Finally got around to testing some internal battery options for the '817 using the common B6 mini charger.

I have been using Panasonic Eneloop Pro (black) batteries for casual operation for a year.

They are rated 2550mAh
I measure: 2500mAh to 7.2V @0.5A (0.9V/cell)
and
2400mAh to 8.8V @0.5A
Total pack weight (inc tape, wires) 249g

Aliexpress batteries "DEAH 3000mAh NiMH LSD" measure at 1850mAh after 5 cycles. (250g) for one set and 2020mAh for another set. I may have over charged the first set once. Certainly not 3000mAh. Capacity is not increasing during the 5 cycles.

I also got some Aliexpress Li-Po 506090 cells sold as 3200mAh. A perfect fit in the battery hole. They arrived marked 4200mAh and test at 2450mAh.
At 186g they are a pretty underwhelming. For all the charging hassle, I will keep using the NiMH.
Also have fairly high R partly due to the individual protection circuits, which have 2x8205A fets in series for 25m x 2 x 3 = 150mR
Calculate to146Wh/kg. Good 18650's should be ~230Wh/kG and L91 disposable lithium rate at 288WH/kg ( but I have not seen any actual test results)

Charging:
from
"A charger that applies a 0.1 C rate for 12 to 14 hours is well suited for NiMH batteries"
i.e. 255mA vs my '817 measured 260mA
"maintenance (or trickle) charge rate of less than 0.025 C (C/40) " This would be 63mA. The radios measured 10mA is below this.
(n.b I calculate 138mA + 14mA trickle , from the schematic component values R1435=4.7 and R1436=47ohm, need to check actual value in my radio sometime)

Expected Life:
Energiser say: "However, battery life is limited to 5 years or less." and "Typical expectancy life is 2 to 5 years."

[BTW, don't leave batteries in during long storage or when they are old. When I bought mine, battery slime had gone all the way up the wires to the pcb and corroded it.]