I am also interested in whether the recommendation not to use rechargeable batteries applies only to 1.2V Ni-Mh batteries. To save on battery costs, I am also considering purchasing AA Battery Rechargeable USB Type-C Lithium-Ion 1.5V.

@Wayne-TP  What do you think about Lithium-Ion rechargebles batteries?

  @PeterL9 

Hi Peter,

After posting last week, I decided to buy some Li-ion batteries as I guessed I was unlikely to get a direct answer from tp-link. I bought Pujimax 3400mWh 1.5v off EBay and they were delivered this morning. Initial finding is that communication between the app and valve is normal. After raising/lowering the target temperature I can hear the valve open/close. Obviously it's too soon to evaluate the battery in terms of interval between full charge and depletion. To my untrained eye, the product appears as well made as European/US brands. I think the onboard USB 'C' socket is a clever idea. 
Fingers crossed I'll be still using these in years to come 😀

Thanks   @SteveWeir 

I think the same, I would like to buy: PUJIMAX 4255mWh with the USB-C.

I found very interesting site (akkutests. de)    whete the author make reviews of Li-Ion batteries. Very interesting to see the graphs.
It would be nice if the Tp-Link could say us, how high is the current load in mA of the valve...

  @PeterL9 

Thanks for the link to the battery test site.

The graphs are very good but my German language knowledge is not good enough.

I see the Li-ion battery voltage drops "off a cliff".  I guess we will have to wait before finding out how the Kasa App battery level indicator handles the lead-up to that event. 

If tp-link supplied the current load in mA, how would you convert that into real world battery depletion?  I have no expertise in this area.

  @SteveWeir

Theoretically, if we knew the current load of the valve in mA, we would know which curve corresponds to discharge (in the graphs, the colors of the curves correspond to different current loads) and we could estimate the battery life...

HOWEVER: I assume that the current load of the valve in mA is not constant - it varies between periods of rest, when it only transmits a signal, and periods of work, when it closes the valve...

If you look at the graphs for cheaper "Chinese" Li-Ion batteries, the discharge occurs in a jump, while with higher quality and significantly more expensive batteries, the discharge occurs gradually... similar to NiMH batteries...

So my assumption is that your batteries will show a full charge and then suddenly drop and be discharged... :-) 

Personally, I wouldn't consider it such a big problem... The important thing is saving money and protecting the environment when it comes to batteries.