Cartridge locked on iSTAT device

I would suggest calling Abbott tech support... They can walk you through any issues and if not , then you need to return.
The number I use is 877-529-7185

Once you put the batteries back in and turn the i-STAT on it should unlock the cartridge it may take a few minutes.  You can also try selecting the #1 button it can be used to eject a cartridge after it unlocks. 

We had to leave the battery out for 48-72 hours so that ALL power discharged from the instrument. Then when we put the battery back in the cartridge was released.

I had this same issue a few months ago. When I called Abbott, they had me try a different (newer) battery and it worked. It was explained by the tech that with an older battery it can show on the iSTAT that the voltage is high enough and should work, but that the battery isn't actually supplying enough power. I hope that's helpful.

We had that issue.  Removing the battery didn't work like I expected.  I did however put a freshly charged battery in, and it let me remove the cartridge.  The battery that had been in the meter had enough power to turn the meter on but not to do anything else.