Ladegerät ist nicht gleich Ladegerät

If you charge a lithium battery, consisting of many cells connected in series, just by connecting the positive and negative lead to your charger, the pack won't charge uniformly, if the cells don't have identical internal resistances. That's why you should charge those kinds of batteries with a charger, that has an integrated balancer.
A balancer monitors the cells in battery pack through a wire directly connected to each cell and if it starts to drift (charges faster or slower than the rest) it can charge the cell directly with a small current.