DIY LiPo Battery Discharger with Halogen Light Bulbs

by Oscar
Published: Last Updated on

I will show you how to build a DIY LiPo Battery discharger easily with 12V halogen light bulbs. It’s capable of discharging 2S, 3S, 4S, 5S and 6S LiPo batteries.

As discussed in the article, how to dispose LiPo batteries, using salt water to discharge LiPo batteries is not a very efficient and effective method. Not only it can take a long time to discharge your battery, salt water can sometimes corrode metal contacts and stop the discharging completely leaving plenty of charge left in the pack.

So I built this light bulb discharger as a better way to discharge my damaged or old LiPo batteries. Just plug in your battery and just leave it in a fire-safe place, when the discharge is complete the light will go out. It can discharge a 2000mAh 3S LiPo battery in less an hour.

Further Reading: What are LiPo batteries and how to handle them properly?

Update (Jan 2018): I have been using this discharger for over 2 years now, it’s still going strong and very reliable for me :) I have discharged over 15 batteries of various sizes and cell counts with it and I have not changed a single light bulb.

LiPo Discharger Circuit Diagram

I am using 6 light bulbs in this discharger with an on-off toggle switch too.

There will be an option for 12V/24V discharging mode:

  • 12V is for 2S/3S LiPo
  • 24V can be used for 4S, 5S and 6S

The max voltage of 6S is 25.2V which is higher than 24V. But it should be fine for a short period of time, most of the times you are discharging half empty batteries anyway.

The bulbs are rated at 20W, so at 12V, each will be drawing 1.67A of current. 3 of them will be drawing  a total current of 5A with 3 bulbs in parallel, on paper it can drain a 2000mAh 3S battery completely in 24 minutes.

However the current draw decreases as voltage drops, and the discharge will slow down gradually. It will most likely take slightly longer than 24 minutes to finish the process.

The light bulbs also has a cut off voltage at about 1 to 2V, which means it won’t completely discharge the LiPo, but the charge left in the pack is so little, I normally just finish it by shorting the positive and negative leads in the battery. (Do this extremely carefully in a fire-safe place) Or you can connect it up to a power resistor.

lipo-discharger-light-bulb-build-circuit-schematics

Components for the Battery Discharger

I am trying to keep it as simple as possible, all the parts should be available on Amazon or eBay. The material should cost less than $10.

lipo-discharger-light-bulb-build-components-parts

lipo-discharger-light-bulb-build-discharge-mode-plugs

Building the LiPo Discharger

PCB screw Terminals

You can solder the light bulbs in the PCB directly, but I prefer using screw PCB terminals because it makes bulb replacement much easier. You can connect as many bulbs as you want and it’s very flexible.

They also allows you to connect other types of load including high current resistors or motors :)

lipo-discharger-light-bulb-build-PCB-socket

Make sure to add a lot of solder because there will be a lot of amps to go though.

lipo-discharger-light-bulb-build-PCB-solder-bottom

12V/24V Discharging Mode Switch

These light bulbs are rated for 12V, so if you are discharging a 4S battery or higher cell count, you need to put these bulbs in series, which effectively will double the voltage rating to 24V.

Here is a simple “mode switch”, which basically is just a bullet connector, either connects to the ground when it’s in the 12V mode, or to another array of bulbs in parallel in the 24V mode.

lipo-discharger-light-bulb-build-discharge-mode-plugs-solder

On Off Switch and Discharge Cables

The on-off switch is not a necessary item, because you can turn the discharger off by unpluging the battery, or the 12V/24V switch connector.

But I think the toggle switch can make this tool a bit more convenient and safe to use, in case something goes wrong during discharging, I can just flip the switch to shut it down if I am unable to unplug it from the battery’s end. (for example, battery is on fire?)

Notice that I am using some pretty long electrical wires, because I want to be able to place the battery as far as possible from the light bulbs during discharging, which should be easier to manage when something goes wrong.

lipo-discharger-light-bulb-build-on-off-switch

lipo-discharger-light-bulb-build-on-off-switch-solder

Future Upgrades

These bulbs can get very warm! I think it might be a good idea to put a fan on top to help heat dissipation. A small fan usually require 5V, which means we will need a step down voltage regulator, but it can be powered by the discharging battery.

I will also put some liquid electrical tape on all the soldering and connections to avoid shorts.

Testing

I tested the unit with just 1 bulb connected.

When turning on the light bulbs, I noticed there was a current surge of 2.6A at power up, and it quickly stabilized at 1.6A. If I reconnect the bulb again, the current surge doesn’t happen any more, probably because the bulb is warmed up. It’s not a biggie, I just found it interesting. :)

These bulbs are rated at 20W, with my 11.9V battery the actual power is 19W so it’s pretty close.

Using the 24V discharging mode, the power drops almost half which is expected because the resistance increases when you put loads in series. So once you have discharged your 4S, 5S or 6S battery below 12V, it’s a good idea to switch to the 12V discharge mode to discharge it more efficiently.

lipo-discharger-light-bulb-build-testing-result

Edit History

  • Oct 2015 – Article Created
  • Jan 2018 – Updated article with a list of build components

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21 comments

darren ceckanowicz 15th March 2021 - 9:40 pm

So I thought I was having great success with this method. Then, I noticed that my solder was “migrating”. It started with one connection and I thought, “that is weird…I swear I finished all the soldering”. Then I noticed that after discharging two batteries the solder was “disappearing” or moving toward the wires. I have a picture. But is it possible there was so much current that it was just heating up the solder?

