9v batteries for Metal Detector

We have just been buying batteries as needed for this, but sometimes we forget etc and the metal detector becomes no good.

Do we all think it would be a good idea to get some rechargeable batteries for this?

3 options:

Cheap NIMH

Li Poly 550mah

Li Poly 700mah (expensive)

I’m not sure theres any great advantage to going more expensive? I think the detector changes its behaviour as the voltage drops a bit, thats why the fine tuning dial is there, but whether better batteries would help or not i’m not sure.

For comparison, a bulk pack of 9v’s:

(not very eco friendly i guess)

Feedback please.

Most cheaper chargers are only time based and can easily overcharge batteries.

Two of the battery chargers have no information about their operation (searching on google),
As such I would be inclined to go with the I-Power option (charger and two batteries to start with?)