I cannot run rpi 3 from 8 battery (14500) Series circuits 29.6v. I am convert 29.6v through dc-dc converter (XL4005 DSN5000) to 5v. The converter output can be 5A. But I cannot take more than 0.5A and rpi 3 cannot start
Here's a link to converter
The PSU recommended is 2.5A for the pi3, so, you'll need to increase the current.
You'll need to find a battery that is suitable to output high current. I know, for example, some 18650 batteries can output 20A on stable discharge.
It will depend on the specific properties of the battery model used.