Low CPU use, but HIGH battery use! How?
Also disk operations get battery power. For example, writing a block into the flash memory works first by burning it, and then a write operation.
Also the communication with your Wifi router or base station can take a lot of energy.
Mobile CPUs in phones are well optimized for low power consumption, but the case is not always so for other parts.
On an ext2-3-4 filesystem, also file read operations transfer into a write by default, because the last access time (last read event) meta-data in the filesystem has to be updated. A sane android would disable it (noatime flag in the mount options) but not always do.