Fiat 500e charging EV discharged battery below "standard" charge level

Disclaimer: i'm a hobbist, not an actual "battery expert", but with common sense you should be able to revive batteries. Please note, lithium based batteries alter their internal cell structure if overcharged / discharged too low / physically abused. The below description is "theoretical", I assume no responsibility (do note, overly discharged lithium batteries present an extreme fire hazard, and should not be used)

In short: you need to get each cell up to a certain voltage level, for the BMS / controller to "accept" the battery pack.

There will be a BMS (battery management system) / battery controller between the battery and the "load" (in this case, let's say the electric motor). If the BMS / controller thinks the batteries have a too low voltage, they will just throw up an error code, and don't allow usage of the battery pack (this may limit the use of the vehicle). As you have access to the battery cells, you will need to check the that each cell is above a certain voltage, for the BMS / controller accept the battery.

In my experience, with hobby grade lipo / liion chargers, battery charging could be performed if the batteries were around 3.2 volts. You will need to check the specific data sheet for the battery (if available), an adjustable power supply (voltage AND amps), a multimeter (to check voltage and amps flowing) and patience to get all the cells up individually.

You need to check the battery data sheet for voltage levels - the above example of getting the voltage up to 3.2 volts was used for batteries with 3.6 or 3.7 volt nominal voltage (or regular lipo / liion).

What you do is just apply 3.2v with a max amp of eg. 200mA to the battery from the adjustable power supply (I'm assuming about 2000mA capacity for a cell, so it's safe charging current imho - to be safe, you can go lower with amps). The aim here is not to charge each cell up fully, but to get them up to a level where the BMS / controller will see the pack as a discharged, but usable battery.

If you cannot get to he individual cells, you can still apply the same approach to a single module - however you'll need to figure out either what the actual voltage of a module is and/or how the batteries are connected within. This is important, as you don't want to apply higher voltage to the module than it supplies normally. Assuming max cell voltage of 4.2volt for a module of 5 cells, where the cells are in series, the voltage to apply would be: 5 x 3.2 = 16 , with a 100-200mA limit. (in a series connection, all the elements receive the same current; if however batteries are in parallel, applying 200mA limit will result in 100mA - 100mA going through each battery, in ideal conditions)

After this you put the battery pack back in the car, and can check if it's ready to accept proper charging / can start. (there's a possibility that you'll need obd2 related code resets etc, so check that you have a reader with capabilities to query the EV modules as well)