Introduction: Make a Simple Battery Discharger for NiCad or NiMH Battery Packs.

Picture of Make a Simple Battery Discharger for NiCad or NiMH Battery Packs.

This instructable will show you how to make a simple discharger for your NiCad or NiMH battery packs for Airsoft AEG's or RC cars, boats, planes, whatever. Draining your NiCad battery packs is an important step in preserving the life of the battery, since NiCad's suffer from whats known as the memory effect. The memory effect is when a NiCad isn't fully charged on its first cycle, then depleted, and recharged again. However, it will not achieve a full charge because it's cells have adjusted to half capacity, or memorized half capacity. NiMH pack do not suffer from the memory effect and do not require regular darining, but if you plan on storing one for an extended period of time, you may wish to do so.

The design is not mine though, it is borrowed from this one here and here. I didn't see the point in paying $5 plus the shipping for something I could easily make for less that amount.

I bought all the supplies(2 12v bulbs, heatshrink tubing and tape) at Wal-Mart for a total of $5.63.

Price list:
2 Pack 12v Bulbs - $2.78
Heat Shrink Tubing - $1.96
Tape(you probably have it, but I happened to have ran out)-$.47

You may be able get spend less at other stores or if you already have some or all of the materials.

Step 1: Materials and Tools.

Picture of Materials and Tools.

You will need the following...

1. Soldering Iron, and of course solder.
2. Electrical Tape
3. Heat Shrink Tubing
4. 2 12v Light bulbs, used for car tail lights and backup signals.
5. 1 Tamiya connector. I used a smaller one from an old charger that fits my battery. Yours may depend on the connector size of your battery.
6. Some 16 gauge wire or similar.

NOTE: Coffee pot not required :).

Step 2: Step One.

Picture of Step One.

Prepare the Tamiya connector to be joined to lead wires.

Since I harvested my connector from an old charger, I had to graft the 16g wired to it. If you do similar, you will need to do what I did here. If you get your connector some where else with wires already attached, you can skip ahead to the next step.

Ok, strip the coating off the two wires on your tamiya connector. MAKE SURE THE POLARITY IS RIGHT SO IT WILL MATCH UP WITH YOUR BATTERY. Ok, that didn't need to be in caps, but is is important that they match up. Usually the round post on the connector is the positive and the squared one the negative. If you use red and black, join them accordingly.

All I did was strip the wires and twist them together, put a little electrical tape over one connection, then slipped a piece of heat shrik tubing over it and then shrunk it. You can solder the connections if you wish.

Sorry about the yellow tinge, I had to take the pictures in my kitchen cause I was to lazy to clear my computer desk, and If I used the flash up close it ruined the picture due to the white counter top.

Step 3: Step Two.

Picture of Step Two.

Ok, the polarity on these bulbs really didn't matter, they worked both ways. Yours may be different, but check anyway with a 9v or with your assembled part from the prior steps.

Anyway, you need to solder the positive(or negative whatever floats your goat,lol)to the bulb. The ones I used had two contacts on the bottom that where conveniently made of solder. All I did was melt them, stick the wire in it, and then solder some more on for good measure.

I found that the easiest way to do this by taking a water bottle, or soda bottle and sticking the bulb on top and holding it in place with a piece of tape. It makes the whole process of soldering to the bottom of the bulbs a heck of a lot easier!

Step 4: Step Three.

Picture of Step Three.

Unfortunately, I accidentally deleted the picture I made for this step, but you can clearly make it out in the one below.

Ok, take the other bulb and lay it opposite to the first bulb. You can use some tape to hold the two together. Put the negative wire in between the the bulbs and solder it in place. This was the hardest part of the job for me because the solder wasn't sticking to one side of the bulb, so I had to make it as best as I could and hold it together with tape. I suck at soldering, but if your good at it, you shouldn't have any problems.

Step 5: Step Four.

Picture of Step Four.

