I'm not sure if it's a great idea to run your primary light off the heater battery. I think that's something that might be better kept separate.
My belief is it depends on how mission critical heat is for the Dive. If you’re doing stuff where you truly need heat, not just using it as a luxury, Having separate packs with an E/O light head certainly provides the most flexibility. I’m using a Fathom heat controller that allows me to switch the controller between any can with an E/O.
On a 8hr Dive, having more heat gives me warm and fuzzy’s (pun intended) considering I know several divers who experienced catastrophic suit floods on similar or longer duration dives, and were incredibly lucky to not go hypothermic or get bent.
One was saved on a 12hr dive by active heat, saying I “it felt like being in a water heater”
Another didn’t wear heat on a 6hr Dive and was shaking uncontrollably and blue by the time he surfaced, and ran very aggressive deco to get out faster. His saving grace was probably that he grew up in Michigan winters.
I personally don’t want to be in the position between choosing hypothermia or DCS.
For the diving I do (Florida caves, 68-72f) I generally follow these rules (I’m also a stick who gets cold easily and always wears 400g thinsulate for anything over 1.5hrs )
- I don’t usually bother with heat unless I’m doing a dive >2.5Hrs.
-On dives <4hrs I’m not reliant on heat, so I just carry one can for light and heat, as heat is just a luxury. I will switch to a backup light on deco, and enjoy my warm nap. I’m also running a Halcyon Focus, which consumes so little power that I’m very comfortable running heat off my 20ah Fathom battery pack for shorter dives.
-On dives >4hrs where heat is a necessity for safety, I carry one can for my light, and stage or carry one can for heat. I’m using a 20ah pack for primary light, and a 30ah pack for “dedicated” heat. This allows me to loose a battery entirely and still have some redundancy in heating.
Just my 2cents