I have installed Ubuntu 12.04 LTS on my Acer Aspire-5745. It is not detecting the battery and showing it as not present if I take out the power cable , Still it is working with the same "battery not present status"
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I had a similar issue that occurred after I updated software. I have an Acer Aspire 5742. I rebooted with linux mint on a usb stick and had no issues with the battery. I then went through the log of the most recent updates. One of the updates was for jupiter. I uninstalled it and rebooted. Since then, I have not had any issues.
Not sure if you have jupiter installed, but if so, try uninstalling it and rebooting. Hope that helps.