Battery Monitor - Shunt modification question

[nixxon]

I have bought a used Victron BMV 602S Battery Monitor (instrument only) that originally came with a 500A / 50mV shunt. The shunt is now gone, due to an accident the seller had with his boat that resulted in water submersion of the engine compartement where the shunt was located.

The Victron manual states that:
"The BMV is supplied with a 500 A / 50 mV shunt. For most applications, this should be suitable; however the BMV can be configured to work with a wide range of different shunts. Shunts of up to 9999 A, and/or 100 mV can be used. "

I have contacted Victron Energy and they state that "Only 50mV or 100mV shunt values can be set". This means that the voltage across a suitable shunt is supposed to drop by either 50 mV or 100mV if the shunt capability is set to i.e. 500A.

There are many affordable 500A shunts on ebay, but they are typically 500A/ 75mV and seemingly not usable because they are neither 50mV nor 100mV shunts.

Possible solution 1:
Based on the manual, it seems that the shunt current can be set in 1A increments: "...up to 9999A".
If I install a 500A/75mV shunt without modification, wouldn't it give almost correct readings if I set the shunt settings on the instrument panel to either 333A/50mV or 667A/100mV?

Possible solution 2 (more mess):
If this doesn't work, I guess I could modify a 500A/75mV shunt if I could increase the voltage drop by 1/3, up to the selectable 100mV setting. The resistance of a 500A/75mV shunt is R = 0.075V / 500A = 0.00015 Ohm. If I increase the resistance of the shunt by 1/3 to 0.0002 Ohm, the shunt should now serve as a 500A/100mV shunt. If I find a shunt with 4 bars across the voltage drop section, cutting 1 out of the 4 bars should be just right, given that all bars have the same resistance. I have found one shunt on ebay with 8 bars across, thus I could dremel off 2 bars out of the 8. (each bar having a resistance of 0.0012 Ohms). The enclosed photo shows a shunt with 8 bars across.

There may be some drawbacks to this solution though:
Maybe not all of the bars have the same resistance, giving an inaccurate voltage drop.
The shunt would theoretically only handle 375A (75% of the rated 500A), but I don't think this will be a problem in my little sailing boat. The biggest current is drawn from the battery when I start my little 9HP Yanmar 1GM10 Diesel engine. I haven't measured it, but I believe it could not be drawing more than 100 Amps from the battery. Usually, I even use my separate auxillary battery to start the engine, and the auxillary battery does not go through a shunt.

I hope solution 1 works, but I would be glad if anyone could comment on my thoughts regarding solution 2 in case solution 1 doesn't work out as planned.

[mtdoc]

Why not just get the correct shunt?

[langwadt]

There is no reason why 1 wouldn't work.  As long as you stay with in the voltage the instrument can handle and can program the gain to show the correct result there is no difference

A shunt is just an fancy resistor.

[Monkeh]

Solution 3:
Buy a 50mV shunt: http://uk.farnell.com/murata-power-solutions/3020-01103-0/shunt-50mv-500a/dp/1339342

[nixxon]

Why not just get the correct shunt?

It is a matter of cost only.
The shunt I was refering to only costs $ 7.80 incl. shipping. The one you linked to costs £ 18.47 (~$28) excl. shipping

Solution 3:
Buy a 50mV shunt: http://uk.farnell.com/murata-power-solutions/3020-01103-0/shunt-50mv-500a/dp/1339342

This one costs $ 27 excl. shipping

If the total incl. shipping is more than $ ~43.75, I also have to pay a $ 15 customs handling fee of and 25% VAT. For example:

Shunt $ 27
+Shipping $ 17
Total $ 44
+25% VAT $ 11
+ handling fee $ 15
TOTAL $ 70

The $ 7.80 shunt totals just $ 7.80

EDIT: Actually, the correct Farnell shunt only totals $ 38,75 incl. UPS super fast shipping.
It is probably way better than the cheaper shunt that will spend more than a month in transit.
http://www.farnell.com/datasheets/1790366.pdf

[nixxon]

There is no reason why 1 wouldn't work.  As long as you stay with in the voltage the instrument can handle and can program the gain to show the correct result there is no difference

A shunt is just an fancy resistor.

One reason solution 1 may not work could be that the manual is misleading. Maybe the shunt currents only can be set in 100A intervals. The voltage is no problem.

[nixxon]

It is actually pretty impressive that tiny (70 g) battery monitors can sense battery current consumption with a resolution of 10 mA (0.01A) through a 500A/50mV shunt. If I understand the concept right, 10 mA current through the shunt generates a voltage drop across the shunt of only 0.000001 Volts (0.05V/500A*0.01A). That is a puny 1 µV measured through a 30 foot 24 AWG (0.2 mm2) cable and two RJ12 connectors.

How is this even possible? In comparison, the Fluke 87V only has a VDC resolution of 100µV.

Victron even claims a Current measurement accuracy of ±0.5 %

Battery monitor info http://www.victronenergy.com/battery-monitors/BMV-600S-and-BMV-602S

EDIT: I found an interesting paper by Keithley: Understanding Low Voltage Measurements that discusses different error sources; Thermoelectric noise, Magnetic Fields and Ground Loops http://www.keithley.com/data?asset=50287