I should have mentioned that the model I questioned Garmin about was the eTrex Vista Cx. Here is Garmin's response:

Quote:

Thank you for contacting Garmin International,

The lithium batteries would work fine within the Vista Cx, without any problems. If you have any additional questions, please let me know.

With Best Regards,

Joey R
Product Support Specialist
Automotive Team
Garmin International

Original Message Follows:
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Hi,

I was recently looking at some information about some of my other camping gear, and read that I shouldn't put lithium batteries in my headlamp because it can cause the unit to overheat. Is there any reason I shouldn't be using lithium batteries in my GPS? I like the weight savings that the lithium batteries offer.