Android Question Data logger and energy support

I have developed a data logger for Android which is connected to the USB port. It works great so far but I have a problem. The data capture process takes more than 24h and I need to keep alive the phone all that time.

I discovered that if I connect a power source to the hardware of the data logger, the phone does not recognise this power source and thus the phones battery is not charged.

Any suggestion or idea to solve this issue?

Thanks!