OK thanks for that reply, first let me say that the reason I want to do this that I use this on my motorbike connected to a 5W usb outlet connected directly to the battery with an inline fuse. So it will usually be plugged into the bike and then when I get off and take it will me it will always be fully charged. It's slightly annoying to unplug and replug the device into the mini jack every time I disembark so often I'll just let it run on batteries and if I'm going for a longer ride I'll plug it into power, so I never really have to bother about getting stuck or thinking about the batteries.
I tried it and it all works but there's some funny business going on and I'm now trying to work out what is causing this. Garmin is not much use because they just robotically parrot that they don't recommend anyone else's batteries be recharged, but parsing that what they are really saying is that they know that eventually someone is going to put ordinary batteries in with the plastic and explode the batteries. So they are just playing it safe.
Now all would be good except for some strange behaviour and I am guessing that it may possibly be due to voltage. OK this is what happened I let the batteries charge up fully in the unit, unplugged it then checked the battery voltage with was 14.4 which seems pretty high for NiMH batteries, but I'm told that they quickly drop down to 1.2 and stay there. FFIW, I have found this to not be the case, I put two sets of batteries in two units today, and etrex20x and the oregon, the etrex had Everready 2000mAh rechargeable, which I think are LSD, and the the oregon had the panasonic pro 2400mAH eneloops. I ran them in both units and for an hour with lighting on and when I got back and checked they were down to 13V and 13.5V respectively. Is this normal now for good quality eneloops? I mean these are old batteries but they seem to hold a decent charge for a decent time. They are charged in a good quality Powerex charger so they are never cooked.
Anyhoo, so after I unplugged the Oregon and checked the Voltage at 14.4V I put the batteries back in with the cardboard and was going to put it back on the bike, but the Oregon would not start up. I opened it up took the cardboard out and put the batteries in it still wouldn't start. I plugged it into a little hand held power supply, (which is in fact what I used to charge the batteries in the Oregon just to be safer) and it started up. When I put the batteries in again, it started up. So I put the cardboard back in and it would not start.
OK I went through this procedure a few times but then it would not even start up from the external power supply. After a few minutes it did start again. But it won't start up with the cardboard in. OK I'm sort of suspecting that this has something to do with the high voltage of 14.4, which isn't really high but it's high for NiMH. Later in the day, after running the batteries down to 13V I put them back in with the cardboard and it did boot up no problem. This makes me more convinced that it could be the Voltage but that could just be a coincidence.
Now here's some more very strange stuff. I put a Fluke 101 meter, cheap but reliable, and I have a high degree of confidence in the Fluke as it reads the same as the voltage on the Powerex. Ok I plugged the Oregon into my little usb power supply which probably puts out about 0.5A and holds about 4Ah of stuff, without any batteries in and no rubber switch pressed down, and I was getting a voltage reading of 2.2V from the +ve to -ve terminals on the right hand side, and very very strangely I also got a similar reading when I read across the two +ve terminals, the one on the top right and the bottom left. This has left me mystified and confused. Pressing the button while checking this again made no difference.
Just for reference I did the same thing with the eTrex and it has no voltage across any terminals when plugged into the external power brick.
I do not know what to make of all this. Oh also note that the photo in the top picture says 2000mAh and 2.4V, which looks like the total voltage for the pair and the single capacity for the battery. Which sort of doesn't make sense, unless these are two 1000mAh batteries but then they wouldn't last 16 hours. It's not possible or is it that these batteries are in fact 2.4V each, for a total of 4.8V surely? especially as it takes normal batteries.
You are right, this button in the middle between the batteries IS the charging enabling switch. All you need is to place some stiff thin metal card (plastic one might be not strong enough and will bend) to be able to charge any pair of NiMH cells inside a Garmin device. Of course, they must be of the same model, make and manufacturing date.
But I personally charge those in a dedicated charger. Just not worth the effort