Windows warns you when your battery charge reaches low, reserve, and critical levels. You can however, change the low, reserve, critical battery level notifications in Windows 7 or Windows 8, through the Control Panel.
Low, reserve, and critical levels in battery
When the charge gets low, the battery icon in the notification area indicates a low-battery level. The default value is 10%.
When your battery charge reaches the reserve level, Windows notifies you that you’re running on reserve power. The default value is 7%. At that point, you need to save your work, and then find an alternate power source or stop using the computer.
When your battery is almost out of power, the battery icon indicates a critical-battery level, and then your laptop goes into hibernation. The default value is 5%.
If you don’t have enough time to act after seeing the low, you can increase the percentage of battery remaining for these settings. This can be especially useful for older batteries, which might not have as much storage capacity.
Change Low Battery Level Notification
1. Open Power Options by clicking the Start button clicking Control Panel, clicking System and Security, and then clicking Power Options.
2. On the Select a power plan page, next to the plan you want to change, click Change plan settings.
3. On the Change settings for the plan page, click Change advanced power settings.
4. On the Advanced settings tab, expand Battery, expand Low battery level, Reserve battery level, and Critical battery level. Change the percentage of battery remaining that you want for each level, and then click OK.
Hope this helps!
How to change Critical & Low Level Battery Action in Windows 8 or Windows 7 may also interest you.