Batteries are charged by different values depending on a number of things, battery voltage, battery temperature, etc. In a Pc in general there’s a quite sofisticated microcontroller continuosly checking battery values and adjusting power flow accordingly. In the initial phase, when voltage in the cells is lower, more current is allowed to flow until voltage reaches its upper threshold. At that point current is reduced to avoid overheating and overvoltage until battery reaches almost full condition and trickle charging is then set. The same applies to all batteries, but as you can imagine is influenced by battery quality and condition as well as the charger used.