I have a GPSmap 76 which has not been used for perhaps 12 years, stored without batteries.
The GPSmap 76 appears to use an internal battery for non-volatile memory and clock, likely to be a MC621 rechargeable LiIon (MnO2) button cell of about 3mAh capacity.
It is not clear when this cell is charged, online comments suggest several actions:
- whilst there are good batteries fitted;
- whilst there are good batteries fitted AND the GPS is on; and
- whilst externally powered, whether or not the GPS is on.
Now you might argue that option 1 is the only sensible one… and option 3 would be the worst only catering for the few people who ever power it externally. Option 2 would not be good for occasional / light users.
So, in my case, the GPS powered up (2023) with a date of 2011, then later changed that to 2004 but with correct time. Classic symptoms it has not dealt with GPS week rollover correctly. This clock error will roll into all tracks logged etc and needs to be fixed.
The user interface does not allow the user to set the date. Searching the net turned up a utility written by Garmin for fixing the date problem on some earlier GPS models, possibly by a NMEA command sequence, and possibly that will work on this one.
I reinstalled the firmware in the vague hope that the firmware installer might also set the clock… but it did not.
The utility mentioned above and found here did set the date to a value that provided the needed correct date block bits.
The GPS was left on for half an hour to collect a current almanac, the power cycled and it came up very quickly with the correct time and location.
The next challenge is whether the battery is good, just discharged. It does seem to function properly after a few hours with external power on and GPS on. It may take more than 30h to charge this cell, so I will leave it running for a few days to try to fully charge the cell. Time will tell if it holds charge ok.
It may be prudent to power this thing every year to maintain the clock state. I have a lot of small bare GPS boards with Ublox chips that need to be powered for a week from time to time to charge the rechargeable cell on them… to this is another on the same task list.
Update: 04/12/2023
After a few days with batteries fitted, the current draw from 3V with the unit OFF is 1.5µA.
Update: 14/12/2023
After a few days with batteries fitted, and 10 days with no batteries, the current draw from 3V with the unit OFF is 8.5µA. I will check it after a month of discharge of the memory battery.
Update: 31/12/2023
It is a month since the batteries were removed from the working GPSmap76. the current draw from 3V with the unit OFF is 12µA, falling to 8µ within a few minutes. This looks more like a supercap than a rechargeable LiIon cell.
On fitting batteries, the date and time displayed were those when it was last powered on a month ago, and it needed a full autolocate start… but after the required 12min or so, it was up and running, including correct year and month.