| Garmin GPSMAP 420 battery replacement |
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Found nothing on this particular unit and Garmin support is frankly not very helpful at all, so I decided to document this.
Most older and possibly newer Garmin units have an internal, rechargeable battery that powers the real-time clock and possibly provdes storage that helps the poor GPS find satellites faster.
My unit was 8-9 years old and the battery had less than 1V left. When this battery fails, one of two things happen: 1 - If the battery is low then the GPS will do a complete reinit on every power up. This takes a very long time. 2 - If the battery is dead, then the GPS will cease to function. The GPS will beep once, whow the Garmin logo, then shut down. The latter reeks of sloppy programming. How hard can it be to provide a reasonable error message?
The battery is located inside the unit and soldered to the PCB, right next to the RTC and clock crystal. Open the case by unscrewing the 8 screws. then remove the two nuts that hold the two connectors to te back cover and remove the cover.
The battery is a Panasonic VL-1220 rechargeable lithium battery. Do not attempt to install a regular CR 1220 non-rechargeable battery! It will cause bad things to happen.
Desolder the battery, using a reasonably small iron, taking care not to heat the battery itsef. Use a small pair of pliers to lift one solder tab at a time whilst heating the PCB pad. The pliers will act like a heatink and stop un-necessary battery heating.
The Panasonic VL-1220 exists in three varieties: Fxx, Hxx and Vxx, ie "flat", "vertical" and "horizontal". The original battery was a "flat" one, but a VL-1220/VCN will fit if you bend the pins slightly. The Farnell number is 1514218. Solder in the new. Possibly replacing the double-sided adhesive tape underneath in the process.
Battery woes.. Lithium batteries are dangerous so there are shipping restrictions on all lithium cells. Problem is that many vendors will apply the same rules to a small button cell, with less than 1g of Li as they do to a 100Ah Li-Ion one.. So, Farnell was no go. Promised to deliver a battery in > 6 weeks. I ended up buying one from eBay and the seller, Akku-Pow3r-Shop, delivered the cell in just a few days
All hope is not lost even if you cannot get the correct battery. In that case, just use a regular CR2032 or similar 3V, but add a blocking diode, preferably a schottky-one such as the ubiqutous BAT-85 in series to prevent charging of the battery. A CR2032 should last 3-5 years
And even more novel way would be to use a supercap. A 2F cap will supply the RTC for a month at 1uA. Only snag is that you will need to provide an alternate charger as the GPS will shut down if the cap is empty, thus prevent charging
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