Originally Posted by green_granite
errrrrrr how do you charge, say, an 11V battery with a 9V charger?
I would not recommend using the wrong charger. However the question here derives from a misunderstanding of how battery chargers work. An 11V battery would be just that, a battery with a nominal output of 11 volts. But a 9V battery charger would be a charger designed to charge 9V batteries. Its open-circuit output voltage will be considerably greater than 9 volts and it will adjust to match whatever battery is connected, over quite a wide range.
There are significant differences between laptop chargers and, say, car battery chargers.
The car battery charger contains the limited intelligence of what current to apply and what voltage to stop at, whilst the battery is dumb. It is advisable to use the appropriate charger so that it charges at the right current and stops charging at the right voltage for that battery. But it is possible to use the wrong charger provided you know what you are doing and do not over-stress the battery.
The laptop charger is dumb but the battery, in conjunction with the laptop itself, is very, very smart. The battery has an internal circuit board that contains a lot of smarts (se diagram in post above)and it communicates with the laptop down the data bus (individual types may vary). Li-ion batteries on fast charge need a lot of attention to keep them safe, and this is why so much smart stuff is built in. It is possible to charge a laptop from the wrong charger (higher or lower voltage) because the battery and laptop will tend to work together to protect the battery from catching fire. But in my experience the use of the wrong charger can permanently affect the logic within the battery and may cause problems even when the correct charger is reconnected. I would advise using the correct charger from the oem (original equipment manufacturer).