An Efficient Tool for Discharging Lithium Batteries

Because everything is battery-powered today, it can be tempting to think that our modern lithium batteries require no maintenance. Unlike older rechargeable battery chemistries, like NiCad, that had memory and would lose capacity if not fully drained and recharged regularly, lithium batteries seem foolproof. But that isn't quite true, because lithium batteries can degrade if they're stored in either fully charged or fully drained states. To keep those batteries nice and healthy, Matt designed this nifty Battery Drainer device that can discharge lithium batteries to exactly the desired level.

Matt built the Battery Drainer mostly for use with hobby lithium battery packs, like the kind you might use for an RC car. But it will work with any lithium batteries. In fact, it will work with all batteries of all chemistry types. That's because the user can configure discharge rates and levels to achieve the optimal settings for each battery. And it gets better: users can discharge multiple batteries at the same time, each according to their own settings. Finally, the Battery Drainer doesn't consume any power when it isn't active. That means that it won't drain your batteries any further than you want it to, so it is safe to leave the batteries connected for extended periods of time.

As the law of conservation of energy tells us, energy can't just disappear—it has to go somewhere. In this case (and for most battery discharging devices), the energy converts into waste heat. For that reason, the Battery Drainer has a massive heat sink and fan. Those help it dissipate the heat when draining batteries quickly.

Battery Drainer requires a fairly complex custom PCB that includes surface-mount components, so this is a project that takes some skill to complete. That PCB hosts a Raspberry Pi Pico development board, an OLED screen and buttons for the control interface, a fuse, and all of the components necessary for the discharging circuitry. To make discharging as safe as possible, Battery Drainer includes a temperature sensor, voltage monitoring, and current monitoring. The latter two are, of course, useful for consistent discharging, but they also increase safety by detecting anomalies like over-current.