Hi,

It's all in the maths. Your Kindle Fire HD 7" has a 4000 mAH battery.

If you are using the charger that came with it, it has a 1000 mAH output.

As a rule of thumb, not quite strictly true but close enough, with these figures it could take up to 4 hours to charge the battery from flat to fully charged.

(4000 mAH divided by 1000 mAH = answer in hours)

A lot depends on how low the battery was when it was started to be charged and also its' age. The older a battery gets (the number of times it has been recharged) affects its ability to fully charge and to hold its' charge.

If you were charging it with a PC connection (500 mAH ) then it could potentially take up to 8 hours to fully charge. (4000 mAH divided by 500 mAH = answer in hours)

Do not be tempted to charge using a charger with a higher output voltage and/or current than what is recommended by the a) battery manufacturer and b) device manufacturer, as the battery could overheat and either be damaged or even explode or start a fire.