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Old 07-06-2017, 03:37 PM   #1
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Default BMS Shunt size: what makes up "load"?

I'd like to get the extra 2nd decimal place on the voltage reading of a Trimetric battery monitor, like the Victron monitor supplies. The special Victron shunt with PC board is incompatible with the exposed underneath battery location of our Travato 59K. To provide 2 decimal places the Trimetric requires a 100A/100mv shunt, rather than a 500A/50mv shunt.

I calculate our usage/load wouldn't exceed 55A in the most unlikely situation. But do I have to consider current sources as well as loads? Solar and AC charger provide under 50A, but what about the alternator? The Travato has a 180A alternator. Is a 100A shunt too small, because current sources are in play, too?

link to Borgart shunt pdf

Bogart says the ratio is what matters, and suggests using a 200A/200mv, or 300A/300mv shunt to get 2 decimal places. Does anyone know where would I could buy one?
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Old 07-06-2017, 07:04 PM   #2
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Originally Posted by Saldar View Post
I'd like to get the extra 2nd decimal place on the voltage reading of a Trimetric battery monitor, like the Victron monitor supplies. The special Victron shunt with PC board is incompatible with the exposed underneath battery location of our Travato 59K. To provide 2 decimal places the Trimetric requires a 100A/100mv shunt, rather than a 500A/50mv shunt.

I calculate our usage/load wouldn't exceed 55A in the most unlikely situation. But do I have to consider current sources as well as loads? Solar and AC charger provide under 50A, but what about the alternator? The Travato has a 180A alternator. Is a 100A shunt too small, because current sources are in play, too?

link to Borgart shunt pdf

Bogart says the ratio is what matters, and suggests using a 200A/200mv, or 300A/300mv shunt to get 2 decimal places. Does anyone know where would I could buy one?
How is an ammeter shunt involved in voltage readings?
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Old 07-06-2017, 08:01 PM   #3
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I see what you are getting at cuising7388...my bad...

per Bogart:
"Requires one of two shunts: 500A/50mV shunt allows amp measurements from 0.1-over 400 Amps. 100A/100mV shunt allows amp measurements from 0.01- over 70 Amps. Larger shunts allow higher current limits. 1000A/100mV shunt (rarely used) may be used (with the “H” shunt setting)."

...So the shunt affects the amp measurement range capacity, not the voltage reading. I misread/hoped since I think 2 digits on the voltage would be useful.

From pictures of the meter possibly the Trimetric can only display decimal 1 digit. It seems more mV/A would provide more "needle deflection". If the meter can't show it then a smaller shunt provides no advantage, unless the calculations are more accurate in the device (FWIW).

Sorry for being mixed up. This will be hard to install on the Travato batteries. Maybe a Balmar SmartMeter is more my speed.
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Old 07-06-2017, 08:32 PM   #4
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Originally Posted by Saldar View Post
I see what you are getting at cuising7388...my bad...

per Bogart:
"Requires one of two shunts: 500A/50mV shunt allows amp measurements from 0.1-over 400 Amps. 100A/100mV shunt allows amp measurements from 0.01- over 70 Amps. Larger shunts allow higher current limits. 1000A/100mV shunt (rarely used) may be used (with the H shunt setting)."

...So the shunt affects the amp measurement range capacity, not the voltage reading. I misread/hoped since I think 2 digits on the voltage would be useful.

From pictures of the meter possibly the Trimetric can only display decimal 1 digit. It seems more mV/A would provide more "needle deflection". If the meter can't show it then a smaller shunt provides no advantage, unless the calculations are more accurate in the device (FWIW).

Sorry for being mixed up. This will be hard to install on the Travato batteries. Maybe a Balmar SmartMeter is more my speed.
For voltage monitoring of AGM/flooded cell batteries, a three digit (100mv resolution) is acceptable for guestimating the state of charge. For lithiums that have a flatter discharge curve, a four digit (10mv resolution) is necessary for a useful guestimate.

A candidate for your project is the Blue Sea OLED meter series. Most digital ammeters require inserting the shunt between the load and chassis. Blue Sea meters can alternatively be inserted looking directly at the source which can a practical advantage when dealing with loads with discrete multiple grounds.

However, since the Travato employs AGMs without the complexity of a BMS, I think the Trimetric unit should work just fine provided you can get the signal wiring from the meter display to the shunt without butchering up the interior panels.
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