|A 12 cell battery is at a higher voltage than most laptop batteries, most stock batteries are 6 - 9 cells. The 12 cell battery would be 12x1.2V=14.4Volts nominally, but when charging this will rise as the battery charges and in order for more 'juice' to be 'pushed' into the battery, the input voltage must be higher than the actual voltage in the battery at the time. That 12cell battery can actually have a voltage at full charge of I believe just over 17 volts. When the battery is very low it is reading lower than the nominal voltage, possibly under 12 volts, if your charging cord puts out only 14volts or 15 volts then the battery will never reach anything near a full charge. To fully charge a 12 cell battery you need a charger output voltage between 18 volts and 20 volts (if that is safe for your laptop's circuits). In other words, the stock charge cord would not likely be able to fully charge a 12 cell battery unless it is designed to do this. IF HP offers this battery, then they need to offer a special charging cord or other charging method. If this is an aftermarket battery, contact the vendor to see what they recommend about charging it.|
[It is unlikely that the 12 cell is set up as two 6 cell packs parallel to each other (same voltage as 6 cell but twice the available current/run time). Although this would extend run time, they would need to be separated and charged separately since any difference in internal resistance would make all of the charge go only to one battery. This would not be practical for a laptop.]
As a matter of interest:
You have to be a little bit crazy to keep you from going insane.