High refresh rate smartphone displays seem to be all the rage these days thanks to a trend kick-started by Razer back in 2017. When I reviewed the Razer Phone 2 for its performance, I found that enabling 120Hz shaved a lot of hours off of its battery life. That device had a 4,000 mAh battery, yet with its 120Hz variable refresh rate enabled it could only manage a measly 5 hours 41 minutes in PCMark's battery benchmarking test. Although it doesn't natively support a high refresh rate, modders found that the Xiaomi Mi 9 can run at 84Hz, but if you choose to do this, your battery life is going to take a hit.

Xiaomi Mi 9 Forums

As a reminder, the hidden 84Hz refresh rate is definitely not meant to be exposed to the user. The Xiaomi Mi 9 was never sold with the intention of running at this refresh rate. Once it's enabled, you can't toggle a button to switch between 60Hz and 84Hz; you need to flash a file and reboot every time you want to switch it on or off. We don't know if this mod will damage your display in the long term, either. Still, due to the interest around this mod, we decided to do some battery benchmarks to see how it affects the battery life. While the results were inconsistent, they were enough to tell us that yes, enabling 84Hz will definitely shorten your screen on time.

Each of these tests was done using PCMark's Work 2.0 battery benchmark. I did these tests over a period of three days, and I charged the device in between each test and let it cool down before running the next one.

As you can see, the 84Hz mode resulted in shorter battery life in each case, though the gap between 60Hz and 84Hz varied. In the case of test 2, the difference is huge with just over an hour and a half between each result. In short, your mileage may vary. You may find that the battery life when 84Hz is enabled is serviceable for your needs, but if you already struggle with the battery life on your Xiaomi Mi 9, enabling this mod is certainly not for you.