Stick with external Li-ion rechargeables and you should be plenty fine. I have done this for years with no issues. These products are offered by several of the manufacturers such as ATN, Pulsar, Sightmark and Armasight and soon one will be available for and supported by another manufacturer.

If a rechargeable battery is employed externally, it has no opportunity to cause damage by gas or corrosion. Remove any battery while the unit is not in use and you will all but eliminate leaking and corrosion issues. That said, I do not use either alkaline or NiMH as a rule of thumb. If I use an internal battery, I use lithium.

I like using externals because I can obtain a large battery capacity in a reasonably compact package. While many TWS will run 4 - 6 hours, sometimes up to 8 or so, on a set of expensive lithium batteries....I can run anywhere from 14 to 22 hours depending on the site and external Li-on rechargeable battery capacity. I don't have to keep buying expensive batteries, just keep on recharging...or even carry a spare rechargeable. I promise that if you try doing it this way, most of you would be done buying batteries.

The OP has ordered his manufacturer supplied external rechargeable. Yes, the manufacturer supports and sells it. I think his real goal was to get away from disposable, one use batteries. He used the term NiMH, maybe not realizing there were several kinds. In the end, I think he has found the solution he was looking for and that is what counts.

Lithium primary batteries do not out-gas, which is why they are highly recommended by virtually all TWS manufacturers for primary use as previously mentioned.

If you use alkaline batteries, you will soon notice your battery contacts are covered with a white deposit covering the electrical leads, this is potassium hydroxide precipitating onto the battery contacts degrading and destroying them as well as the internal electrical circuits inside the thermal video camera assembly. Potassium hydroxide is a strong alkaline base and very corrosive to virtually all metals. It is NOT something you want to have in your TWS under any circumstances.

NIMH batteries, which the OP asked about in the title of his thread, should not be used in any thermal device.

Buuuump. A google search for batteries brought me back to THF. Can't get out of the wormhole I guess.

I am considering using Kentli AA lithium rechargeable batteries in my HD19. That is what a lot of the guys who run the X-Sight use, and they are having great results and no problems.