Hello Rémy, and welcome to the forum.
The first step in understanding your battery is to measure it.
To do that you need a OBDLink LX or MX or MX+ Bluetooth adapter which plugs into the car's OBDII connector, an Android phone or tablet with Bluetooth, and an app such as CaniOn or hobdrive.
Your car should have an OBDII port - look under the dashboard above your knees somewhere around the steering column. You must find it and know you have it before you buy anything.
The car itself does not have Bluetooth - the Bluetooth is inside the OBDLink adapter. Be sure you get a genuine OBDLink LX, MX, or MX+ Bluetooth adapter, as cheap imitations do not work.
Once you have the hardware, install the app onto your Android device and follow the instructions when plugging in the OBDLink adapter and pairing it with your Android device. Do NOT try to use the OBDLink software.
Once you have paired the OBDLink adapter to your Android phone or tablet, open the CaniOn app or hobdrive app and hope it links up.
I am only familiar with CaniOn. It is very programmable - you move from screen to screen by swiping sideways. Once you get it working, you can configure the individual screens by selecting from the menus above.
The key information you are interested in is battery capacity (in Ah) and individual cell status (voltage) which with CaniOn is seen on the screen with the 88 vertical bars. Ideally these bars should all be at the same height, or at least within 0.01v of each other.
Let us know how things turn out for you.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV