Can 100A shunt survive 5.7l engine starting?

[EugeneR]

Is anyone starting a large engine like a Mercruiser 5.7 through the BM1 battery monitor's 100A shunt? Does it work?

I've waited months for Nasa to come up with a larger shunt, as they told me they are looking into, but I need to get something soon given the amount of stuff the kids want to run these days.

I am reluctant to buy other brands because they do essentially the same thing at several times the price; except, they have bigger shunts but then a bigger shunt should not have to cost £100 more...

 

[Amulet]

Conventionally the starter circuit doesn't go through a shunt. I think you may have a problem. I have a tiny 10HP single cylinder diesel. I decided to shove it through the Nasa shunt. (I actually have two BM1's in a symmetrical two battery system.)

When I start the engine the BM1 reads about 170 amps, which is more that it's supposed to be capable of. Nasa told me it might bugger the shunt but not the instrument to send more than 100 amps through it. The shunt does not get even noticeably hot if you touch it while starting the engine.

With a big engine you could be drawing 500 amps, which'd be asking for trouble.
Also, the extra resistance could actually make it hard to start the engine I guess.


I guess that the instrument is safe cos, at the end of the day you cannot create more than 12 volts across it even if you melt the shunt down.

 

[pvb]

Shunts will accept high instant loads, so you should have no problem starting the engine. Usually, however, boats will have a separate starting battery, and most people use a battery monitor to see what the domestic battery is doing. If you only have one 12v battery/bank, you should still be OK with the Nasa shunt.

If you're not entirely happy about overloading the Nasa shunt, you could substitute a higher-rated shunt. The Nasa BM-1 needs a shunt rated at 50mV per 100A, so you need to keep this ratio the same - ie if you get a 200A shunt it must be rated at 100mV. As it happens, the Sterling 200A shunt is rated at 100mV, so you could use one of these instead (they're about £50 and are quoted as having a 500A capacity for 10 minutes max).

 

[Ruffles]

Is anyone starting a large engine like a Mercruiser 5.7 through the BM1 battery monitor's 100A shunt? Does it work?

If you're worried about the heat generated you could use two pairs of paralleled shunts in series. This will give the same reading on the meter but handle four times the power. Four times the cost though!

 

[pvb]

If you're worried about the heat generated you could use two pairs of paralleled shunts in series. This will give the same reading on the meter but handle four times the power. Four times the cost though!

Errr... don't think so. Think about the wiring to the meter....

 

[VicS]

Errr... don't think so. Think about the wiring to the meter....

I think you are going to have to explain your objection to me as well. I cant see the difference between doing as suggested and using the 200amp shunt that you have suggested.

Not caught Skipper-Stuitis have you

 

[pvb]

Not caught Skipper-Stuitis have you

That's enough! You can't post things which Stu might find offensive (which doesn't leave much room for manoeuvre). If you're not careful, he'll report you to Dan the Man. On the other hand, if he's really got you on "Ignore" he won't read your post (or mine!).

 

[pvb]

I think you are going to have to explain your objection to me as well. I cant see the difference between doing as suggested and using the 200amp shunt that you have suggested.

OK, you have 2 terminals on the meter which are connected to the shunt. They read the tiny voltage created across the shunt when current flows through it. The Nasa meter works on 50mV per 100A. The 200A shunt I suggested gives 100mV at 200A - ie it's the same basic ratio, so will give a correct reading with the Nasa meter.

Now, think of the "two pairs of paralleled shunts in series" proposed by Ruffles. How would you wire the sensors, and what would they read?

 

[VicS]

Wire as below. Still the same resistance and therefore still the same mV to amps ratio.
Just a little group of four shunts in a series parallel circuit. The interconnecting wires would have to be heavy enough not to cause any errors.

It'd be cheaper to just buy the one 200amp shunt that you suggest and more sensible too with less connections to cause trouble

 

[pvb]

OK, 10/10 [size=-4](plus a gold star)[/size].

 

[Billjratt]

I don't like shunts.
They are extra connections in a crucal part of the circuit.
I put the meter sensewires across the common negative wire between the batteries and the starter motor. (and moved the outgoing and incoming feeds to the motor end)
Obviously, the resistive value of the "shunt" depends on the length/area of the cable, however, it will probably be higher than a real shunt, so it will be necessary to trim the meter with a potentiometer.
This is easily done with a known wattage load - like all the lights!
No problems with cranking or anything else.
And, ever so cheap. Ye ken?

 

[halcyon]

I don't like shunts.
They are extra connections in a crucal part of the circuit.
I put the meter sensewires across the common negative wire between the batteries and the starter motor. (and moved the outgoing and incoming feeds to the motor end)
Obviously, the resistive value of the "shunt" depends on the length/area of the cable, however, it will probably be higher than a real shunt, so it will be necessary to trim the meter with a potentiometer.
This is easily done with a known wattage load - like all the lights!
No problems with cranking or anything else.
And, ever so cheap. Ye ken?

Why do people go to so much effort spending money on accurate monitoring equipment, then do a lash up to provide data for it

Why not just fit a Hall Effect shunt, just measures the magnetic field on the positive battery terminal post, no current carried, no breaks in the cable, no extra connections, cannot be overloaded, no volt drop, no heat genaerated, and no need for twisted pairs.

