DIY Resistor Discharger for Draining/Disposing Batteries

Before throwing away damaged or old LiPo batteries, you should first drain them completely  until there is no charge left. In this article I will show you how to build a high power resistor discharger.

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Simply connect the power resistor, or multiple of them, to your battery. Leave it for several hours or even overnight, it will discharge the battery completely eventually.

Although the discharging is quite slow, it’s gradual which is relatively safer for damaged batteries. There is no heat build up in the battery and so no puffing.

Further Reading:

• The many ways of discharging LiPo Batteries
• How to dispose old/damaged LiPo safely?

How Does Resistor Discharger Work?

It costs less than $2 to build one. The parts I am using are:

• 5W 47ohm Radial Ceramic Resistor:
  • AliExpress: https://s.click.aliexpress.com/e/_Dl16CvR
  • Amazon: https://amzn.to/3LbOX3m
• XT60 Connector
  • RDQ: https://oscarliang.com/product-xzn1
  • GetFPV: https://oscarliang.com/product-hwx7
  • Amazon: https://amzn.to/3l6oonc
  • Banggood: http://bit.ly/2AuxHnC
  • AliExpress: https://s.click.aliexpress.com/e/_Ddy57H7

This will discharge a 4S battery at roughly 0.3A  (16V / 47ohm = 0.34A), or 5 Watt (0.3A x 16V = 4.8W).

You can speed up the process by connecting a number of resistors in parallel using a splitter. If you are using 3 of them, it will discharge 3 times faster at 0.9A.

It’s not that fast compared to using Halogen light bulbs (each bulb is rated at 20W), the process can take a few hours to complete for a 4S 1500mAh pack. The good thing is you don’t have to sit next to it and watch, so it’s not a big problem.

Since the resistance is constant, the discharging current and power will drop as the voltage of the battery decreases. Therefore it’s a good idea to leave it to discharge a bit longer. Note that the resistance actually goes up a little as it heats up, so it will actually discharge a bit slower.

For higher voltage batteries like 5S and 6S, you might want to search for resistors, or that of higher resistance.

Warning: Make sure you don’t touch the resistor during discharging because it can get very hot.

Building a DIY Resistor Discharger

• Solder the resistor directly to a male XT60 (or XT30, or JST if you want to use it for smaller batteries) – polarity does not matter
• Cover it with some heat shrink tube (30mm width)

Checking Voltage

After discharging, you want to check the voltage of the battery to make sure there is no charge left (voltage is close to 0V).

A thing to keep in mind is that most battery checkers don’t work well when voltage drops below 3.0V per cell, and will report “no cell present”. It’s best to use a multimeter to check the voltage in this case.

You can pick one up cheaply if you don’t have one:

• BM18C: https://s.click.aliexpress.com/e/_DCXqsYf (recommend, feature-rich and good value)
• INNOVA 3320: https://amzn.to/2QkwGss (cheap and basic)