Can I put the charge button on alkaline instead of NIMH since I have a external battery charger and don't want the scanner to charge when plugged into AC power.
Yes. Use a pointed object to set the battery selection switch to ALK. This is on page 23 of the manual. This will get expensive if you listen to battery power a lot. I keep plenty of extra rechargeable batteries, which I charge with an external charger, in my radio go bag. It's still always good to keep some alkaline batteries available for just in case.