I no longer have the config file for 2.6.21.5-smp (it left with my upgrade from 12.0 to 12.1), so I can't check the options in the kernel (sorry). Looking through /etc/rc.d/rc.modules-2.6.21.5-smp (which is still hanging around unused from 12.0), there is no line for the battery module. That would probably mean that it is either compiled into the kernel (unlikely) or that it is not compiled in/didn't exist in 2.6.21.5 (not sure which). It is possible that the module was called something else in 2.6.21.5 or that it didn't exist. It does exist in 2.6.24.5-smp (/lib/modules/2.6.24.5-smp/kernel/drivers/acpi/battery.ko). Switching kernels is quite easy and involves a simple edit of /etc/lilo.conf (make sure you run `lilo` as root after editing lilo.conf to write to the MBR). Note that the generic-smp kernel (and the generic non-smp kernel) requires an initrd -- see /boot/README.initrd for more information.