Now, you need to complete the circuit. Cut a piece of wire long enough to reach around to the bottom of the other bulb. Make sure you strip enough coating off to make it easy to solder. I was able to slip one end of the wire under the first connection on the first bulb and fold the wire over and solder it in place(sorry no pic of that, got deleted). Or just do whatever is easiest for you.

After you secure it to the first bulb, flip it over and solder it to the second bulb.

Trim any excess, and wrap the middle in electrical tape.

Step 6: Test It Out.

Picture of Test It Out.

Connect your battery pack to the discharger and make sure it lights up. If your using double filament bulbs like I did, make sure both are lit.

Congratulations, that's it, your done!

You can add more bulbs to the circuit if you have a larger capacity battery. Mine is a 8.4v 1100 mah battery.

Oh, and be very careful where you put this thing when its plugged in, the bulbs get very hot!!!


stavart (author)2014-01-29

Can you use this method to totally discharge Lipos(that aren't good anymore), for safe disposal?

hansonsux (author)2010-03-29

The NiCad "memory" is a myth.  repeadedly discharging any battery will only use up it's cycle life faster.  There are however times where you could occasionally deeply discharge a battery - buy for memory is not one of them.

shannonlove (author)hansonsux2013-03-03

It's a myth with the technology used in the last 10-15 years but in the early 90s when NiCads were very new, it was a real effect. However, it was eliminated fairly quickly by the manufactures but somehow the concept really "stuck" with people.

m1sterb0b (author)hansonsux2010-06-15

actually, "deep discharging" a NiCd battery is bad for it. Most cell phones or devices that use NiCd have a circuit designed to only discharge it to about 90% on PURPOSE. ----> explains it a bit better!

hansonsux (author)m1sterb0b2012-08-26

I know, repeatedly deep discharging is to be avoided, but there are times when it can be done. Cells have a certain number of times they can be "repeated" chemically, which equates to their cycle life. If the cells are deeply discharged, then it takes away more cycle life, or wears them out faster. But, deeply discharging occasionally isn't much harm. Of course, better quality cells can survive this "damage" better then cheap ones.

* Why people still continue to perpetuate the "memory myth" is beyond astounding. In doing so, people will heavily deeply discharge their cells believing their actions will make their cells healthier, but in reality they are doing more harm than good in the process.

m1sterb0b (author)hansonsux2012-10-16

I know part of it is to be blamed on ignorant people who work at stores that sell devices that use these batteries. I don't know how many times I've been in an apple store (servicing an ipod or getting a new one or something) and I've heard the "genius(s)" behind the counter talking to those who just got a computer/ipod, or those complaining of poor battery life to "completely discharge it until the computer shuts off and then plug it in and charge it back up". I know people who, after hearing that, do that once a month, and then in 6 to 12 months complain that they can't keep their computer on battery for more than an hour when they COULD leave it on for about 4 hours when they first got it!

Amperis (author)2012-03-05

great instructable, I leave a link on the common factors of the discharge of the battery: Description and treatment Of sulphated batteries:

de0509 (author)2012-01-14

why not get a flashlight that uses what size batteries you want to discharge, then you can play with the flashlight all night long. it would be more fun. but still, whats wrong in making a new toy right? congrats for the project

rcisneros (author)2010-09-24

Wow. There are so many wrong things said, it would take to much time to correct them all. Go to to get the whole truth about deep discharge, memory, etc. They go through each of the facts about nicad, nimh and lithium.

Macworldwizardz (author)2010-06-25

Thats Good. I bet you'd make a fourtune selling this.. :)

shad0w88 (author)2010-01-10

i have the same bulb, so ur saying that one terminal is the two points on the bottom and the other terminal is the outer shell?..

i've done this is it right?.. 

i plugged this into 7.2v battery and it wouldn't light up??

