I built this circuit (Simple Lithium-ion / Lipo Battery Charger) about a year ago and I remember that it worked fine. Recently I built it again but this time I’m having some problems. No matter what value resistor I use for R1 and R2 (currently combined into a 1 ohm resistor), the constant current always kicks in at about 7mA. In the attached picture, good.bmp, ch.1 is output voltage and ch.2 is the reference pin on the TL431. The output load is 1000 ohms and as you can see, it works as it should. In the picture, bad.bmp, the output load is 500 ohms and obviously something isn’t right. I’ve tried 3 different LM317’s and TL431’s (actually 2 of them are KA431’s) and I get the same results with all of them. R5 is a 50k potentiometer and R4 is 56k resistor. I have 100uF on input and output. Any idea what could cause these results? The input voltage stays at a steady 16v and nothing gets warm.
good.bmp (219 KB)
bad.bmp (219 KB)