Fast charging? ...a calculation example

Every now and then the question is asked about fast charging a power battery. Here is an example calculation and some information on the subject.

Initial situation

Battery: BQ983, blue
Configuration: 13S6P cells
Cell: LG INR18650MJ1Nominal capacity: 3.5 Ah.
Recommended charging current 0.5C (1.75 A), max. 1C (3.5 A)
CR246 charger: Charging current max. 4.5 A

Invoice

The 4.5 A of the charger is distributed over the 6 strings = 0.75 A per string/cell. This corresponds to 43 % of the recommended charging current (0.5C) or 22 % of 1C.

From the battery's point of view, higher charging currents would therefore be possible without 'stressing' the battery.

Is it possible to fast charge in one hour instead of the regular five?

To do this, we would have to charge with 5x more charging current, i.e. 22.5 A. Using the same calculation, we now arrive at 3.75 A per string or cell. This would also just exceed 1C (3.5 A), the maximum charging current of the cell.

What are the arguments against fast charging?

• No manufacturer supplies chargers with 22.5 A charging current for their bikes. The thing would be too big, too heavy and too expensive.
• S-pedelec batteries are usually pushed to the limits of their specifications when riding. Stressing them during charging is detrimental to their service life.
• Stromer batteries roughly degenerate at 1% SoH per 1,000 km. After 25,000 km, or the theoretically achieved SoH of 75 %, a new battery would be due. Fast charging would be on top of this and would further shorten the life of the battery. This would probably not be a problem for occasional drivers, but it would be for commuters with longer journeys to work.

> See also BREMSEN - 'Recuperation and recuperation' - Good to know