Discharges when power is OFF

No, this is not normal. It should keep the charge much longer. However, a thing like this happened to me as well (once) and I think the reason was inadvertently clicking "Sleep" instead of "Power off", as those two fields are close to each other on the touchscreen, and it is relatively easy to hit the wrong one. So next time make sure you actually hit "Power off".

And one more thing, somewhat off-topic. It is not healthy for the battery to store it at 100% for prolonged time. If done occasionally, then this should not affect the battery condition significantly, but generally this should be avoided with li-ion batteries.

Thanks tmk2, I'll take your advice and do a very careful and special trial before I go any further..

As regards storing li-ion batteries long-term, thank you for pointing this out. To get the full story I checked on-line at "https://batteryuniversity.com/learn/article/how_to_store_batteries" and here is what it says:-

Lithium-based

There is virtually no self-discharge below about 4.0V at 20°C (68°F); storing at 3.7V yields amazing longevity for most Li-ion systems. Finding the exact 40–50 percent SoC level to store Li-ion is not that important. At 40 percent charge, most Li-ion has an OCV of 3.82V/cell at room temperature. To get the correct reading after a charge or discharge, rest the battery for 90 minutes before taking the reading. If this is not practical, overshoot the discharge voltage by 50mV or go 50mV higher on charge. This means discharging to 3.77V/cell or charging to 3.87V/cell at a C-rate of 1C or less. The rubber band effect will settle the voltage at roughly 3.82V.

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Li-ion cannot dip below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical short. (See BU-802b: Elevated Self-discharge.) If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion batteries that have been under stress may function normally but are more sensitive to mechanical abuse. Liability for incorrect handling should go to the user and not the battery manufacturer.

Comment:- its a bit difficult testing for the above when the battery is closed up inside a GPS unit.  Hopefully much of this is taken care of by the unit's sotware.

Cheers
Adrian

Send it back to garmin and get an 830!!