If your Mac reverts to a date in the past when you turn it off, or forgets its Startup Disk or Network settings and time, then it means that the PRAM (parameter RAM) memory is getting reset, and the primary reason for this is that the PRAM backup battery is drained. The solution is to replace the battery.
When replacing a PRAM battery, it is important to orient the new battery in the same direction as the old one (+ and – terminals) or to attach the connector in the same way.
Not all Macs have a PRAM battery – some Macintosh laptops rely on the main battery for PRAM power, and have a capacitor on-board to keep the PRAM alive for a couple of minutes while you switch batteries. Whether it has a PRAM battery or not, a Mac notebook is not designed to have its main battery run completely out. You should always recharge the main battery as soon as it reaches the battery-low warning signal from OSX (the exception is when you are doing the calibration procedure on a new battery).
PRAM batteries have had various shapes and sizes over the years.
Mac PRAM batteries 3.6V Lithium + 4.5V Alkaline
Desktop Macs use one of two types:
1/2 AA 3.6V Lithium battery (half the length of a AA dry cell) Note that this is not a Lithium-ion rechargeable battery
Used in many Macintosh desktops, PowerMacs, iMacs, eMacs, PowerMac G4 and G5
4.5V Alkaline Cube shaped battery with 2 wire cable For many old Performa and LC models
Mac laptops have used a staggering array of different battery designs, some of which are proprietary to Apple and can only be bought as service parts for $40 – $80. Don’t make assumptions about one battery fitting a MacBook with a similar name – the batteries differ from generation to generation, and also between 15″ and 17″ models.
MacBook Pro 15in A1211 PRAM Battery 922-7913
MacBook Pro
A1150 1.83, 2, or 2.16 GHz Core Duo Installation Guide
Apple Part Number 922-7190
A1121, 2.16 and 2.33 GHz Core2Duo Installation Guide
A1226 and A1260 2.2, 2.4, 2.5, or 2.6 GHz Core 2 Duo Installation Guide
Apple Part Number 922-7913
MacBookPro 17 inch CoreDuo + Core2Duo PRAM Battery 922-7505
MacBook Pro 17″
Models A1151, A1212, A1229, and A1261 (excluding Unibody models)
Apple Part Number 922-7505
Installation Guide
MacBook 13 inch PRAM Battery 922-7369
MacBook
A1181
1.83 GHz or 2.0 GHz CoreDuo Install Guide
1.83, 2, or 2.16 GHz Core 2 Duo MacBooks (exluding SantaRosa models) Install Guide
Apple Part number 922-7369
PowerBook G4 17 inch 1.0 GHz - 1.67 GHz Lo Res PRAM Battery 922-6763
PowerBook G4 Aluminum 17 inch 1.0 GHz -1.67 GHz Battery and USB module
1 GHz model Apple Part Number 922-5767 (820-1391-A)
1.33 GHz model Apple Part Number 922-6064 (820-1537-A)
1.5 GHz model Apple Part Number 922-6395 (820-1618-A)
1.67 GHz model Apple Part Number 922-6763 (820-1814-A)
Installation Guide
1.67 GHz Hi-Res / Dual Layer model Apple Part Number 922-6978 (820-1824-A)
Installation Guide
PowerBook G4 15" Aluminum 1.5 GHz and 1.67 GHz PRAM Battery
PowerBook G4 Aluminum 15 inch models
1.5 GHz and 1.67 GHz models
A1106 1.67 “low res” screen models before October 2005
Apple Part Numbers 922-6714 & 922-6711 (820-1686-A)
Installation Guide
Powerbook G4 15" Aluminum 1.0 to 1.5 GHz PRAM Battery 922-6007
1.0, 1.25, 1.33 and 1.5 GHz models
A1046 and A1095
Apple Part Numbers 922-6007 922-6000
Install Guide
PowerBookG4 Titanium 15" Onyx & DVI PRAM Battery
PowerBook G4 Titanium Onyx and DVI models (not 400 MHz and 500 MHz Mercury model)
Apple Part Numbers: 922-4631, 922-5205
You might think that the PRAM battery would last longer if you don’t use the machine,but the opposite is true. The battery is being drained whenever the machine is unplugged, so a machine in storage will tap out the PRAM battery sooner than a machine in daily use.