Windows 7: Acer notebook doesn't notify on low-battery and abruptly hibernates

Hi,

I have been using my Acer Aspire 5742G notebook since Mar'11. It's specs are as below -
Core i5 (1st gen) 2.27 Ghz processor
8 Gb (1333 Mhz) DDR3 RAM (2X4Gb - Transcend JM1333KSN-4G)
500 Gb SATA HDD
DVD Multidrive
Win 7 HP SP1 (updated all critical updates till date)



For the past week or so I have been facing a strange issue. The laptop abruptly goes into hibernate mode (occasionally shuts down too) when it touches 10% of battery charge, or close to it. Sometimes system does show the pop-up notification of low battery at 10%, when I immediately start charging to avoid any issues. Also, I always stop charging the battery when it shows it's Fully Charged in notification area. Have monitored battery charge cycles for the past 2 days and found charging takes about 1.5 hrs and charge lasts for about the same time (depending on how much I use it, i.e. playing music/video DVDs, MP3 songs on laptop hard drive, etc.).

Can someone explain why the system goes into hibernate mode, or shuts down abruptly at 10% remaining battery charge ? Acer Support had advised me earlier to recalibrate the battery thru some steps, which I tried a couple of days back too (i.e. fully charged, then fully drained battery twice). However, this hasn't helped. Of course, the laptop is just over 1.5 years old now, so does it mean I need to replace the battery ? Would help if someone can suggest a good (freeware!) battery diagnostic tool (Acer doesn't have or provide one ! ), which can confirm the health of the battery, and if it does need to be replaced.

Sorry for this long post, but just thought I should give all relevant details to preempt follow-up queries (which are welcome even otherwise).

Thanks for your time and consideration.

Regards,
Ramesh