As I had too much time on my hands, I made a video on the battery replacement for my DNS-325.
I used Duracell 307/357 Silver Oxide battery, which you have to specifically look for. The original battery was Maxell LR44 which is widely available everywhere (and cheaper). The duracell has 50% more capacity than the Maxell so I opted for the Duracell.
Prior doing the battery update, set your filesystem fsck interval. Remove the time based fsck as powering down the NAS will make the clock reset and when restarting, there will be fsck because the last fsck date is so far from the resetted clock time.
Video available at:
Duracell battery: