On the HF ones you can take them apart and there are a couple of coin cell batteries in there. They aren't made to be changed but you still can if you're stubborn like me. If I remember right they are each 3v coin cells and wired up in series so 6v total. They are what makes it switch instantly, and then the solar cell kinda takes over once you are welding. I took my coin cells out and soldered in a 9v connector and then added a clip to hold a 9v Battery above the lens. Some of the more expensive models of auto darkening helmets actually have little access doors to change your coin cell batteries out when they die.
Got asked for more Christmas ideas. Never had an auto dark helmet. What one out of these 2 would you choose for under a $100. Or something completely different.
edit: I have a large head oval shapes if it makes a difference.
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I have an older Optrel that I really like, the batteries died and I had to do surgery on it with a hot knife but I could not get the replacement coin cells to work, I even bought a little coin cell enclosure. Pretty sure the voltage was right but maybe it did not supply enough juice? Anyway I ended up soldering in a RC hobby type connector and velcroing in a 6V 700mAh Rechargeable Ni-Cd Battery Pack. Feels a bit sketchy having cells that close to my face but It worked and I have had no issues out of it since.
Are those Optrel’s worth it? ~$500 for a helmet seems ridiculous unless you’re making money in one