Let's make some assumptions:
- the 6-cell battery has around 72 Wh which breaks down to single cells that have roughly 3.4 Ah each)
- the charger is a switch-mode charger, so we must regard that the charger can push more amps to the battery as it get's on the input
- he cells are all empty, they could be down to let's say 3.0 V that is 9 V for the series of 3 groups
- they are just not yet considered deep-discharged so the charger does start in fast-charge mode (if they are lower than 3 V, according the the datasheet of samsung cells, IT equipment chargers should employ pre-charge with I translate to reduced current charge up to 3 V)
- the computer is off - maximum power available to the battery
- power supply is 65 W @ 20 V, with 90% efficiency that could give 58.5 W for charging, that means at 9 V no more current than 6.5 A is available in the first few minutes of charging
- the efficiency may decrease a bit at lower load, but we ignore that
- the cells are charged in a string 3S2P with balancing
If we assume that the two parallel cells are in good shape they consume each half of the 6.5 A so 3.25 A in the beginning of the charging process.
If 3.6 V per cell are reached (pack at 10.8 V) the 58.5 W translate to 5.4 A max. so the current per cell is already below 2.7 A.
Justme's comment about larger power supplies is useful. The charging can and will be faster. But it is not said that it will charge with 120W, because the charging system will limit the current to what the original cells can handle (and the charging system itself also has a current limit!).
If it would use 120W for charging that would mean 12 A of current under the same assumptions as above, so 6 A per single cell.
There are also 170W power bricks available with the same jack so you can connect them to any mating Thinkpad - without harming the battery. (My P52 supply has 170 W (20 V and 8.5 A) and it can safely charge a Carbon X whose original supply has only 65 W (20 V and 3.25 A).
- The new cells should have the same rating than the original cells
- we see with the 65 W brick the cells are more or less charged with 1C
- we just can and do not know the max. charging rate limit of the original cells, but we can assume that with a larger power supply they may get charged with up to 2C
- If the original cell type is unknown, take new cells from a trustworthy supplier and manufacturer that allow continous charging at 2C rating. For 3.4 Ah cells this means 6.8 A max per cell, which means >12A for the string which is not possible from a 120W supply. Thus you are on the safe side.