i have a 5000mah 19wh and 3.8 li-ion battery i am connecting it to 2000 mah 3.7 volts li-ion battery in series to get 7.5 voltage.Now my question is that what will be the output capacity of the batteries 5000mah or something different?

 

It will be the capacity of the lower rated one. As the (3.7V 200 mah battery) gets to the end of the discharge cycle you can expect to get much less than 7.5V out of the series combination. If the internal resistance of the battery goes up you might get less than the 3.8 volts the other one is capable of. BTW don't under any circumstances try to charge them both in the series configuration.

 

And also, don't let the smaller one become fully discharged or it will be in danger of being charged in reverse -kind of a bad idea.

 

Thank you brother for responding to me.