Can I use my Ni-MH batteries until the device completely stops working?

It is not recommended as this could lead to over-discharging the batteries, which is one of the main causes of shortening the service life of your batteries.

How about digital cameras which automatically shut down when the battery runs out of capacity? 
There is no need to worry about over-discharging the batteries. However, if there is a battery indicator, it is recommended to charge the batteries when the indicator shows a remaining capacity level of +-25%.

How about appliances (e.g. torch lights) which do not shut down when the batteries are low on capacity? It is recommended to charge as soon as possible when there are signs of the appliances not working appropriately or working slowly. Otherwise over-discharging the batteries could occur, which will critically degenerate the batteries’ performance and may seriously damage your batteries. 

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