Hello! I am working on a simple project, a device to discharge my lipol batteries. Fully charged, they have 4.2V per cell, and I need to discharge them to 3.85V per cell for storage. I am using 2 cell batteries, so they have 8.4 V fully charged. This is my circuit, which I built on a breadboard:
The idea is, that the TL431 will provide 3.85 volts, which is compared by the comparator(LM2903) to the battery voltage divided by two (2 cell battery). If the battery voltage is higher than 3.85 V, the mosfet will turn on and burn the energy on a power resistor (the resistor is actualy only 4.7 ohms, forgot to update the schematics). The problem is, that the comparator only puts about 2-4 volts on the mosfet gate, which is not enough to turn it fully on. If I disconnect the mosfet and measure the comparator output voltage, it is at saturation, about 8V, but when I connect it to the mosfet, it drops. I measured the voltage drop of the battery when directly connected to the 4.7 ohm rezistor, it dropped from 8V to 7.99V, so the voltage drop of the battery should not play a significant role. And another observation, I just tried to connect my only 3S battery (11.1V) to the circuit, which basically means that on the noninverting input, there will be about 6V instead of 4V and it worked flawlessly, mosfet turned on completely. So the problem seems to only arise when the voltages on the comparator inputs are close to each other. What am I doing wrong?
Home
Help
Search
Login
Register
