I'm sure this is very basic for users of this site, but I would like to mention it anyway. Recently I was trying to get an old notebook computer going again for a friend and one of the problems was a very expired cmos battery pack that can no longer be replaced. For some reason I still don't understand, this computer used a 7.2 volt rechargeable NiMh pack for the cmos.
After thinking about it for awhile and recalling all the ancient PCs with still-working lithium button cells I have encountered, I couldn't see any reason why a rechargeable battery was needed at all just for the cmos. Of course it is not a good idea to recharge lithium button cells since they tend to explode, so I included a diode in a circuit with two buttons in series. The six volts from the buttons is enough to keep the cmos alive and the diode protects them from the charging current. I have tested this and so far it seems to be working perfectly.
So is this a good idea? If so, I submit it as a tip for anyone faced with a simlar problem. If not, I would appreciate knowing what is wrong with it.
After thinking about it for awhile and recalling all the ancient PCs with still-working lithium button cells I have encountered, I couldn't see any reason why a rechargeable battery was needed at all just for the cmos. Of course it is not a good idea to recharge lithium button cells since they tend to explode, so I included a diode in a circuit with two buttons in series. The six volts from the buttons is enough to keep the cmos alive and the diode protects them from the charging current. I have tested this and so far it seems to be working perfectly.
So is this a good idea? If so, I submit it as a tip for anyone faced with a simlar problem. If not, I would appreciate knowing what is wrong with it.
