Will the Eneloop Pro work or any NiMH at 1.2V?
What are you using for batteries in your Uni-T UT139C?
Just standard AA Energizer currently , also use the Duracell AA
I have had issues over the years with NiMh 1.2v cells in different devices eg Camera gear etc , so I avoid them nowadays. I do have some 1.5v+ Pure Energy Rechargables but was worried about leakage as the Batts sit along time in a Meter so I didn't use them.
According to the Specs at 2.4v it will work but I am not sure if accuracy is effect as it turns off at 2.2v according to the Specs.
Perhaps try emailing Uni-T and ask for more details especially the Upper limit and if you can use the 1.8v Lithiums...
KB.