jonnyd91

I purchased the following battery and charger.

• Thunder AC680 Professional Dual-Power LiPo Balance Charger/Discharger w/ AC Adapter for 1-6 Lipo/ 1-15 Nimh USB to PC Software
• SKY LIPO 5000mAh 40C 7.4V Hardcase Lipo Battery Pack ROAR Racing Approved

The past 2 times i have charged the battery the ending voltage on the charger was 8.4 however when I measured the voltage on a multimeter it read lower.

Last charge I did was a balance charge. When the charge was done the ending voltage was 8.4 and the cels were 4.19/4.20.
When I measured it on the multi-meter it read 8.32V and 4.16/4.16 per cell. I also noticed the last time I put the battery in storage mode it read 7.6v off the charger and 7.52 volts on the multimeter.
That is the same .08V discrepancy. Is this an issue? I guess it better it reads lower on the multi-meter then higher. Would there be a problem with the battery or charger?

I could have sworn the very first time i balanced charged the battery the voltages checked out. Battery has only been charged 4 times. For reference I checked the voltage of a new AA battery and it read 1.6V.

tacx

My Feedback: (1)

Not an issue. Measuring devices will vary. Unless you are using high end equipment that has been calibrated with lab equipment your going to get differences in readings.

chuckk2

There is a bit of an accuracy problem in measuring 4.2V.
First, most "pocket" DMMs read 4.2V on a 20V scale.
For giggles, using 1% accuracy on a 20V scale gives us .2V.
Many DMMs do better, so I'll use .1% Usually, there is an additional % of reading as well.

In actual practice, most of the DMMs have a display that reads 4.2V as
4.20V. This is automatically +/- one LSD, or .01, which can worst case, get added to the
.02. So now we have a possible error of +/- .03 before getting into the additional % of reading.

Leaving that for a minute.
I have three identical pocket DMMs that read 4.2 as 4.20 (two decimal places)
Using a 4.2000-4.1999 precision voltage standard, the DVMs actually read 4.20 (best one) 4.19, and 4.17- 4.18
Typically you cannot use the voltage standard to directly check a charger or battery checker's accuracy, since they load
the cell to some extent by intent. A DMM is thus necessary as a transfer measuring device.

One battery checker and one of my chargers will read to three decimal places.
The charger and the best DMM readings are within .005 of each other at 4.205 from the charger.
The battery checker is within .010 of that in most cases. (good balance connections, etc.)
I did find out that a lipo balance connector's pins may not be making as good a contact as
one might like. Cleaning balance connector individual female pins one by one, re-springing the wiping parts of the pin contacts,
and so forth can make a big improvement in both the accuracy of cell voltage readings, and improve the overall
battery balance when balance charged.

In conclusion, a reading of say 4.18 to 4.22 in a cell that is exactly 4.20V is quite possible.

chuckk2

"Not an Issue" Well, yes and no. It's common to accept a reading as perfectly accurate, even though it's not.
When all the possibilities stack up, you can see significant differences. What they mean can vary between nothing in reality, to something that is a possible problem.

A long time ago, I had a QC related task to develop a way to measure contact resistance on a go/no go basis for a production line.
The device containing the contacts was a miniature push button momentary or latching switch with gold plated wire wipers
and a ceramic sliding part with sputtered gold lands. It was intended for a very high reliability, low voltage, low current application.

Accurate laboratory measurement was complex, involved some expensive equipment, and time consuming. (As you mentioned)

When all was said and done, a very stable current source, a limiting diode (for open circuit voltage limiting)
and very accurate small value resistors, along with a bridge circuit and an op amp got the job done.
Accurately and reliably measuring milli or micro volts is not that easy, unless you limit all the "gotchas".