The Nora battery box accommodates simple non-proprietary, relatively low cost, 3.7v 7000mAh 1s2p 18650 battery packs. Weight 100g.
2 cell arrangement minimises helmet weight and provides plenty of capacity for well skilled caver to easily cope with 24+ hour underground trip. Spare capacity is of course better carried elsewhere, not on your head, for the longest of trips. For such trips you would need to carry extra capacity if you had 3 or 4 cells on back of helmet too, so the weight (and cost) penalty just keeps stacking up. Larger battery packs makes little sense with advances in LED and battery tech.
Note how the battery pack protrudes from the open Nora 4 box and have plenty of clearance around them, making them easy to remove for charging, or of course swapping out on those super long multi day underground trips.
Connectors protected within well designed battery box are less prone to failure than external connectors on lamp cable, when used in harsh environment such as caving.
Nora battery packs are a simple 1s2p side by side 3.7v arrangement, with robust rc connector, and protection circuitry. No fanciful proprietary arrangements to tie you to Nora cell packs.
Lil' Monkey battery packs are manufactured from high quality industrial cells, or can be readily constructed from 18650 cells if you know what you are doing.
Knowing some fundamentals about li-ion batteries (important) will help you get the best from these.
Edna 1s1p 3.7v 3.5Ah battery packs can also be used with Nora battery box. Useful option for reserve, or shorter / lighter weight trips. Weight 50g.