Charge Indicator Circuit help

[IMtheS]

Guten Tag!

I'm planning to do a project, wich contains a 3Cell Li-Ion Battery Pack with a Charge/Balance/Protection board (13.5V Charge Input).
The main load is controlled by a MCU that needs to know when the battery is beeing charged. The LED will be replaced by an optocoupler, connected to the MCU.

I came up with this circuit:

Will this work fine? Any suggestions?
(I'm pretty new tho electronics)

Cheers IMtS

 

[Harald Kapp]

This may work or not, depending o the tolerances of the components. difference between 12.4 V and 13.5 V is not easy to ditinguish with a zener diode only. A more precise circuit would involve a stable reference and a comparator. A TL431 or TLV431 has both in one chip. Chapter 10.2.1 in the datasheet shows a suitable application.
With a little modification it should work:

Use the additional resistor from the REF pin to ground to create a voltaeg divider with Rin such that for 13 V the input voltage is higher than 1.24 V (the internal reference).
Calculate Rsup such that Iled = (Vsup-1V)/Rsup.
Vsup is the supply voltage, so it is either 12.4 V or 13.5 V which will give you a bit of tolerance in the LED current, but that shouldn't matter. Or use the 5 V (3.3 V) of the MCU as power supply.