Vmoto and Social Media, page-1568

I'll say it again, current charges a battery (Q = I x t). The resulting energy stored in the battery is given by E = QV (where V is the open cct voltage). A battery cannot convert voltage into current during charging.

"The voltage a battery is charged at is not limited to the output voltage"

Agreed - in fact it has to be higher.

In understanding battery operation, two important parameters are open circuit voltage and internal resistance. A battery cell can be schematically represented by an idealised DC voltage source equal to the open cct voltage, in series with a resistor equal to the cell's internal resistance. Unfortunately, neither parameter is constant. Both change with age, and vary with temperature and level of charge, and the internal resistance also varies with current.

However, this representation is a simple way to understand how terminal voltage varies with the direction and amount of current. The terminal voltage will be higher than the open cct voltage during charging, and lower while discharging. The difference between the terminal voltage and the open cct voltage, multiplied by the current, gives the power loss due to the internal resistance.

If we plug in the numbers we have been using (just as an exercise - this is not a realistic scenario for a bunch of reasons I won't go into) we get:
220 - 60 = 160V dropped across the internal resistance.
The current that would be required to create this condition would be far greater than the output of the fast charger, but we can calculate a notional charging current using the charging time, battery voltage, and battery capacity:
2.2kWh/0.5h/60V = 73.3A.
We can then use this to calculate the power dissipated across the internal resistance:
P = 73.33A x 160V = 11.73kW.

Our discussion has prompted me to investigate how these charging systems actually work. Unfortunately the most promising articles were behind paywalls, but I did manage to find this general schematic illustrating the power circuitry of the various types of charging systems:


So in the DC fast charging cct we have 3 phase full wave rectification, followed by active regulation, all using N-channel enhancement mode MOSFETs - represented by this symbol:


In fact these are used in all of the charging ccts.

What is not shown, is where the inputs to the MOSFET gates are coming from. All we have is an arrow (coloured yellow above) indicating further circuitry. However, this tells me all I need to know. MOSFETs are used in this kind of cct in switch mode at high frequency. The gate inputs come from control circuitry that regulates the output of the power cct. A step-down DC-to-DC converter works the same way. Basically the voltage output is regulated by the balance of current flowing into the capacitors with current flowing out of them. The current flowing out is determined by the load (in this case the battery under charge) and the current flowing in is regulated by the switching of the MOSFETs which provide a stream of charging pulses to the capacitors.

The fast charger will only be able to deliver it's maximum rated voltage at it's maximum rated current. The voltage will naturally drop at higher currents. This relationship can be represented by a voltage vs current output curve. When the charger is connected to a battery, it's output voltage and the battery terminal voltage logically become one and the same (ignoring voltage drop across connecting cables). So in the absence of regulation, the charge current will be that which corresponds to the battery terminal voltage on the charger's maximum output curve. This will provide a measure of passive regulation, but the current will likely be excessive (unless the curve has been tailored to match the specific battery type - effectively regulating at a fixed point). And once fully charged the battery voltage will climb, limiting the current, but at the cost of overcharging the battery to the point of destruction in a fairly short time. So as a minimum the voltage would need to be monitored to detect the point of full charge, and disconnect the charging.

So I am satisfied I understand this now. The voltage rating of the fast charger is probably just the maximum voltage output, but it will regulate it's output to suit the new battery. I'm guessing it will somehow communicate with the battery under charge for charging instructions, probably digitally (that's how I would do it). It is fundamentally the current that is regulated. The voltage will be determined by the battery. When not connected to a battery the output of the charge cct (as shown above) will have a natural tendency to drift high, but I imagine there is protection against this in the interest of safety.

All IMO.