmar using a clamp on DC ammeter. The current exceeded the 200 amp maximum scale of the meter.
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Old 05-03-2012, 17:22   #11
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Re: YANMAR 2GM20F STARTING AMPS

I don't want to get off the subject, but saw again a referance crimping the connections on a battery cable ! I have always solderd the connections and used shrink wrap ! I do the same thing on all my boat wireing. I always had trouble with crimped conections, is there neww crimpimg stuff availble these days ?? just asking !
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Old 05-03-2012, 17:37   #12
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Re: YANMAR 2GM20F STARTING AMPS

Oh dear. crimped vs soldered falls under the same as guns and anchor
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Old 05-03-2012, 18:59   #13
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Re: YANMAR 2GM20F STARTING AMPS

Quote:
Originally Posted by JSG View Post
I have a YANMAR 2GM20F in a 1987 Hunter 28.5. I am moving the location of my batteries and I want to make sure I use the proper battery cable size to avoid an unacceptable voltage drop. I heard that a good rule of thumb is to figure 2 amp per cubic inch and I figure this to be a little less than 80 amps. Does anyone know for sure how many amps are needed to start this engine?

Thanks
This may help...

2QM Starting Current
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Old 06-03-2012, 01:36   #14
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Re: YANMAR 2GM20F STARTING AMPS

That's the inrush current. I'd think you'd want to have an idea how long that lasts to select a fuse. The Blue Seas link posted above showed maybe 40 msec of high inrush, then 2x to 3x for another 200-400 msec.

I'd be interested in seeing the fuel shutoff pulled while turning over the engine to get the steady state current draw while loaded.

John


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This may help...

2QM Starting Current
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Old 06-03-2012, 06:18   #15
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Re: YANMAR 2GM20F STARTING AMPS

Time to get back to basic electrical theory.

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Originally Posted by barnakiel View Post
Agreed. I would use even thicker cables as the 1kW may at times be drawn at around 10 Volts thus I would size the wires at 150%.

b.
If you are referring to 10 Volts at the battery terminals, then you must remember the "other 2.7 volts" have been dropped (lost) inside the battery (its internal resistance) so it is not really correct to consider only a P=VI calculation using 10 volts and 1 KW as the battery isn't able to supply high current if it's internal resistance is significant plus you have to factor in the cable resistance.

Quote:
Originally Posted by cal40john View Post
I have a 3GM30, the 1GM, 2GM and 3GM all have the same starter, but obviously will load the starter differently. I have been thinking about fusing the starter battery, found conflicting information on the web, so I measured the voltage drop of a measured length of the starter wire and calculated 220 amps after the inrush settled out. I had the person turning the key also hold out the fuel stop for the duration of the measurement.

The service manual for the GM series of engines has a set of performance curves for the starter. At the maximum output of 1100 watts the graph shows the terminal voltage to be 9 volts and the current to be about 225 amps. This suggests to me that the input power of around 2000 watts for 1100 watts of output gets an efficiency about 50%. Considering that you're not supposed to crank the starter for very long so you don't burn it up, a low efficiency doesn't seem improbable. I found one website calculating starter currents in general suggested a 50% efficiency.

Using the linear rule of thumb from my data point, and that my engine is 1.5x the displacement of the 2GM, that means 150 amps might be what the 2GM draws.

Another data point showing at least 200 amps of current draw for a 27hp(same as 3GM30):
From post #13 of this thread I found:
Engine Starting Video / Real World Amp Load Data

1. I tried to measure the starting current on a 27 hp Yanmar using a clamp on DC ammeter. The current exceeded the 200 amp maximum scale of the meter.
Cal40john is heading in the right direction.
The starter motor is series wound so if it is highly loaded (therefore low RPM), the current drawn from a constant voltage source is correspondingly high. In fact, if stalled (0 RPM), the theoretical current (from a constant voltage source) is infinite and thus the torque is also infinite. Yes of course this won't actually happen but the point is a series wound DC motor when loaded) will generate as much torque as the battery can deliver (before discharging) or the motor burning out. Conversely when the starter motor is unloaded (say spinning the engine over while decompressed), the RPM of the starter motor will be high and the current will be proportionally low.

So measuring the current is not a good way to work out was cable is needed as it can vary greatly all due to the load on the series wound motor. In fact for the first few milliseconds until the starter motor develops some RPM, the current is limited by the CCA of the battery as the starter will want to draw thousands of amps.

My humble advice is to use the nominal power rating of the motor (1 KW), add your own fudge factor (say 50% or whatever makes you warm and fussy) and spec. your cable for a <3% voltage drop. Upsize to the next cable size if you really are conservative! Regardless, the cable will handle the current for much longer than the starter motor windings.

The fusing aspect is another kettle of fish and is a more problematical; maybe that is one of the reasons it isn't always addressed!
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