Laptop resumes from standby after critical battery warning

Hi everyone, I have an Acer Aspire 3630 laptop running XP with SP3. The original battery was replaced earlier on this year as it was lasting less than 20 minutes. I replaced it with one from a third party manufacturer I found for sale on ebay. ("Buyer beware," I hear you cry!) Initially it displayed a projected working time of around 90 minutes. It now only lasts around 20-30 before a warning comes up suggesting a switch to mains power. However, about 3 or 4 minutes after the warning the computer goes into standby but immediately springs back into life and can work for another 20 minutes before repeating the process. Then, without warning, it dies and a full reboot after plugging into the mains is required. This process happens most but not every time I use the computer on battery. The alam settings are: 7% for low battery alarm and 3% for critical battery alarm. I would like a solution which, if possible, allowed me to use the full battery life before entering standby. Thanks for your attention.