1. I have a battery, a charger & a load as in ckt attached.
2. Have to glow led when charging happens. While charging current decreases while battery getting full.
Have conencted a small resistor in battery path whenever current will flow across it from charger to battrey, there will be drop across the resistor, which appears across opamp input. Since opamp is in open loop configuration even a small drop will saturate the opanmp & will glow led.
3. Will it work as intended? Havent tried in actual yet.
4. Most of time bothe charger and battery will be connected to load, once in a month mains may go then system shifts to battery.


Probem may arrise:
1. There is small vid diff due to offsets, so opamp may be saturating even if no current is flowing, so how to avoid this situation
2. Any other low cost solution based on voltage measurements or current measurement circuit will work only?
3. Low cost is must have, low power is better if available.
4. Any other better way to do that with/without cost constraints.

 

there are a lot of gotcha's with your circuit. Your better off with a high side currrent sense amplifier. See https://www.analog.com/media/en/technical-documentation/product-selector-card/Currentsense.pdf

and a comparator. e.g. https://www.analog.com/en/products/lt6700.html#

There is a single version available too.

 

You would need a rail-rail type of op amp such as an LMC6484..

You can insure the op amp is off with zero charging current by adding a 1k resistor in series with the op amp plus input along with a 1 megohm resistor to ground from the plus input.
That will give a zero current offset of 12mV to keep the op amp output off.

 

Attached is circuit.
1. Now sense moved to negative side, this will remove +ve rail common mode range of opamp, LM339 opamp has gnd in common mode range.

2. At +ve input of opamp, a small divider of 1K & 1M, is created so that voltage at +ve side is always lesser than of -ve side of opamp.