The 2.6.21 linux kernel branch has broken support of ACPI battery - on some notebooks (for example, IBM ThinkPad T23) it doesn't detect batteries at all (/proc/acpi/battery is empty). linux-2.6.20 doesn't have this bug. Reproducible: Always
FYI. Related upstream bug: http://bugzilla.kernel.org/show_bug.cgi?id=8651
Can you reproduce this on the latest development kernel, currently v2.6.22-rc6?
According to the upstream bug, 2.6.22 fixes this. If it doesn't, or you'd be interested in helping us backport the fix to 2.6.21, please reopen this bug.