Very interesting thread as I am in the process of exchanging several “9V” 6F22 NiMh batteries that I use in my multimeters, thermo sensors, wall scanner, fire alarms etc.
I found that one cannot trust specifications like “true 9V output”. It is best to buy some models and test for yourself.
I have bought 2 types:
https://www.fasttech.com/products/1812000
This has 2 cells Li-Ion in series. The voltage is 7.2V and 8.4V when fully charged. There has to be a balancing circuit inside or else it will fail early (the charger for these has been called crap). My multimeters give up between 6.5V and 7.5V so this type of battery is not good for all of them. I think I read that the output end voltage was protected at 5.5V.
http://www.banggood.com/ZNTER-S19-9V-400mAh-USB-Rechargeable-9V-Lipo-Battery-p-1070703.html
This has 2 cells of Li-polymer in parallel and a booster circuit that deliver a very steady voltage in the range 9.3V to 9.5V (measured 5 pieces). It took 930mAh to charge one of the batteries with the build-in USB charge circuit. I intent to see how much it takes to refill after a certain time of no-use (booster keeps working?). This battery type can also be had in 1.5V versions, like Kentli (that HKJ tested positively), but with the smarter USB charging. The cells is said to have all nesessary protections and are “promised” to 3000 cycles. That has yet to be seen.
I have asked M4DM4X for a deal on ZNTER but had no answer yet.