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Mysekai 16th October 2020 - 2:46 pm

Not knowing much I soldered a single 75w halogenbulb to a pair of wires with an xt90 connector for the battery.

I tested the 3-in-1 discharger with resistors but on 6s they were too hot so I went simple with one
75w halogen bulb.

The bulb lights but goes out well before the battery is discharged to zero. Will it fully discharge with a 75w bulb? Will it take a long time compared to using 25w?
Is a single 75w halogen bulb too much? Should I have gone with 25w?

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Sandor 22nd June 2020 - 3:01 pm

Hi Oscar,

why can’t be used the same method for single cell?
I splited up my 3S battery because one cell is bad and would like to safely dispose only that one…

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Oscar 22nd June 2020 - 4:31 pm

Light bulbs have a cut off voltage, it stops discharging before voltage drops to zero. Use this for single cell DIY Resistor Discharger for Draining/Disposing Batteries

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Justin 29th December 2018 - 10:52 pm

Question about odd behavior. I built the circuit as shown above and it works with the lights lighting up when connecting battery. However the discharge rate seemed extremely high (checking with multimeter via balance cable). I stopped the process and the. my multimeter showed the voltage climbing back up after disconnecting. The battery was already bad (puffed) but looked to have puffed a little more. Any ideas regarding this behavior?

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Oscar 1st January 2019 - 12:16 pm

Discharge current depends on the power rating of the bulbs you use, and how many do you use in parallel.
If you want slower rate, try power resistors so that you have more control to the discharge current by using higher resistance.
It’s normal voltage go back up after disconnecting, voltage drops when it’s under load.

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Justin Stefano 1st January 2019 - 5:42 pm

Thanks for the quick reply. I have used 20w bulbs wired in Parellel. The voltage seems to drop a 1 volt per 10 – 15 seconds. I have also removed two bulbs and this still occurs. The voltage will drop 5 volts or so in a minute then climb back up to close to the original. It als seems to occur one my voltage drops below 9v or so. It seems to work as expected from 11v to 9v then drops rapidly.

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Jeff D Valento 14th December 2018 - 4:17 pm

Hi Oscar,
I dispose of old lippo batteries for our RC Club. Your article was exactly what I needed. I built your discharger (I had to learn how to solder first – not a techy!) with a few changes (gave up on the solder wire track, changed the XT60 for a toggle switch, and aligator clips on the discharge cables, as I deal with a wide variety of batteries and connectors). So far, I have dispose of 97 batteries with 15.6 hours of drain time. Discharger works perfectly and no bulb failures. Also built the ceramic resistor discharger as some batteries do not drain down to a low enough voltage to safely short out battery leads.
Thank you for a simple and effective design.
Jeff

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Oscar 20th December 2018 - 2:33 pm

that’s great to hear! thank you for taking the time giving me feedback.

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Mark 9th November 2018 - 10:45 am

Hello Oscar, thank you for this tutorial.

How did you fit the switch and the XT-60 to the prototyping board? (or are they just held on by the solder?)

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Oscar 23rd November 2018 - 3:58 am

XT60 is just held by solder :)

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Joe 16th July 2018 - 1:35 am

” The light bulbs also has a cut off voltage at about 1 to 2V, which means it won’t completely” quite the opposite, as these bulbs turn off (light wise), the filament gets lower ohms (couple ohms). For 4S, I just use a 1K quarter watt resistor (1 cent) and leave it on for a week outside.

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Daniel K 8th August 2017 - 3:15 pm

What size PCB is that?

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Oscar 11th August 2017 - 4:02 pm

i think its about 10cm x 10cm.

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Miguel 13th July 2017 - 1:58 pm

What kind of watt meter are you using? And how to connect an alarm buzzer to that?

I’m very inexperienced but I’d like to build a gadget like that. MTIA !

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Oscar 16th July 2017 - 3:01 pm

this is the watt meter i was using.
This is just a simple discharger designed to be used for disposing LiPo batteries, so I wasn’t using a alarm buzzer.
You could just simply plug a voltage alarm in the balance lead, however these alarms normally only go off at 3.5V percell.

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Earl 30th November 2015 - 11:52 pm

How low is it safe to discharge a lipo? I have a damaged one I need to dispose of

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Oscar 1st December 2015 - 4:21 pm

depends on why you want to discharge it… if for storage, best to discharge them to 3.8V per cell.. if you want to disposal them discharge them as close to 0V as possible.

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Catio 27th October 2015 - 11:54 pm

Hi Oscar
that’s too simple!
Please add an Arduino / µc with auto voltage detection to the discharger which switch the bulbs with mosfets. ;-)
Thanks.

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Oscar 28th October 2015 - 5:32 pm

hehe… no need to overcomplicate a simple idea :D

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Catio 30th October 2015 - 9:57 pm

Hi Oscar,

sure, it would be more a challenge for “because it is possible”.
I’m going to buy* an electronic load, so it will be useful for this too.

(to less time and knowledge to built it)

Reply