DIY Resistor Discharger for Draining/Disposing Batteries

by Oscar
Published: Last Updated on

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.

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:

How Does Resistor Discharger Work?

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

how to build resistor discharger steps

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: Banggood | Amazon

Or you can invest a bit more to get a decent one like this: http://amzn.to/2xIaV9Z

Too Lazy to DIY?

it seems like Banggood took the idea and started selling a ready-made version of this. They even took the beginning of this tutorial as their product description without permission LOL. Well, it is slightly cheaper, and less work than building it yourself.

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12 comments

Kranthi Kumar Reddy Banala 18th September 2020 - 1:02 pm

Hello,
I want to discharge to lithium polymer batteries of 11.1V and 14.8V to their cut off voltages. Both batteries having 20C.
I want to discharge using resistors. But i do not know the value of resistor to be used. Can you please help me.

Reply
Oscar 29th September 2020 - 10:50 pm

You can use the one in the tutorial.

Reply
Peri 18th June 2020 - 11:31 am

It’s crazy they just copied the whole thing!

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wd 16th February 2020 - 12:46 pm

I used one sold on Banggood (3-in-1) on a 3s 3200 hardcase recently. At the end the hardcase was puffed but the seals around not broken. It was not puffy at all before starting discharge. Is this normal?

Also the battery was quite hard at the end of the process. I wasn’t about to smack it on anything to see what was up. Is this hardening normal?

Reply
MH 13th December 2019 - 4:16 pm

Instead of shorting the batteries could one just leave a resistor attached permanently?

Reply
Claude Haridge 17th October 2019 - 4:49 pm

Is there a safe voltage per cell at which point discharged batteries can be safely disposed? Say the discharge brings the voltage down to 0.5v per cell; is this safe to dispose? I’m wondering if anyone has researched when a LiPo battery ceases to be a fire hazard.

Reply
Oscar 22nd October 2019 - 3:04 pm

I always short the positive and negative leads to completely discharge the cells before throwing them out, but of course only do this when you get the voltage down to as low as you can first.

Reply
Gary 6th August 2019 - 2:26 am

I am looking to discharge several new Lifepo4 3.2v 200ah batteries to check their resistance. Can I hook up a 50w 25ohm resistor to discharge a little faster? My goal is to time the discharge on the top balanced batteries to insure they are all good/matched batteries. Would it take long with batteries this size? Are there any challenges with taking this approach?

Reply
wd 18th June 2019 - 12:18 pm

I just bought the one from Banggood. It discharged my 4s LiPo perfectly. My question is:

Is the Banggood one good for discharging 6s as well? I’m new to this. Each resistor seems to be 15ohm.

Reply
Oscar 27th June 2019 - 5:40 pm

Should be fine as long as you keep the resistor cool, the only problem with higher voltage is that the resistors can get really really hot :)

Reply
amisoftau 21st November 2018 - 5:21 pm

Just extrapolating from the 47 Ohm for 4S – about 5W; I get the following:
3S – 33 Ohm 5W
4S – 47 Ohm 5W
5S – 68 Ohm 5W
6S – 100 Ohm 5W

Reply
Michael N 15th July 2018 - 1:15 pm

You should encourage your readers to recycle unwanted batteries instead of throwing them out. In many areas it’s illegal to throw them away.

Reply