How to make a battery draining circuit?
Without knowing the details of this battery, it would be unwise to discharge it.

If it's a multi-cell NiCd or NiMH, for example, then it is constructed of multiple batteries in series to give the required voltage.  If you discharge this type too low, than you will deplete the weakest cell first.  When this happens, it's polarity effectively changes (only this one cell, mind you), and that cell is destroyed.

If, instead, it's a lithium chemistry, they can respond violently to being taken too low and then charged again.  Lithium cells generally have integrated circuitry to prevent this from happening.

If you can find out what kind of battery it is, there could be a good solution.  But don't try to discharge it too low until you're sure it's ok!



Edit: OK, I see your update.  A NiMH cell is 1.2V, nominally.  With 7 cells, this is an 8.4V battery pack.  Once the voltage gets down to 1.0V per cell, then 95% of the energy has been discharged.  This link has some good info.

One manufacturer cautions against discharging a cell below 0.85V.  However, I would only go down to 7.0V total (1.0V per cell), to make sure that the weakest cell has a little bit of cushion.

I think that the best thing to do, however, is to simply use the battery pack in your device until it starts seeming underpowered.  At this point, you've reached the standard depth-of-discharge that the battery charger is expecting...

I hope this helps!