Which are better for the F5 -rechargeable batteries or Alkaline batteries ?
Just comparing the numerical values of rechargeable batteries and alkaline batteries will not answer the question.
An example:
A normal Alkaline battery has more than 2000 mAh and 1.5 volt.
Modern rechargeable batteries have up to 1800 mAh -but only 1,2 volt.
The mAh value says how much energy the battery can store.
A higher mAh value means a higher value of stored energy .
The mAh value can be compared with the capacity of a car gasoline tank.
You can drive more miles, with a higher tank capacity.
With a higher mAh value you can use a portable cd-player longer.

The voltage (indicated in volt) is the strength which the battery needs to "pump in" the stored energy into the F5 .
The electrical resistor (summary of all in the F5 installed electric parts ) against the battery power is to high for a single battery.
The added strenght of 8 batteries is neccessary to overcome the electrical resistor and enough force to " pump in " the stored battery energy into the F5.
The F5 "take"from the battery the stored energy. Over the electrical circuit flow the unused energy, back to the battery .
If you compare the numerical values of Alkaline batteries with the values of the rechargeable batteries, you will find out that the Alkaline energy source is stronger .

But the Alkaline battery isn't the better solution for the F5 .

Why? You can learn this here.