[MOVED] Op-amp based current sensing

Hi, I’m trying to build a Li-ion charger and finally found a chip that suits my needs (namely low cost, low complexity and one that can charge a 12V or 4-18650 cells in series Li-Ion cells). The LT1513 works great because it can operate above, below or at the battery voltage. This reduces the strain on building a power supply with high voltages. The only issue I’m facing is charge termination. The LT1513 has a shutdown pin that shuts the charger down when pulled low. I looked at some charger protection and controller ICs but they are too complex for my liking and do not give me options to switch battery configurations. The next best thing I thought of was an op-amp based current sensing and using the output of the op-amp to shut down the charger once the charging current has reached a specific level.

I’m fairly new to op-amps and was hoping someone might guide me a bit in the direction of current sensing op-amps and some of the most commonly used ones. The typical charger current is 500mA and the current sensing should be at 10% of this charger current i.e. 50mA.

Without thinking about this too much, you could have a small resistor in the charging path. You could then use an opamp, configured as a differential amplifier, to measure the voltage across this resistor(which is proportional to charging current). You have to consider the voltage levels involved to get this all right (common-mode voltage). But it's Sunday and I'm not thinking too hard.

a schematic for the common method of using differential amplifier with series resistance . the output is Vout=A(I*R) , A is the gain adjusted by resistors check wikipidea on opamp for more details .

link for schematic: