2019 CX-9 Battery low, likely needs replacement and liftgate not working

Hi folks. I have a 2019 CX-9 which yesterday failed to start on me for the first time.

Summary of what happened:

- Car failed to start. Dashboard indicator lights kept flickering. Dashcam powering on/off quickly.
- Used multimeter to check car battery voltage, and it was low, about 7.4V.
- Used an emergency jump-starter and got the car started within minutes.
- Drove around for about an hour and no problems ...
- ... except that power liftgate not working at all. No beeps even.

What's weird is though moments before trying to start the car, I wanted to load some stuff in the trunk and used the button on car's rear to successfully open and close the power liftgate. But right afterwards, car won't start as mentioned above. Since then, power liftgate hasn't worked even though the car itself powered on.

My dilemma now:

Since the car is 4.5 years old, I assume car battery just needs replacement, which I'm fine doing myself. But I don't know how to fix the power liftgate issue. Some folks online have mentioned that disconnecting/reconnecting the car battery helped 'reset' the electronics, and got the power liftgate working again. However I then came across another set of online posts where people said that when they disconnected & reconnected their CX-9's car battery (or installed a new battery) without using a 'memory saver' (https://www.amazon.com/dp/ (commissions earned)), their car's electronics showed malfunction warnings because the car didn't expect all its electronics to *completely* loose power (example 1, example 2). So I'm worried that if I try to disconnect/reconnect the battery to try to 'reset' the power liftgate, I might face other issues.

Any suggestions on what I should do ? I can take my CX-9 to a Mazda dealership to have them replace battery and fix the power liftgate issue, but they'll most likely charge me a whole lot more for something I could probably do myself.

Needs suggestions on what to do.
No need to take the car to a dealership to replace a battery, they will charge you an arm and a leg for the battery and the labour - it's just not worth it. The first thing I would do is change the battery to eliminate that as a potential cause for the liftgate issue. Replacing the battery could very well resolve the liftgate issue by itself. I've replaced my battery with one from Costco and they work great, but battery prices have gone up significantly, and depending on where you live, Costco's warranty may not be as good as it used to be. So you might be better off shopping for a battery at Walmart instead. Ensure that the battery is installed correctly. There are lots of videos and guides on the internet to use for reference.

After battery replacement, you may have to reinitialize your power windows. For this you just roll each window all the way down, then roll them all the way up, holding the switch for 3-5 seconds after the window closes. Do it one by one to make sure it works. Besides that, you may have to reprogram your radio favourites, and you'll lose your current trip meter records. No big deal.

If the liftgate still won't open, check to see what the car does when you try to open it with the button on the dash, the outer switch on the liftgate, and the keyfob. If it beeps when you try to open it, it's probably just a blown fuse. If it doesn't beep, try manually opening the liftgate from the inside (instructions in the Owner's Manual). Gently open it all the way, then close it all the way. Try the liftgate again, and if it still doesn't open, disconnect the negative terminal for 5-10 mins, then reconnect it and try again.
Thanks @sm1ke for the detailed reply.

But what are your thoughts on any other things resetting when the battery is disconnected ? See the following two threads as an example:

I totally agree that I should just replace the battery on my own, and I feel it's better and cheaper this way. But then I fear this may cause other things in the vehicle to not work correctly. Resetting things like trip meters, radio favorites is totally fine, but what about if it resets other things, like car odometer reading, or the VIN number, some sort of car electronics configuration/calibration done by factories for any car functions (e.g. radios, sensors, etc.) ?
It's very unlikely that any of that stuff would happen with the CX-9. In the first example the owner has i-Stop, which requires a special battery. The CX-9 uses a regular battery. The other malfunction mentions a parking brake error message, but others mentioned that they may just have to put the car into maintenance mode and then exit maintenance mode. Very simple to do. But again, this was on a 2016.5 CX-5.

For your peace of mind, I will mention that I once forgot to turn off my dashcam when I left for a week long trip. The battery was dead when I came back. I recharged the battery on a trickle charger and started the car. All I had to do was reprogram the windows and radio presets, and I my trip meters were reset. Odometer readings, i-Activ safety settings and preferences, windshield and camera calbrations, everything was working fine. Unless you accidentally connect the positive wire to the negative terminal and the negative wire to the positive terminal during the battery install, you should be fine.

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