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Topic: Anyone wire a generator starter through a shunt?  (Read 824 times) previous topic - next topic
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Re: Anyone wire a generator starter through a shunt?
Reply #25 –
Quote from: 73gitane  - 
Oh well... in the end the 500 amp shunt is going to let me know when the battery is fully charged or if it's getting alarmingly low. If I ever have the need to track down a small parasitic load under 100mA I can always bring out my small portable  meter that has a current clamp.
BTW, just because you may lose some resolution in the readout with the 500A shunt, the measurement should be just as accurate at the input to the integrating processor, since the shunts are probably similar in precision. When you are putting a lot of charge current into the batteries with a long drive or an upgraded fast-charge converter, the larger shunt will run cool and not lose accuracy.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #26 –
Quote from: Steve  - 
BTW, just because you may lose some resolution in the readout with the 500A shunt, the measurement should be just as accurate at the input to the integrating processor, since the shunts are probably similar in precision. When you are putting a lot of charge current into the batteries with a long drive or an upgraded fast-charge converter, the larger shunt will run cool and not lose accuracy. Steve

Speaking as an electrical engineer who has bought many meter shunts: The signal the processor gets with the 500A shunt will be 20% of the size it would be with 100 amp shunt, and it's accuracy will be reduced in proportion. The accuracy of the 100A shunt will still be within it's specifications, even at 100A currents, by design. That's what makes it a meter shunt, and not just a bulky, low ohm resistor. Of course, if your charging is going to exceed 100 A, use the 500 A shunt.

For your purposes, it might not make any difference which you use, as there are other factors that will have more affect on the accuracy of your amp hour measurement.

**Added 9/21: The above remarks have some errors, so be sure to read my corrections further on.
2005 Jayco 24SS
Re: Anyone wire a generator starter through a shunt?
Reply #27 –
Here's a nice article from Bogart engineering detailing 100 & 500 amp shunts.

http://www.bogartengineering.com/wp-content/uploads/docs/SHUNT%20Info2.pdf

I'm using the stock (2) 6 volt battery set up. This article says the 100 amp shunt can handle up to 75 amps. I'm not sure what the worst case draw from the alternator would be if the batteries were way down.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #28 –
You won’t get anywhere near the 75 amp  limit with lead acid batteries. Mine start at 40 amps and begin dropping immediately.

I have both lead acid and lithium batteries with a separate monitor for each. The lithiums are at the limit every charge. No problems in the last two years but I’ll check temperatures next time. Thanks for posting the information. I had forgotten about it.

That 40 amp limit is from a dedicated battery charger capable of 14.4 volts. The engine alternator charges at around 13.7 volts as does the standard converter.
Harry 2006RB
Re: Anyone wire a generator starter through a shunt?
Reply #29 –
Quote from: Eric Greenwell  - 
Speaking as an electrical engineer who has bought many meter shunts: The signal the processor gets with the 500A shunt will be 20% of the size it would be with 100 amp shunt, and it's accuracy will be reduced in proportion. The accuracy of the 100A shunt will still be within it's specifications, even at 100A currents, by design. That's what makes it a meter shunt, and not just a bulky, low ohm resistor. Of course, if your charging is going to exceed 100 A, use the 500 A shunt.

Here's where the confusion may arise: A 50mV/500A shunt specified at 0.25% accuracy will read 0.1 mV at 1 Amp, with a reading uncertainty of  +/- 2.5 mA. A 100mV/100A shunt specified at 0.25% accuracy will read 1 mV at 1 Amp, and the reading uncertainty is +/- 2.5 mA. So The reading resolution has increased by a factor of 10, but the accuracy is the same. Unless you need a readout with 10mA resolution, the heat dissipation of the 500A shunt is a significant advantage with NO loss in accuracy.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #30 –
Quote from: Eric Greenwell  - 
The accuracy of the 100A shunt will still be within it's specifications, even at 100A currents, by design. That's what makes it a meter shunt, and not just a bulky, low ohm resistor.

This part (above) of what I said is wrong, so I thank 73gitane for posting the Bogart article. Years ago, when I was buying shunts for various data systems, the limits on continuous current were not in the catalogs I was using, and still aren't, so I looked in a couple other places to verify what was in the article. This Murata Power company link  is a comprehensive list of shunt specifications  and how to determine what continuous and intermittent currents you can use.

https://www.mouser.com/datasheet/2/281/dpm_shunts-25677.pdf

Their shunts have a maximum 125 deg C limit, a 66% of nominal  current rating for the continuous operation at 25 deg C, and a requirement to mount the shunt "vertically" (for best air cooling) to meet those ratings.

