Does Yongnuo YN622C TX/RX accept rechargeable NiMh AA batteries?

They do not "need" 1.5 volts, you get a tiny bit of oomph due to the minutely greater voltage.

Here is one quote in reference to the particular model you are using: "Due to their higher voltage it's often best to use good Alkaline Energizer or Duracell batteries rather than rechargeable NiMH batteries.".

Here is another quote, from SLRLounge "Best Rechargeable AA Batteries for Flash and Photography – The Ultimate Guide Part II":

"The short conclusion was that the Eneloop XX and Standard Eneloop not only performed better than all of the alkaline batteries, they were also more economical in the long run. However, if you had to use alkaline batteries, your best bet would simply be the standard Duracell AA alkaline batteries.".

SLRLounge's reasoning to use a (particular) Flash to test rechargeable batteries is: "The Vivitar 285HV has a very long full power to full power recycle time just by itself. This makes it wonderful for these kinds of tests since the results are a bit more exaggerated, and also because it allows the batteries and flashes to stay a bit cooler since the recycle times are more spread out.".

My opinion: Ray-O-Vac.

Everyone has an opinion of what to do and how to do it. So that this question is not opinion based, the fact of the matter is that rechargeable batteries will save you a small fortune, the power difference is negligible, and it's allowed for if the voltage regulator is doing it's job; plus it's environmentally friendly.

Do your research, choose a charger from the manufacturer of the batteries you decide upon (as opposed to a 3rd party unit) and be prepared to shell out U$40 to get started. You'll get your money back in 6-12 months if you use a lot of batteries, the only 'expense' is the charger and the higher cost of a few sets of batteries. You can swap them out when there's still a 1/3 of a charge left without penny-pinching them to the end because they are rechargeable.

I've been using the YN622C-TX and YN622C transceivers for about the last four years. I think I have gone through maybe three or four sets of alkaline batteries in the TX.

The transmitters and receivers don't use much energy at all. The transmitters are very low powered radios that only transmit a few milliseconds per exposure. The receivers are even more miserly in their power consumption. I do use the receivers much less frequently now that I have a couple of YN685 flashes with built-in receivers.

For the high drain flashes I use 2400mAh NiMH rechargeables. I've got enough NiMH rechargeables now that when the set that is in my YN622C-TX now dies I'll probably go to rechargeable batteries in it, too.

Like the instructions with most products from Yongnuo, sometimes the instructions lose a little in translation. I have a feeling someone misinterpreted the User Manuals that say they don't come supplied with AA size batteries (excluded) to be saying certain types of batteries are not to be used.

The User Manual that came with my YN-622C transceivers says:

Open the cover and install two AA batteries (excluded)¹ according to the + and - marks, rechargeable batteries of 1.2V can be used. Replace the batteries when the product does not work stably.

The User Manual for my YN622C-TX says:

Installing the 2 AA (1.5V) batteries (excluded).¹

There's no mention at all regarding NiMH or other types of rechargeable batteries.

The specifications in the user manual for the YN685 say:

Power Supply: 4xAA size batteries (Alkaline or Ni-MH battery)

_{¹ 'Excluded' here means the batteries are not supplied in the package with the transceivers. It does not mean the batteries listed are excluded from use!}