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CR1632 is the battery you need, you should be able to find one around town somewhere, if not, I'm sure Lexus will have them. As far as I know there isn't a way to charge them without some sort of specific charger/equipment.
Replacing the battery is so simple even a cave man with all thumbs can do it.
Check on eBay and order a strip of them, they are cheap. Throw one in the car as a spare, and freeze the rest. You'll never be without a battery again.
The Fobs are strange. Once in a while, it says the remote battery is weak. Then nothing after that. And I know my Fob battery is good cause it is new (6 month old)...