Im building a test box that requires a rechargeable 9V NIMH battery, could I SAFELY charge this battery with a universal 9V AC/DC adapter??

 

Im building a test box that requires a rechargeable 9V NIMH battery, could I SAFELY charge this battery with a universal 9V AC/DC adapter??

No. It might be fine for the faster "bulk" charge phase, but you'd have no way to know when to taper off and terminate charging. Pop.

 

You can safely trickle charge it at 1/10 the rated amp-hour capacity. However, this way, you will not charge the battery to full capacity.

 

NiMH at 0.1C? I thought that was the max continuous trickle for NiCad, not NiMH, which is less tolerant to overcharge.

 

Energizer and a Japanese Ni-MH battery manufacturer say to trickle charge at no more than C/40. They say C/10 for Ni-Cad. But a Ni-MH battery has 4 times the capacity of a Ni-Cad so the trickle current is the same.

Some "9V" Ni-MH batteries are actually 7.2V (6-cells) or 8.4V (7-cells).
They are fully charged at about 9V to 10.5V.

Your 9V adapter might not have a regulated output voltage which might be 18V when the load current is low. That will destroy a 9V Ni-MH battery.