So i found this battery : http://www.ebay.com/itm/3-7V-Li-ion-Rechargeable-Battery-for-Nintendo-3DS-Tool-/160573338176?pt=Video_Games_Accessories&hash=item2562eab240
and i hoped this would be a better fit than the Ds lite battery.


and I know the middle pin is for sensing if the battery isn't too hot or overcharging and i wondered which pin(s) have this function in the ti nspire?
As the iphone battery has 4 pins and 2 of them are non + and -, so i thought as the ti nspire also has 4 pins maybe they correspond or something like that.

also picking out which pins to connect which pins isn't a problem as i have the right connector for it