[I recently purchased a used Pentax 645 in excellent plus condition. I don't know how old it is, but I wanted to change the lithium battery so that it wouldn't die on me unexpectly. I thought that the lithium battery was vital in its function, but apparently the camera will function fine without it and use the 6-AA batteries. Anyway, I opened the bottom chamber with tweezers, took the old battery out, put a new one in, and the liquid crystal display would not come on. It's as if there's no battery in there. I changed them back and forth (the old and new, thinking perhaps the new battery was dead somehow) and tried everything, but the liquid crystal display doesn't come on. But I put the 6-AA batteries back in and the camera works fine anyway. The manual does say that this lithium battery is actually a backup source anyway. Does anyone know why the lithium battery (both of them) will not work? Perhaps the contacts are not clearly connecting to the battery somehow. It worked fine until I changed it. The camera seems perfectly functional, meter and all, without it anyway. I'll make sure not to let the main batteries completely die in the middle of a roll though. Why would Pentax make such a stupid design anyway where they claim you need to bring it to their service center to replace? Any advice from people who have changed the lithium battery themselves would be greatly appreciated. Maybe there's something I didn't do right when changing it. Feel free to email me privately as well. Thanks in advance.] I recently posted about the problem I thought I was having with my Pentax 645 lithium battery. Since it did not work, I removed it from the camera and closed the chamber. Now there's no lithium battery, yet it still works perfectly. I really don't see the importance of it anyway. I will not let my batteries because exhausted and will change them when they are weak. In case of emergencies, there's always that manual rewind method anyway, but I don't plan on needing it. It doesn't seem that important that the liquid crystal display goes out while changing the batteries. I don't really see why Pentax would want to put it there in the first place. As far as I can see, no other medium format camera has a "backup" battery like that. I don't claim to know it all, but the camera appears to be working fine without the lithium battery. If anyone knows something I don't in regard to why the lithium battery might really "need" to be there, I'd be happy to hear from you. Thanks.