On the iPod Nano 7th Gen battery connector, what is the TH contact for ?

On the iPod Nano 7th Gen battery connector, what is the TH contact for ? Is this part of the charging circuit?

I have replaced the battery, everything seems to be working fine. But when I come to charge the battery.... initially I get a flash of the charging icon and then it changes to the charged icon, but the battery voltage reads 3.7V only. I don't know if the battery is charged or not! Thank you for any help on this...