Can i connect a 9v, 600 mah battery and two 1.5v, 2500 mah batteries in series and connect it with arduino uno r3 board, is it okay for the board or not also for what time would it last for. As I am connecting a 9g servo motor, an 1602 LCD display with i2c and 2 or sensors, and only 9v is not enough as the LCD keeps turning off and the servo doesn't work at all. Pls help
- Always show us a good schematic of your proposed circuit.
Hi, @adarshtechs
Welcome to the forum.
Putting batteries of different capacity in series is not a good move, the batteries will not discharge evenly.
Tom.. 
Why would you want to do that? An Uno will happily run on 9V. It will however not run very long on this type of 9V batteries:
I suppose your problem is not the voltage but the current. And more voltage makes it even worse when you use the internal linear voltage regulator of the R3 board (Vin).
You should never use the UNO R3 as a power supply if you need more than a few mA. Your servo and your LCD need a separate 5V supply ( maybe a separate buck converter to convert the 9V to 5 ). But 600mAh will definitely not last long.
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You can, but that would end with destruction of the 9V battery.