Draining a battery is easy. Just put a load across the terminals, maybe an incandescent bulb or a beefy power resistor, and wait. What’s quite a bit trickier is doing so safely. Put too large a load on, or leave it connected for longer than necessary, and you can end up doing damage to the cell. Not convinced he’d always remember to pull the battery out of his jury-rigged discharger at the opportune moment, [Jasper Sikken] decided to come up with a simple tool that could automatically handle the process with the cold and calculating precision of silicon.
At a glance we can see the major components you’d expect in a discharger: a fairly simple PCB, four ceramic power resistors, a holder for a single 18650 cell, and a rocker switch to connect it all together. But wait, what’s that a TP4056 charging module doing in there?
While its presence technically makes this device a battery charger, [Jasper] is actually using it for the onboard protection IC. With the charging module between the cell and the power resistors, it will cut the connection when the voltage drops to 2.4 V. Oh yeah, and it can charge the battery back up if you connect up a USB cable.
[Jasper] says his little tool works great, with the resistor array putting just enough load on the battery to pull it down quickly without getting so hot that they’re dangerous to have exposed. He estimates the BOM for this gadget runs around $2 USD, and is considering offering it as a kit on Tindie in the near future.
[Pawel] himself notes that the device isn’t something the average person would necessarily need, but it does have its applications. There are times when working with various battery chemistries that it is desired to have them held at a certain state of charge. Also, such devices can be used to measure the capacity of batteries by timing how long they take to discharge when placed under a given load.
The build is one that takes advantage of the available parts of the modern hacker’s junkbox. An Arduino is used with an N-channel MOSFET to switch a resistive load. That load consists of load resistors designed for automotive use, to allow cars originally designed for filament bulbs to use LED indicator lights without the flash frequency speeding up. The resistors are 10 ohms and rated at 50 W, so they’re just about right for ganging up to discharge small LiPo batteries in a short period of time.
[Pawel] has tested the basic concept, and has things working. Next on the agenda is to find a way to get rid of the excess heat, as the current design has the resistors reaching temperatures of 158 °F (70 °C) in just a few minutes. Use some of that power to drive a fan?