Electronics > Projects, Designs, and Technical Stuff
"Battery discharger" issue
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bajticzek:
Hello, for my first post I'd like to introduce myself. I have finished electronics secondary school ~15 years ago, sadly I didn't attend university (girls were more important to me back then) although being accepted to one. I like to tinker a lot and I also work as electrician for all these years + I help my friend in his workshop as an autoelectrician. Right now I am in need of your help. I am working on a small project - battery tester - which basically discharges Li-Ion batteries in a controlled manner. Please have a look at this simplified schematic. All components are 1:1 as I soldered them to a perfboard.
I am powering the circuit from Rohde&Schwarz bench power supply which has 4 isolated outputs. One output is used for power, another one works as a tested battery. If I connect the supply output to BAT+ and BAT- terminals and gradually increase the voltage, everything is fine, I can read the voltage from ADC without an issue. Around 4.1 volts the mosfet starts to conduct. I can see there is a noise at opamp output which tends to become stable at 4.3V and is growing steadily. At 5 volts there is ~100mA current flowing through the mosfet although the value from DAC is set to 0 volts. So basically the voltage measurement works fine and no current is flowing up until one point.
In another scenario I connect the BAT- pin to common ground. Now there is no current flowing in full tested range, the ADC reads voltage fine from 0 to whatever voltage I try (in safe margin). But another issue arises - if I disconnect the "fake battery" (switch the PSU output off) - the voltage between terminals BAT+ and BAT- immediately jumps to 3.8~4 volts (I guess it depends on the mosfet parameter variations, it is a 4-channel device). If I leave the "battery" disconnected and short BAT+ and BAT- pins, there is of course no voltage across the terminals now and the circuit current consumption increases by 3mA. This wouldn't be an issue, but I need to detect whether the battery is connected or not before starting the discharge cycle. And if I have this 4V reading, then it cannot be done.
Any thoughts and any kind of help is very welcome. I can give you more information if needed.
bajticzek:
Whole assembly looks like this:
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