I've opened up one that died, they're just 18650 batteries. You can buy the 4x battery sleds at Tayda, and the MP1584EN chips on eBay for cheap, about 50 cents to a buck each. For a couple bucks and some wire you can easily roll your own, and have spare batteries for your flashlights
I'm using the prototype of this drawing below to run my pedals. I used 4 battery sleds, talk about overkill. I've only had to recharge twice in the last year, lol...
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