I have a Motorola V550 cell phone. When I use it as a modem, it drains
its battery quickly. So, I decided to make an auxiliary power supply
for it, to be used when I am away from AC power.

The phone's AC adaptor supplies 5.81 volts when the phone is not
plugged in, and 5.61v when the phone is charging. The phone starts
charging when supply voltage reaches about 4.8-4.9 volts.

I bought a battery pack that has about 6.7 volts when fully charged.

I would like to know if, in your opinion, it can be simply connected
to the cell phone directly. Or is 6.7 too much.

If, in your opinion, 6.7 volts is too much, an easy option to consider
is to just add a diode like N4007 for a 0.7 volt voltage drop.

Any thoughts will be appreciated etc.

i