WARNING: If you do this hack, you CANNOT charge through your main discharge lead anymore. The reason is that the charger will think the battery is at 4.5 volts / cell and will refuse to charge it. You MUST charge ONLY through the balance lead. Here is the charger that I use to do that:
Get the Pololu step-up regulator here:
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You might be familiar with the Fatshark Goggle Hack, where you build a 2S pack out of commodity 18650 lithium cells. The result is a pack not much larger than the Fatshark 1800 mAh pack, but with almost 2x the capacity. But there's a catch! The Fatshark goggles themselves will shut down when the battery gets down to about 7 volts, and they start beeping at you around 7.2 volts. Which means you can't actually use a good chunk of the battery's capacity!
In this video, I show you how to install a 9v step-up regulator in your battery pack, so that the output voltage stays at 9v ... well, pretty much forever. And that's a good and a bad thing. Because it means you can draw your batteries all the way down to 3.0 volts per cell and get their FULL capacity, but also it means you can draw your batteries all the way down to 1.5 volts per cell and destroy them, and you'll never know the difference.
If that doesn't scare you off, proceed.