My Sony z7iii Camera sometimes sits for long periods (weeks) in its camera bag.

I began having the problem of the camera 'forgetting' the date & time when I'd power the camera on.

I learned that the internal ('real time clock') battery only gets charged when :

1.) The camera is turned-on (with the regular battery in the 'battery well') as when the camera is being used

or

2.) When the camera is turned-off and a fully charged 'regular' battery is in the camera's 'battery well' and the external charge cable is plugged-in (and connected to the AC charger) (Typically I've found that the tiny  amber 'charging' LED comes on for a brief period when the external charger is first plugged-in but then it goes off. Apparently it is only then that the internal battery starts to be charged ... when the amber LED goes off.)

When I looked into the situation I found that I was not the only person having the problem. Other a7iii owners had asked how to charge the internal battery and then Sony explained to leave the camera plugged into the charger for at least 24 hours. (I don't believe that this is covered in the Owner's Manual.)

I have managed to charge-up the internal battery (at least enough such that the date & time is maintained for awhile) but I now question if 24 hours is long enough ?

In late June (2025) I left my a7iii Camera plugged into the external charger for several days and 6 weeks later the camera still remembered the date & time (though the clock's accuracy isn't all that good)

Does anyone have any evidence as to how long the a7iii needs to be plugged into the external charger to fully charge (as when the camera came from the factory) the internal battery ?

(signed)

- Paul, WB5AGF

I can't really help you with that but unfortunately, the internal batteries do degrade over a period of time.

Your battery is shot and no amount of charging will rejuvenate it. Had my internal die on mine. Set time at startup and would have to reset again whenever the battery got swapped*. Dead to the point it wouldn't stick no matter how fast the battery got changed.

Instead of resetting each swap it was quicker to take pic of clock or slate then batch adjust all file attributes later on.

*I put up with the time/date dance until the camera went in for servicing.

I used to access the 'memory' batteries in my Maxxum 7000 and replace them. Blast from the past. Those were pretty easy though; one board located under the battery grip and a soldering iron did the trick.

As near as I can tell it takes a special battery that we can't lay hands on anyway. Here's a link to get the process rolling if you want to replace it:

 Product Repair - Electronics Support | Sony USA

Here it is (almost) mid September (2025) and the problem has not reoccurred (i.e. my Sony a7iii Camera is still remembering the date & time when turned-on).

A few days recharging the internal 'RTC' (real time clock) battery seems to be adequate.