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Old 11-12-2012, 08:52 PM   #1
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Running starter through a shunt.

Any idea what the amp draw on a starter for a Lehman 120? I am looking at a Victron battery monitor that comes with a 500 amp shunt. Will it handle it? I will be using my (4) 6v batteries as a house/starter bank. My genset will have a seperate battery and will be my backup.
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Old 11-12-2012, 09:22 PM   #2
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Any idea what the amp draw on a starter for a Lehman 120? I am looking at a Victron battery monitor that comes with a 500 amp shunt. Will it handle it? I will be using my (4) 6v batteries as a house/starter bank. My genset will have a seperate battery and will be my backup.
Just interested to know why you are starting your main engine through a shunt.
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Old 11-12-2012, 09:36 PM   #3
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I want to be able to measure the total usage of the battery bank.
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Old 11-12-2012, 11:11 PM   #4
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I want to be able to measure the total usage of the battery bank.
Thanks, makes sense.
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Old 11-13-2012, 05:27 AM   #5
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Will it handle it?
Compare the cross sectional area of the shunt with that of the wire supplying the starter. If the shunt is the same or larger ... or close, it will handle it. Unless of course your starter wire normally gets hot when you crank the engine, then your shunt will too.

If the battery monitor circuit is calibrated to read up to 500A and the draw is larger then your consumption figures will be off.

Look up the locked rotor current for that starter and you will know if the information you obtain about starting power is accurate
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Old 11-13-2012, 07:27 AM   #6
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Terry:

Although it will probably work ok, that is not a good way to use your batteries. Sounds like you have a single battery bank of 4 parallel/series 6V golf cart batteries. GC batteries are not designed for short high current starting loads. But they work ok, right?

A better scheme is to add a separate Group 31 starting battery for the Lehman. You will want to isolate it from the house batteries so when you are on the hook drawing from the house bank you don't draw down your starting battery, maybe to the point that the engine won't start.

There are a number of ways to do this. The simplest is to wire the two battery banks to a 1,2,All switch that you use to isolate the banks.

David
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Old 11-13-2012, 07:46 AM   #7
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How come when someone asks what time it is they get advice on what kind of clock to buy?

The question is rhetorical ... this is the web after all.
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Old 11-13-2012, 07:51 AM   #8
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Compare the cross sectional area of the shunt with that of the wire supplying the starter. If the shunt is the same or larger ... or close, it will handle it. Unless of course your starter wire normally gets hot when you crank the engine, then your shunt will too.

If the battery monitor circuit is calibrated to read up to 500A and the draw is larger then your consumption figures will be off.

Look up the locked rotor current for that starter and you will know if the information you obtain about starting power is accurate
Rick, I've never heard of LRA's given for a DC starting motor, only AConditioning compressors which also state FLA's (full load amps).
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Old 11-13-2012, 08:16 AM   #9
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42MT Starter motor specifications :: Delco Remy

The specs may read as follows:

5. DATA
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 v dc
Torque (lock min) . . . . . . . . . . . . . . . . . . 26 1b-ft (at 500 amps, 3.5 volts)
Pinion speed (no load) . . . . . . . . . . . . . . . . . . . . . . . . . . 7000-10700 rpm
Pinion rotation (facing drive-end) . . . . . . . . . . . . . . . . . . . . . . . . clockwise
Number of teeth on clutch assembly (pinion) (one tooth blank) . . . . . . . . . . . . . . . .12
Mounting data:
Number of mounting holes... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Diameter of mounting holes . . . . . . . . . . . . . . . . . . . . . . . . . 0.6592 in.
Mounting hole circle diameter. . . . . . . . . . . . . . . . . . . . . . . . . 5.75 in.
Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.96 0.12 in.
Diameter (field frame) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.56 0. 04 in.
Weight (approx) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 lbs
4
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Old 11-13-2012, 09:35 AM   #10
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Terry:

Although it will probably work ok, that is not a good way to use your batteries. Sounds like you have a single battery bank of 4 parallel/series 6V golf cart batteries. GC batteries are not designed for short high current starting loads. But they work ok, right?

A better scheme is to add a separate Group 31 starting battery for the Lehman. You will want to isolate it from the house batteries so when you are on the hook drawing from the house bank you don't draw down your starting battery, maybe to the point that the engine won't start.

There are a number of ways to do this. The simplest is to wire the two battery banks to a 1,2,All switch that you use to isolate the banks.

David
Although it's not a direct answer to the original question, I agree that this is not the best plan.

I would go with djmarchand's suggestion except that I would use a battery combiner instead of the manual switches.

I don't see the value in measuring the current draw of the starter in normal use because it's used for such a short period of time.
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Old 11-13-2012, 09:52 AM   #11
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I had that very set up 120 and victron bm, starter worked perfect tru the shunt
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Old 11-13-2012, 09:53 AM   #12
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I wouldn't do it. One starter hang up and your amp draw will go thru the roof and possibly burn up your shunt. Seems like a high risk that could leave you stranded. You want to carry an extra shunt? That might be a solution.

