[deanhills]

so my idea is to supply the phone with power to maintain 50 percent charged state and keep using the otg. i mean even if you use external power to run otg connected devices, the phone will use energy and keep discharging.

OK I think I get it now.  Something like this - B or C:



I found this discussion  - Can you use USB OTG and charge at the same time?


Some of those who responded were confused.  Some said "no".  Others said "depends on the device".

Apparently one has to check first whether the device is otg compatible - which I'm sure you've already checked - but just for the benefit of others who are looking at doing this - one checks it with a Google app:
https://play.google.com/store/apps/detai...otgchecker

You then need a special cable that works with otg.

For the simultaneous charging to work your tablet must support "accessory" mode and you have to get an accessory charge adapter.  Gets further tricky with how to get the right voltage.  I'm sure you'd be able to swing it being a super tech.  Would be great to let us know when you've found a way to accomplish your goal.

Here is a best answer to the question I found (got a green tick against it) - note your device needs to be able to be compliant with "accessory mode":

In case (3) the tablet will take charge as from a regular 500-mA port. However, there will be no any USB connections.

To have your tablet to be charged and simultaneously be in host mode, a special arrangement is required. In USB charging specification it is called "accessory charger adapter". For some more details you can see this item. However, your tablet must support this "accessory" mode, which may not be the case.

Source (check for green tick against comment): https://superuser.com/questions/1136336/...ve/1136353