I had my S3 OLED open after bad cap and hard drive repair so I figured I'd change out the CR2032 clock battery at the same time.
This is just info for future reference.
The unit has been running since around Mar 2007, so that makes around 6.5 years.
Given the length of operations, I figured the battery was probably pretty close to being done or maybe was already done. When I rebooted, certain values in system information like "Guide cache to" and VCM connection info would reset to December 31.
It turns out those values reset to December 31 regardless of the clock battery. The only job of the clock battery appears to be to, surprise, keep the clock going when power goes out. So the only reason to replace it (other than being proactive) is if your "Today's date"
resets to "Tue Dec 31, 2002 4:00 pm"
when you pull the plug.
Even if it does reset, you don't really need to replace the battery
as upon first connect to mothership, it will correct the clock, but it might result in more background indexing than necessary. Also you might miss season passes for up to 24 hours until the first connect to mothership resets the clock to current date/time. You can alleviate that by forcing a connect to mothership after pulling the plug but may not be able to do that if you have a power outage and don't have access to the unit. If you have your TiVo connected to a UPS, there is even less reason to replace it.
Anyway, if you decide to replace the battery, be CAREFUL when you remove it. The "positive" retainer "arm"/"clip" is not designed to be lifted up. If you try to do so, there is a good chance you will BREAK the arm. Instead, insert a small jeweler's flathead screwdriver underneath the clock battery and twist around 30-45 degrees to get the battery slightly lifted, then push/slide the battery towards the side of the receptacle OPPOSITE of where the retainer arm connects. The battery will just pop out.
Though it isn't absolutely necessary, I found it easier to get clearance to remove the battery after removing the hard drive cage.
To insert the new battery, just tilt it slightly to get under the retainer arm, and push/slide it back in.
The original battery was Panasonic CR2032 from Indonesia. It tested 2.98 V after 6.5 years. My "Today's date" had never reset after pulling the plug, and the CR2032 is listed as supplying 3V, so it probably had a couple of years left.
The replacement battery tested at 3.25V.
Upon first boot, your clock will be reset to Tue Dec 31, 2002 4:00 pm. The first connect to mothership takes longer than normal. A bunch of time fields in the system information will reset to Dec 31. Everything is working fine after replacement and subsequent connect to mothership.
It is interesting to note that TiVo doesn't set its time at startup. It only appears to set the time when it connects to the mothership. If I had not forced a connect to mothership, the time would probably be wrong for 24 hours and season passes wouldn't record. I imagine this is what someone might experience if their clock battery was dead.
In conclusion, IMO it isn't really necessary to replace the clock battery, and I wouldn't bother in most cases, certainly not until your "Today's date" starts resetting to the past. I did it because I was curious and had the box open already.
Hope this at least prevents someone from breaking their clock battery arm just for being curious.