When I reconfigured my DC system last year, I was told to size my wire to the MAXIMUM current draw it could see. 000 gauge wire is what I chose for the starter circuit with a single Group 31 battery. Keep in mind that a shut is just a few step away from being a fuse. The CCA of the battery gives you a good idea of what the max current the battery can provide. That much will toast a shut.

Without trying to tell you what kind of clock to buy (RickB), I agree with David, with a big motor like that, you *should* have a separate (un-shunted) battery for the starter. Lots of sailboats start the engine with the house bank, but these motors are usually small compared to a Lehman and the house bank is large. That is a large rotating mass to get going from a dead stop and a ton of compression to overcome.

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Old 11-13-2012, 10:16 AM   #13
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Not really my area but, why do you want to know the amps used when starting? It seem you would be more interested in how many amps remain?

If you want to know the the amps it take to start, buy a clamp amp meter, which you should have on the boat.

A cheaper way of knowing the amps left is a volt meter. The voltage can tell you roughly the % of battery amps left. Some volt meter has % on the meter. That is how we did it way back when, and I still due today by the voltage. No sence spending a lot of money for some thing that can be done with a volt/amp meter.
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Old 11-13-2012, 10:50 AM   #14
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There is no such thing as measuring the "amps" remaining in a battery. "Amps" (short for "amperes") is a measure of current flowing in a circuit at a particular time. Think of gallons per minute running through a pipe or hose.

"Amp Hours" (X amps for X hours) is an approximation of the power available in a battery or the power that has been used from the battery.

Simply put, if a battery has a capacity of 100 amp hours (AH) and you draw 20 amps for one hour, you have eighty AH left in the battery. In reality, it's nowhere close to this because the voltage drops as the power is consumed and the battery will be permanently damaged if much more than 50% of the capacity is used before recharging.

In the real world, figure on using 50% 0f the capacity before recharging so a 100 AH battery can provide 50 AH. 50 amps for one hour or one amp for 50 hours.

Can you estimate the remaining power of a battery with a volt meter? Estimate is the key word here. I wouldn't want to have to count on it.
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Old 11-13-2012, 12:24 PM   #15
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I wouldn't do it. One starter hang up and your amp draw will go thru the roof and possibly burn up your shunt.
If the cross sectional area is the same the shunt is no more likely to burn up than the existing wire.

As a matter of fact, if you want to you can use the existing wire as a shunt if it is long enough to provide the resistance necessary to provide the (milli)voltage required by the power monitoring system you plan to use.
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Old 11-13-2012, 01:00 PM   #16
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If the cross sectional area is the same the shunt is no more likely to burn up than the existing wire.
Fair enough. Since I don't know what a shunt is made of or how delicate it is with 500+ amps surging thru it, I guess I am being careful. I mean, I assume they have a rating on it for a reason. The Victron shunt looks like about five strips of metal and looks a lot like it would act like a fuse to me. I do know that the monitoring system knows what the resistance is SUPPOSED to be (that's how the whole thing works... as we all know), but I suppose a follow-up question would be that if a shunt got hot enough, would it alter the resistance enough to change the results?

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Old 11-13-2012, 01:16 PM   #17
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OK, guys. Ignore the recommendations to upgrade his electrical system, ignore the cross sectional area BS (how are you ever going to find that info?) and look at the attached spec from Xantrex for the shunt that is supplied with their Link 10 battery monitor. It is typical of what you will find in most battery monitors.

Shunt Type 500 A / 50 mV (included) - Link 10

That means that at 500 amps (about the most current you are going to pull from a normal starter) the shunt will produce a 50 mv (millivolt- one thousandth of a volt) voltage drop. I'll bet the terminal connections have more voltage drop than that.

At 500 amps the shunt will consume 500*0.05 = 25 watts. It will get a little warm after 10 seconds of cranking, but it won't harm anything. Even at the maximum current supply capacity of a Group 31- about 1000 amps, it isn't going to do any harm.

There are smaller shunts. Blue Sea makes one that has a 50 mv drop at 50 amps. That one might get a little toasty passing 500 amps.

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Old 11-13-2012, 02:10 PM   #18
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Old 11-13-2012, 02:20 PM   #19
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ignore the cross sectional area BS (how are you ever going to find that info?)
It ain't rocket science and it doesn't take a Chinese work ethic either.
If one wishes to work with electricity and wiring on the DC system of one's boat, knowing X-sectional area of the conductors used is far from BS. It is the basis of wire sizing and current capacity.

Quote:
500 A / 50 mV (included) - Link 10

That means that at 500 amps (about the most current you are going to pull from a normal starter) the shunt will produce a 50 mv (millivolt- one thousandth of a volt) voltage drop. I'll bet the terminal connections have more voltage drop than that.
It doesn't matter what the drop across the terminals is, the shunt measures the drop across a known resistance and that figure is used to calculate current.

You don't even need a shunt, figure out what length of your battery cable equates to a resistance of .001 ohms and measure the shunt voltage at those points.
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Old 11-13-2012, 04:43 PM   #20
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