Brian

 

[pvb]

Why not just fit a Hall Effect shunt, just measures the magnetic field on the positive battery terminal post, no current carried, no breaks in the cable, no extra connections, cannot be overloaded, no volt drop, no heat genaerated, and no need for twisted pairs.

Can it just be connected to a Nasa BM-1 meter?

 

[VicS]

I don't like shunts.
They are extra connections in a crucal part of the circuit.
I put the meter sensewires across the common negative wire between the batteries and the starter motor. (and moved the outgoing and incoming feeds to the motor end)
Obviously, the resistive value of the "shunt" depends on the length/area of the cable, however, it will probably be higher than a real shunt, so it will be necessary to trim the meter with a potentiometer.
This is easily done with a known wattage load - like all the lights!
No problems with cranking or anything else.
And, ever so cheap. Ye ken?

Its a way of doing it I suppose.
You say that is what you do but you seem uncertain whether or not the the length of cable and its cross sectional area will produce a resistance more than the shunt.

It will in fact require a cable a little longer than 0.33m of 6 AWG, 0.65m of 4 AWG , 1.0m of 2 AWG or 1.25m of 1 AWG. ( If I have my arithmetic correct) So achievable.

Calibration with lights of "known wattage" will not be good enough because you do not know the exact wattage or more importantly the total current consumption with any degree of accuracy. However a load such as that and a decent ammeter would enable the adjustment to be made.

Not sure the idea will meet with universal approval though.

 

[William_H]

Shunts

A shunt is just a resistor if i9t developes 100mv at 100 amps then at 100 amps though it it will dissipate 10 watts. Likewise the 200amp for 200 millivolts will dissipate 40 watts at full current.
So you look at the shunt. If as I suspect it is a decent sized lump of metal like 10cms long 1cm wide 2mm thick then I would suggest it would easily dissipate the 10 watts or even 40 watts without getting so hot as to melt or damage the surroundings.
You can easily try it with the higher current and check the temperature of the shunt after the high current is passed. Of course long periods of high current will be a greater heating problem.
All this assuming that the meter itself will not be damaged by the higher voltage produced on overload. ie the 100mv goes to 200mvs at 200 amps.
olewill

 

[rogerthebodger]

The problem with shunts is that you are adding an extra volt drip in a high current line which will effect all equipment down line.

I have 3 off 500A 50mV shunts trying to drive a 3.5 digit DVM which has a 200mV full scale input. I need a 10:1 amplifier to get the reading right.

I am going to replace with a hall effect "shunt" as halcyon recomends but finding one that will run off +12VDC is quite confusing and as said will it will also reduce the unnecessary volt drip.

 

[VicS]

The problem with shunts is that you are adding an extra volt drip in a high current line which will effect all equipment down line

. But a single shunt giving a 50mV drop wont have a serious effect on anything will it, It's significantly less the 3% generally accepted for loses along cables.

Voltage loses do add up so some caution is needed and of course putting a high starter current through one will give a proportionally bigger volts drop than 50mV.

the latter perhaps another good reason for the battery monitor to be on the domestic bank while the dedicated starter battery is not monitored.

 

[Amulet]

Despite all the ingenuity and wisdom in this discussion, I have a suspicion you are asking for trouble. My starter is over the BM1 specification by about 70%. Your engine is roughly 15 times bigger than mine. I am sure that the starter current doesn't scale in a simple relationship to the engine capacity, however it is quite conceivable that you will be over spec by a factor of 5 or even 10. I'd get someone with the necessary meter to measure the starting current (I believe DC clamp-on meters exist but are expensive).

In your place I'd cave in and have a separate battery circuit if I was even close to my guessed current of 500-1000 amps. N.B. You can, if you insist, use the same battery for starter and domestics if you have the starter bypass the BM1. It will then still give you volts and amps as a decent guide to battery state, but will be no use for "% charge" and "time to run". (Actually I think it might recalibrate itself each time the battery gets up to full charge, but someone else on the forum will know better than me.)

By the way, the mutliple shunts solution assumes that the wire between the shunts is of zero resistance if it is to give the correct reading. My understanding (maybe wrong) is that even a pretty tiny resistance could produce a noticeable error in the readings.

 

[pvb]

Despite all the ingenuity and wisdom in this discussion, I have a suspicion you are asking for trouble.

Your suspicion is unfounded. Firstly, as I posted earlier, shunts have a high capacity for short bursts of overload. Secondly, a Mercruiser 5.7 starter motor is usually rated at 1.4kW - so it's not going to draw anything like 1000A! Thirdly, even if it did draw 1000A, the 200A shunt I suggested would still be OK; read what Mr Sterling says about it in his latest catalogue...

 

[VicS]

Take some care with these starter motor specs.

I think you will find that the rating is the mechanical power delivered, the power consumed will be higher also I think you will find that the ratings may be at a terminal voltage lower than 12volts, which means the currents will be higher than you'd expect.

I have figures for a 1.1hp (810watt) motor.

its normal cranking current is 185 - 220 amps with a terminal voltage of 9 volts. That's a lot higher current figure than you get by dividing 810 by 12!

Stalled the current is 400-490 amps at 7 volts.

Still even when extrapolated to 1.4 kw less than 1000 amps though!