Why do LithIon battery have three contacts?

[IsaacG]

Just wondering... why is it that LithIon batteries got three contacts?

 

[jstuedle]

Li-ion batteries if over charged or charged too rapidly can catch fire. The third contact is for the charger to monitor charging rate and charge state.

 

[ferny]

Maybe no-one likes them. Or they've only just opened their Facebook account and they haven't told their friends.

 

[IsaacG]

@jstuedle: Thanks for the useful, informative reply.