Mittwoch, 6. Januar 2016

Improved Data Logger for Charger Measurements

I had some spare time during my vacation so I finally built a more solid version of my shunt and voltage divider "breakout" board which I used to measure the performance of LiIon chargers with.
I soldered everything onto a small breadboard and adopted the programming a little ...

Improved Data Logger for Charger Measurements.
As you can see, the plenty of loose cables are gone. Everything is statically soldered together so that no distortions should be possible anymore.

During the first tests, some error spots are still left. The two DuPont cables that lead from the charger to the board don't make solid contact, need to solder my own ones. And when moving the plugs in the USB ports of the computer, the voltage and current delivered are changing a little as well.

The first test: The DuPont cables for connecting
 the charger need to be replaced.
But the main goal was to minimize other distortions and that worked great. I now use a 10 mOhm resistor which can stand 1W of heat dissipation and has a <= 1% error. The ADC (ADS1115) is connected via 10 kOhm resistors to its ends, ensuring very little bias. The voltage divider consists of two 100 kOhm resistors, where the connection to the ADC has again a resistor of 1 kOhm. The OpAmp for fetching the amplified differential voltage from the shunt is removed, I'm now using the built-in PGA of the ADC to get a good resolution. 1mA current over the shunt equals 10 µV, while the resolution of the ADC is ~7µV. I should probably add some oversampling as this signal has true noise and can improve this way.

Update 22:00h: Soldered wires with better contact to the board. This helped the resistance of the whole circuit to drop by 10% to now ~90 mOhm (including the battery with its inner resistance).
Also used a USB hub so only one connection is used on the computer. Turns out that the plugs show similar behaviour on the hub when the computer is moved - you can clearly still see every movement test.
The charger seems to get even less disturbed looking at the charging curve. >1A until 4.20V are reached and then the current drops slowly, while the voltage rises up to 4.25V for a short time. This is all within LiIon specs and looks quite nice.

Keine Kommentare: