View Single Post
Old Jun 28, 2010, 06:56 PM
jj604 is online now
Find More Posts by jj604
ancora imparo
jj604's Avatar
Melbourne, Australia
Joined Jul 2005
8,250 Posts
The Quanum telemetry unit - a useful idea: Part3

How accurate is it?

It doesn’t matter how clever it is, if this thing isn’t accurate enough it’s a waste of time. There are a number of cheapo Wattmeters out of the Chinese copy factories that are so inaccurate they are dangerous. A meter that lies to you is a menace in my view and often worse than no meter at all.

Cell voltages
The easiest thing to test is the accuracy of the individual cell voltages. Using a 40,000count 0.05% accurate DVM I checked the cell voltage over the range 3.3 to 4.2V. The display resolution of the Quanum is 10mV – that is it reads to 0.01V best precision. One cell was within 0.8mV of the displayed reading over that range (i.e. much less than the resolution), the worst was 25mV different. In other words the very worst case was less than 3 display digits out. Perfectly acceptable.

I then measured pack voltage, current and mAh for two discharge rates on a 5000mAh 3S battery. The first was a short run at about 5A discharge to check accuracy at low current, the other a full discharge at 20A. I put the Quanum in series with my EmeterII which was recently calibrated and I know to be fairly accurate. This means the current through both is identical so the mAh should be as well. The voltage measured will be slightly different because of the small drop in the leads.

The x axis labelled “Time” on the graphs is just when readings were made so is not necessarily a linear time scale.

Pack Voltage
The pack voltages at both 5A and 20A were measured and compared with the Emeter over the whole discharge. They were very close.

Current error at 20A shows the Quanum current plotted on the y axis and the Emeter on the x axis. The black line is what the reading would be if the Quanum agreed with the Emeter exactly. The Quanum reads about 0.8A low at around 20A. That’s 4% error.

This is the one I was really interested in. mAh at 5A and mAh at 20A shows the comparison between the mAh readings of the two meters. I plotted an error graph as before for the 20A run mAh readings. See mAh error at 20A. The actual readings were 4774mAh for the Quanum and 4576mAh for the Emeter which is 198mAh difference at the end of the discharge which took about 13.5 minutes. That translates to a current reading error of 198*60/13.5 = 0.88A. The 5A test gives a higher error but the numbers are low and the test was fairly short.

Conclusion is that the major error with this particular Quanum is in reading the current but it’s less than an Amp for the 20A test which translates to about 4-5% error.

Be nice if it was better but still perfectly usable as an in-flight battery usage gauge. Interestingly, the Quanum shows about a 0.2A current when none is flowing. See picture NoCurrent. The Hall effect sensor used in the Quanum has a much better accuracy than this - it is used in the AEO-RC Wattmeter P1 which is quite accurate - so what would make this thing really nice is a user calibration feature.
jj604 is online now Find More Posts by jj604
Last edited by jj604; Jun 29, 2010 at 03:38 AM.
Reply With Quote