18650 flashlight suddenly goes dim in all modes. Please help!

[Booga]

Hi all, I have an MTE SF-23 that takes 18650 batteries.
The issue is that it seems to suddenly go dim, sometimes it's immediately dim when I turn it on, sometimes it will be at "normal" brightness for a few seconds before then going dim. This happens in any mode (low, medium, high, flashing...) and once it's happened it remains dim when I click through the modes.

I've tried the batteries in another flashlight and they work fine, I've tried removing the rear cap and touching a key across from the body to the battery and the problem is the same. Cleaning with rubbing alcohol has made no difference.

This is about the limit of my skill/knowledge. Does anyone have any ideas or advice for what I can try? Thanks

 

[thermal guy]

I don't think that flashlight has a protection circuit in it are you using protected batteries? If so they may be tripping.

 

[Booga]

I don't think that flashlight has a protection circuit in it are you using protected batteries? If so they may be tripping.

It happens with both protected and unprotected batteries. (I think some are unprotected anyway, they're certainly a few mm shorter than the ones I know are protected.) I can take a pic of all the batts tonight and hopefully someone can confirm.

 

[Hooked on Fenix]

Return it if you can. Replace it with a quality light. Something similar happened to a Jetbeam headlight after I dropped it once. It was never the same again. I switched to a Nitecore headlight. When I dropped it, it damaged the floor, but the light still worked (HC60 headlamp).