There is a derating formula for ambient shunt temperatures between 25 C and 125 C. For example, a 100A, 100mv shunt in a compartment with 120 deg F (50 deg C) air will have a 50 amp continuous rating, not 66 amps. The 500A shunt my inverter/charger uses can easily handle the 90-100A currents it ocasionally uses with almost no derating.

2005 Jayco 24SS
Re: Anyone wire a generator starter through a shunt?
Reply #31 –
Quote from: Eric Greenwell  - 
Speaking as an electrical engineer who has bought many meter shunts: The signal the processor gets with the 500A shunt will be 20% of the size it would be with 100 amp shunt, and it's accuracy will be reduced in proportion.

**Added 9/21: The above remarks have some errors, so be sure to read my corrections further on.

Eric, I think your implication is that the above statement is NOT in error, so to avoid confusing the forum, lets first examine the math:

The 500A shunt has a resistance of 0.05 / 500 = 0.0001 Ohms at 0.25% tolerance. So, the actual resistance could be as high as 0.00010025 Ohms. With a 1A current through the resistor, the reading across it would be 0.10025 mV. Note then that the error reading 1 Amp would be (0.10025 / 0.10000) X 100 = 0.25%.

The 100A shunt has a resistance of 0.1 / 100 = 0.001 Ohms at 0.25% tolerance. The actual resistance could be as high as 0.0010025 Ohms. With a 1A current through the resistor, the reading across it would be 1.0025 mV. So the error reading 1 Amp would be (1.0025 / 1.0000) X 100 = 0.25%, again.

Now the other issue to consider is the way the signal is processed by the Victron when confronted with two equally precise dc voltages that differ by a factor of 10. If the analog input were equally sensitive to both, the weaker signal would fall in a less sensitive region of the input, and will likely be noisier as well. So this would produce a less accurate reading on the signal from the 500A shunt. However the common design solution of adding an amplifier to switch ranges, then losing the noise in the A/D conversion will eliminate most of that issue, which we can assume the Victron design accomplishes. It is safe to say there is no reason to expect any significant loss in accuracy when using the 500A shunt vs. the 100A shunt. This is important, because errors add up when integrating up to 225 A-hr or more of charge.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #32 –
Quote from: Steve  - 
Now the other issue to consider is the way the signal is processed by the Victron when confronted with two equally precise dc voltages that differ by a factor of 10. If the analog input were equally sensitive to both, the weaker signal would fall in a less sensitive region of the input, and will likely be noisier as well.
Without knowing the details of the Victron input, or testing it on the bench, we can't determine how it processes the input voltage. My guess is it's a 0-100mv input that assumes the range will be determined by the shunt you use, and the scaling is done digitally, as you describe in partial quote of your remarks.

I still think capacity variations due to battery temperature, age, and current draw will matter more than a even a few percent errors in the battery amps.
2005 Jayco 24SS
Re: Anyone wire a generator starter through a shunt?
Reply #33 –
I’m glad you guys are having fun. I don’t have a clue!   — Jon
(Former) ‘06 TK “Albatross.” And (former) Vespa 250.   Alas, no more; both are gone.😕 Great memories remain! 😄
Re: Anyone wire a generator starter through a shunt?
Reply #34 –
Quote from: Jon & Loni  - 
I’m glad you guys are having fun. I don’t have a clue!  — Jon

Steve will be happy to explain it all to you when we met at the December Caravan.
It makes perfect sense to me, sorta maybe.

In the real world. either shunt will be perfectly adequate and produce similar results, as long as the shunt is large enough to pass the maximum expected amperage flow.

In our 2003 LD, the charging and battery system were upgraded years ago with four batteries, 400-watts of solar, a PD9270 converter (the largest that would still fit in the stock converter's location) and upsized cabling.
In driveway testing, a maximum of 72-amps was seen coming from the alternator and 68-amps from the PD9270. With  the solar charging system producing 18-20-amps, on a good day, the potential flow could be higher . The readings taken from a Xantrex battery monitor and a Fluke clamp-on amp meter. 
In this situation, the loads are high enough to absolutely need a 500-amp shunt, which is good since both the Xantrex and Victron battery monitors come standard with 500-amp shunts.
It has been a long time since the last time I saw a Trimetric monitor

Larry

Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
 
Re: Anyone wire a generator starter through a shunt?
Reply #35 –
Yeah I like to nerd out about stuff like this. The downside is I  probably agonize about it more than I need to :)
Steve
2003TK