Op amp help

[johnny78]

hi Guys
i was trying to fix my battery charger & i want to understand how the Op amp detects the battery level & start or stop charging

i've tried to build this simple lm358 circuit & yes when i connect it on my charger it works as expected
when the battery level is under 13,8v the output led turned on & when it reach 15v it turns off
But
when i connect the wire from R3 to battery negative the led keeps off & when i connect it before the shunt resistor it works
why is this happens ? & what is the setup of the lm358 connection name is ?
& how can i isolate the lm358 power from the reading source?

thanks guys
Johnny

 

[danadakk]

Remove

 

[johnny78]

after some readings its a low side current sensor circuit
Trying to understand how this works

 

[danadakk]

I think there may be a schematic error absence of load ground point....?

LM358 has no fdbk from output so being used as a comparator.

Vni is at ground so switching the LM358 would need Vshunt thru R2 & R3
divider to move Vinv of LM358 to move below ground, so again circuit
diagram not right ?

 

[johnny78]

hi
i've checked the circuit again & there is 2 mistakes
the c1 capacitor as fdbk i think & the power source of the lm358 is 15.1v fixed & the chargers positive has its own source
the Led is to indicate empty battery connected
i thought it was checking the battery voltage But it looks like its checking current draw

thanks for help

 

[johnny78]

hi
i've checked the circuit again & there is 2 mistakes
the c1 capacitor as fdbk i think & the power source of the lm358 is 15.1v fixed & the chargers positive has its own source
the Led is to indicate empty battery connected
i thought it was checking the battery voltage But it looks like its checking current draw

thanks for help
View attachment 198639

i was searching for such circuit to detect current flow but i cant find anything else current measuring which its not my case i guess