GPSMAP 66i Version for using AA batteries

Lithium batteries in a bank are going to be lighter and/or have more recharges than Alkaline or NiMH.

If you can change batteries you can plug in a bank, I can't see what the fuss is.

Garmin are right, the internal lithium will provide significantly more peak current than any external AA's. 

Changing batteries yourself and particularly in the field runs risk of compromising the integrity of the connectors (dirt, corrosion) further limiting or even stopping current in an emergency transmission, and likewise changing the internal batteries likely compromising the water sealing. 

This is all quite common in safety devices, for example automatic openers in parachute systems. The right battery for the job & keep it sealed and working when it really needs to.

By the way there is also a holder available with an external power cable,

One week? I'm a bushwalker and for the last 35 plus years spent multiple weeks alone off track in remote areas....including your south west NZ Fiordlands.

The issue is not about what charging method is better, it's about the power source being able to provide the peak currents and operational duration required.

This is not simply a low powered passive receiver like the 64, 66i has an active transmitter that needs to fire precise packets into space either continually for social reasons and 100% reliably when required in an emergency.

Nothing you could insert in AA format would come close to doing what the internal battery will do, and many AA's you will find on the market are much worse. Even if an AA was marginally capable, any dirt, corrosion or residual pitting on the contacts can degrade the higher current a device like this requires. And that is a risk.

Most people get the convenience of AA's, I have a GPSMap76csx & 78sc that use AA and I have AA adapters as a backup for my Rino 650's. Don't get fooled simply because a device externally looks similar or is sold in into the same market space. The 66i is a different beast, it's a great unit and needs the appropriate power source.

FYI my background includes electronics technician on military communications equipment.

The issue is not lithium vs AA batteries, it is the fact that garmin makes a device that has a limited lifespan.  I have had to replace two Rino lithium battery packs, once on each unit over a 6 year period.  If they were not replaceable, they would have been garbage.  Maybe I am missing something, but the garmin specs are not removable or replaceable.  Is this actually the case?

No.

6 torx screws and 5 minutes of your time.

Batteries are readily available from multiple popular online sources.

I've gotta say, I was on the fence about Gpsmap 66i, but the battery situation made it a firm NO for me. I understand the peak current argument, but there's absolutely no excuse for making the battery not user-replaceable. This is planned obsolescence and there's no other way to look at it. So what should I do when the battery inevitably degrades? Should I throw away the device, or just risk it and hope it won't fail in an extreme situation? I couldn't find any spare batteries on the Garmin website either, so if you want to change your battery, you'd have to resort to unofficial and unproven sources. And this is simply not acceptable for a device that you rely on in possibly life or death situations.

The newer 750i has a removable LiIon battery pack. So you can carry spares. However, it’s hard to recommend the 7xi for hiking use. It’s huge, heavy, and a power hog.

Hopefully future generations of GPSMAP and Oregon devices will have user replaceable Li-ion battery packs similar to the new Montana 7x0 and Alpha 200i, as well as the Rino 75x and its predecessors.