I greatly enjoyed the previous comments about battery life in the Xhdata D-808 radio and how battery life varies so much and was motivated to run some tests of my own.
I ran some tests on my own various 18650 batteries. First I found the radio battery indicator would flash and the radio would not stay on at 3.403 volts, indicating I should test batteries with a load simulating the radio until they were discharged to about 3.40 volts. I found the radio tuned to a station, backlight off (the light greatly increases current draw), with low to medium volume drew about 50 mA and used that current for my test. I used two 150 ohm 1/4 watt resistors in parallel with an on/off switch to give such a load across the battery, tested outside of the radio of course. I bought four new Samsung "36G 10A 3600 mAh" batteries off eBay, charged them one time initially for the test. Those four batteries all lasted over 52 hours with the 50 mA load before dropping to 3.44 volts where I stopped the test for those batteries. The 2000 mAh battery that came with the radio, which I did not recharge after it had been charged maybe 6-8 months previously and taken out of the radio and let sit (its no-load voltage was 4.079V) lasted 28.5 hours. A Tecsun-branded 2000 mAh battery, also previously charged and let sit 6-8 months earlier, with no-load voltage 4.179V, lasted 31 hours and 15 minutes. Two old "GTF 9800 mAh" branded batteries that I'd used in flashlights were much poorer; both lasted only 11 hours.
The Samsung 3600 mAh batteries gave very long life but they are not inexpensive, almost $10 each with shipping and tax. Maybe other brands of 3600 mAh batteries would work similarly but I can't say.