Op amp help

johnny78

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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
 

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