Alternator output

[Fred Mc]

I am looking at a combo amp/volt meter that uses a shunt. Will a 200 amp shunt be sufficient for 2- 6 volt golf cart batteries in series? My bus is 12 volts.

Thanks

Fred

PD 4106

[Sean]

The shunt should be sized for the largest charge source and maximum load.  It has nothing to do with the number of batteries.

If your alternator is 270 amps, for example, then you need at least a 300-amp shunt.  Likewise if you have an inverter with a 5,000-watt surge rating, you will need a 500-amp shunt.

-Sean
http://OurOdyssey.BlogSpot.com

[Fred Mc]

The meters also come in various ranges. Seeing as how the prices are the same is there any advantage/disadvantage of a 200v300A meter over a 20v300A?

I doubt the voltage in a 12 volt system would go over 20 V.



Thanks

Fred

[Len Silva]

The ammeter doesn't care what the voltage is (within reason), they are two separate functions.  When you purchase the ammeter, be sure to specify the cable length from the shunt to the meter.  Do not cut or re-terminate that cable, just coil up any excess.
The ammeter across the shunt is actually measuring millivolts and the cable length is an important part of the meter's calibration (much the same as the recent thermocouple discussion).

[Sean]

The meters also come in various ranges. Seeing as how the prices are the same is there any advantage/disadvantage of a 200v300A meter over a 20v300A?

I doubt the voltage in a 12 volt system would go over 20 V.

If this is a digital meter, it probably makes no difference.

If this is an analog meter with a dial and a pointer, then you want the 20v item, as you will get a much more accurate reading.  With the "normal" range of a nominal 12v system being 11-15v, that will be 20% of the meter's total deflection (between 55% and 75% of full scale), whereas on a 200v meter it will be just 2% of the total (between 5.5% and 7.5% of full scale).

Even some digital meters will be less useful on larger scales.  A 200v meter with 3 digits in the display might show only whole numbers from 0 to 200 (in other words, no places after the decimal point), whereas a 20v meter with the same number of digits will read from 0 to 20v in tenth of a volt increments, much more useful on a 12v system (there is an enormous difference between batteries at 12.0 volts and batteries at 12.4 volts, but a meter with no decimal will shoe them both as "12").

You should find out how these meters display fractional volts before making your choice.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Fred Mc]

On the issue of cable length,Doug,I am looking at a 20V300A digital meter with a shunt. But it only has a 30 cm(1 ft) lead from the shunt to the meter. I want to go from the battery compartment into the bus. Can this be accomplished. I understand the length of the wire is important. Can you extend the wire but do a recalibration of the meter.( i.e. if the wire is 30 cm long and the battery reading is 12.5 that corresponds to a wire that is 10 feet long and the reading is 11.5)

Thanks

Fred

[Sean]

Fred, whether or not the meter can be calibrated to the shunt depends on the make and model of the meter.

Also note that shunts measure current, not voltage.

The larger the wire gauge you use to connect to the shunt, the lower the error will be.

-Sean
http://OurOdyssey.BlogSpot.com

[Fred Mc]

Thanks Sean.
Fred.