Ammeter/ Volt meter adjustment

My cheapo combo volt/Ammeter was the 100 amp version and it was for a project no longer in my possession. It was Not able to read anything below 0.78 amps no matter how much I tried to calibrate it.

I had the Shunt mounted on the battery (-) Stud, and it had been filed flat and polished with 2000 grit sandpaper on all ring terminal mounting surfaces, and with a Dollop of CAig Deoxit shield applied to keep it at maximum conductivity.

It was just not a precise instrument.

For my project's purposes, it was adequate. I told the owner to NOt stop charging until about a half hour after the Amps read 0.0 at 14.4v. It would read 0.0 at 0.78 amps actual, and needed to taper to 0.625 amps before reduction of voltage, but it was close enough, and the 40 initial amps was more than enough to meet the Lifeline gpl-31XT 20% minimum charge rate when drained to 50%, or less.

It was able to read a 100 amp charging source pretty accurately though, compared to my clamp on Ammeter.

Must be the same one I got:

http://www.ebay.ca/itm/DC-0-300V-200A-Shunt-Voltage-Current-Panel-Meter-Digital-LED-Voltmeter-Ammeter-/271729368180?hash=item3f44551c74:g:~~0AAOSwc0FUqBWS

Note the display resolution is 1 amp. First you have to get the festering thing to show any amps at all! Landyacht warned about that. I get sporadic readings of amps, mostly says zero. Not sure what the matter is --connections ok.

Do I need to use fatter wire from display to shunt for the amps to read anything? The Tri was with a 500a shunt- this is a 200. I am using the same wire the Trimetric has at that distance (15 ft?) from the shunt. I got the voltage calibrated easily enough.

My meter reads from 0-200 volts and 0-200 amps. The shunt is 200 amps and 75 mv.

Your shunt is a resistor. Your ammeter is actually a voltmeter reading the drop across your shunt. If you know the shunt's value (usually expressed as Amp to millivolt drop) you can read the voltage across the shunt and calculate the Amperage. My shunt is 50 Amp to 50 mV; so if the volt reading across the shunt is 35 mV, my meter should read 35 Amps. The most common shunts are designed for 50 or 100 mV at max current reading.

Doug

what is your full scale ammeter reading? Do you have another vom? Measure your vehicle battery. Divide about your half scale ammeter reading into that voltage. Buy a resistor that size. Doesn't have to be exact. Once you have a resistor divide the resistance into your exact voltage. If you put the resistor in series adjust the pot for that amount. When you buy the resistor you will know about how many ohms. . It is a quick way for fairly close calibration of a meter. We're not looking for rocket science.

Post your full scale ammeter reading.

I square times R = P
E/R = I
E/I = R
I.R = E

Is the shunt wired to the battery post so that it reads all of the battery amps?

Gjac wrote:
Hi BFL13, is the 7 pin connector that you are referring to for a towable? I have a MH.

Yes. Can't say about a MH

Hi BFL13, is the 7 pin connector that you are referring to for a towable? I have a MH.

You can get in the ballpark by using an inverter with a 100w bulb in a lamp as the load, and the "divide by 10 rule" for the assumed amps draw of the inverter. 10 amps for the 100w bulb.

I have one of those meters and have not calibrated it yet. I plan to compare the amps on the meter with the amps showing on my portable battery charger's display.

BTW I would not be surprised if your 7-pin amps is only 5 amps. Mine starts out higher but very quickly tapers to around that.

A potentiometer for adjusting the voltage and amperage readings? That just seems wrong. Did they expect you to have a metrology laboratory nearby?