DC is better then AC and both depends on how much the battery is already charged, temperature of the battery, the infrastructure (charging cables for example). The range is somewhere from low 80's for low amperage AC charging on cold weather using a low quality granny charger cable to high 90's for a warm battery on a dedicated high power DC charger.

This of course doesn't include losses in transmission from the power station and in electricity production.

Charging goes like this:

  - AC converted to DC (with power factor correction, usually means AC stepped up 1st)
  - DC converted back to AC (but higher voltage) and MUCH higher frequency
  - AC transformed to lower AC voltage (still higher frequence)
  - AC rectified to DC (filtered and stabilized), DC voltage lower

If there is DC, the very 1st part can be omitted.

That's not true. Tesla works like this: 1. You have a high-voltage DC bus that is basically connected to all battery modules. Modules have individual BMS modules and can connect/disconnect to that bus. 2. If you're doing fast charging, the charger connects to that bus directly (you can hear contactors closing), matches the voltage and pushes the current. 3. If you're doing AC slow charging, the charging module on the Tesla simply boosts the voltage to the bus level via a PWM-based power supply.

That's it. A pretty simple system.

Wait! what? why? as part of the DC to DC conversion? how does it affect efficiency?