Charging battery from another battery ?

I want to charge 5V battery from 12V battery. I attached 7805 voltage regulator with 12V battery. How many 5v batteries can i charge with this 12V battery ?

What is the battery chemistry?
It is not normally advisable to charge batteries without a proper charging scheme, which you won't get from a simple series regulator, particularly if you want maximum life and performance from your batteries. Additionally, putting batteries in series or parallel to charge them can create problems, since the state of each battery could be quite different, or one might even be faulty. I would suggest that you look at some of the battery charging chips available from companies like Texas Instruments.
Cheers,
Richard

Adding to what Richard says, DON'T try to charge Li-ion battery its liable to blowup without proper current control.

As for how many 5V batteries you can charge, it depends on the mAH rating of the battery on the 12V and the 5V side. In short, it comes down to energy ratings not the voltage levels.

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10000mah Power Bank

Assuming you want to charge a USB battery pack that has a 5V input and already has a charge controller built-in, using a 7805 is not a good idea.

Even if you have enough output current, you are going to burn up nearly 60% of your 12V battery capacity in heat in the 7805.

You should look at the OKI78SR-5 switching replacement for the 7805. It can provide 1.5A of output current with almost no heat output.

If you are building a one-off unit, take a look at the switching regulator boards available from Amazon or elsewhere. A typical 5V USB battery pack requires 1-2 amps of input current during charging so you can calculate the number of units you can charge concurrently by dividing the max current for the switching regulator by the input current requirements of the USB battery pack.