If your brother uses Nitecore 16340 batteries with 950mAh, and set his usage to 3.0v instead of 3.7v (batteries are 3.7v, but that uses more juice), he will get MUCH better battery life in his AGM thermal. It's what I run in mine.
These batteries fit with no problem, but DO NOT order the ones with the built-in charging port, they DO NOT fit in the AGM, they are too tall...Ask me how I know.
Luckily I only bought 2 of them, and I can use them in one of my Streetlight flashlights, but the regular 950mAh 16340's in the link below fit perfect and run perfect. I have about 8 of them (4 sets), and carry 2 fresh ones in the thermal, and then 2 extra sets in a Store-a-Cell to keep them together, and to keep them from touching each other in my pocket and discharging.
Next-Generation 16340 Battery From Nitecore’s latest battery update, the NL169 is a high-quality 16340 Lithium-ion rechargeable battery. It has increased its capacity by 46% to an impressive 950mAh capacity, compared to its predecessor NL166. Made with Nitecore's hallmark safety features and...
www.nitecorestore.com
Stor-A-Cell for CR123/16340 Batteries