marlok key system
You obviously didn't spend much time researching the web before posting your questions. I learned the following by looking for Marlok, key, and then Millenium Marlok. It's an interesting system.
The Marlok key system is made by Millenium Group, now part of ILCO.
The key works by shining infrared light through the key to read codes recorded into the key at the time of manufacture. The black stripes would appear to be IR windows. The key itself is a nickel alloy.
Since the reader is an optical reader, dirt can interfere with proper operation. If you get too much dirt into the lock/reader, you'll probably have to replace the reader assembly of the lock. If the key is too scratched or marked up, it would have to be replaced. The IR path has to be clean and clear.
Each door has a control unit that has been pre-programmed from a central location to allow a particular key to open the door. The door control unit is located near/at the door itself - probably just inside the hinge side of the door.
It is a digital system controlled from a central PC that is used to download the approved codes to each door control unit.
The system looks to be hack proof. Since the key system is optical, no magnetic tampering would be effective. You would have to know a valid key code to tap into the system and modify the information stored in the door control unit, or you would have to access the door solenoid wires directly.
If the unit worked better before the battery was replaced, it sounds like either the new battery is defective (terminal voltage too low, amp rating of the battery too low, or resistive connection at the battery terminal), or the power supply was damaged during the battery replacement.
https://www.kaba-ilco.com/lodging_systems/pdf/Millenium_9000_Brochure_[m1890-01-06].pdf - page 4
**broken link removed**
https://www.kaba-ilco.com/lodging_systems/products/ml9000_specs_info.asp