I have a battery application. With an 11.1 V battery input, I need to maintain a 12 V, 2 A output as the battery discharges.
I bought a 1500 mAh, 11.1 V LiPo battery pack with a 35 C rating and I fully charged and balanced it.
https://www.amazon.com/FLOUREON-1500mAh-Battery-m4-fpv250-Shredder/dp/B06Y5VJ13Y/ref=sr_1_4?s=toys-and-games&ie=UTF8&qid=1539483472&sr=1-4&keywords=floureon+11.1v+3s+lipo+battery+1500I also got an LM3481 SEPIC mode dev board.
The test setup
Battery feeds the input of the boost converter, the output of the boost converter goes to an electronic load.
Load is a 1.2 A constant current load.
The problemThe test starts off fine, but quickly becomes out of control. The battery input current keep going up and up, within 7 minutes and battery current reaches almost 4 A and rising. The output stays at 12 V at 1 A.
What I think is the issueI think this is due to the internal resistance of the battery. Once the current starts to flow, the voltage drop across the internal resistance will cause the input voltage to drop, therefore the boost converter will have to draw more current to keep the output current going, then the resistance would drop even more voltage, causing more current to be drawn. This just keeps happening in a "runaway" unstable system.
Possible SolutionRegulate the input current. Put a max input current based on the current load. If the input current gets too high, control the boost converter via a DAC to lower the output voltage, and keep the input current at that max value.
But that max input current draw will depend on what the load is. If the load requires an input current of 500 mA, I would want the input current to limit to be around 500 mA, but if the load changes, so does that set point.
Questions1) Is this a common problem using boost converters with batteries?
2) Did I just buy a bad battery?
3) What would work for this application?
4) This battery that I bought seems like it has really high internal resistance. Like 1.65 Ω (for 3 cells in series). I put on a 3 A load and the battery voltage drops very low!
Sorry I don't have exact numbers on that, I didn't write it down. What is a normal internal resistance for a LiPo battery? For a 35 C rated battery, there is no way it would be able to provide 35 times its capacity and have any voltage left over for the load. It is a 1500 mAh battery.