S1L3N7 SWAT (author)shad0w882010-01-11

The bottom terminal should be the positive (+) end. The side should be the negative (-). Although, it really doesn't matter what the polarity is because it's an incandescent bulb. Maybe your battery is to weak to light the bulb. Also, check your solder connections. It's most likely a bad connection, those things are difficult to solder to. Or, your battery is too weak.

arvindhimalayan (author)2009-08-29

time pas

petederidder (author)2008-12-11

Pete says: I used one std. 12v auto bulb and discharged some 700mAh RC packs. It took about 30 minutes to discharge to 1.05v/cell with a peak temperature of ~91 degrees F. With two bulbs the dishcarge time was ~13 minutes and I got a peak temperature of over 120 degrees. Use one bulb and be kind to your batteries.

fwjs28 (author)petederidder2009-03-05 said

Derin (author)fwjs282009-07-22


S1L3N7 SWAT (author)Derin2009-07-22


fwjs28 (author)S1L3N7 SWAT2009-07-22


fwjs28 (author)Derin2009-07-22


S1L3N7 SWAT (author)fwjs282009-03-05

He meant "standard", but I guess you already know that.

fwjs28 (author)S1L3N7 SWAT2009-07-22


kanamin (author)petederidder2009-06-26

I'm guessing the temperature is coming from the internal resistance of your cells dissipating heat. RC packs with larger cells typically have a lower internal resistance, and you can discharge at higher current. Most RC dischargers I've seen use 10 of these bulbs.

knight2067 (author)2008-08-19

Because incandescent bulbs use resistance to generate light, polarity doesn't matter. With an LED, it does.

Derin (author)knight20672009-07-22

Press the reply button.

joinaqd (author)2008-12-29

why dont you use a JOULE THIEF instead?its better,plus you can get the light on for longer...

S1L3N7 SWAT (author)joinaqd2008-12-29

Because the the purpose of this is to drain the battery to ready it for a recharge, not to draw out all the remaining energy, like a joule thief. I suppose it would work, but would likely take longer and do damage to the battery, like reverse charge or something. Thanks for the suggestion though.

IX Smith XI (author)2008-07-21

how long do you have to wait to discharge

S1L3N7 SWAT (author)IX Smith XI2008-07-23

Not too long. You just have to watch it till the lights go dim, it shouldn't take long.

hp79 (author)2008-06-02

If you keep this on until light is completely dead, it'll cause a reverted voltage on some of the lower capacity cells in your strip of the battery pack. Then you have a bigger problem. It would be okay for a parallel hooked single cell though.

theRIAA (author)2008-04-25

nice! i've seen these online for like $20, way to stick it to the man. just be sure you know what they're for before making one.

S1L3N7 SWAT (author)theRIAA2008-04-25

WOW $20!! That must be for the ones with like 6 bulbs. But yeah, make these only if you intend to use them to drain battery packs, or if you want a crazy bright hand held flood light! I plugged in mine and was blinded for like 5 minutes lol.

richms (author)2008-04-24

Unless the pack is matched, this will allow the cells to be damaged thru reverse charging unless you are there to monitor it via a volt meter and disconnect it when the voltage drops below what indicates that at least one cell is down.

S1L3N7 SWAT (author)richms2008-04-25

Yeah, its a matched 7 cell pack. This thing is supposed to leave less than 1v in each cell. You unplug it when the light gets really dim.

nf119 (author)2008-04-24

Isn't the memory effect only on the NiCd? I am not sure but I think it doesn't help to fully discharge NiMh, it may actually shorten their life by wasting charge-discharge cycles.

S1L3N7 SWAT (author)nf1192008-04-24

Yeah, I think your right, but it may help if someone wanted to drain one for long term storage.

nedfunnell (author)S1L3N7 SWAT2008-04-24

It's best to store NiMH at around 80% charge.

S1L3N7 SWAT (author)2008-04-24

Ah, what the heck. I submited it to the contest anyway.

joejoerowley (author)2008-04-24

Cool! Great Instructable! You might be able to enter it into the go green contest because allows batteries to last longer. Idk though. Thanks Joe

S1L3N7 SWAT (author)joejoerowley2008-04-24

Yeah, I was thinking about that but since its not really my design it may be inappropriate.

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More by S1L3N7 SWAT:How to replace a broken analog joystick on an XBOX 360 controller.Make a simple battery discharger for NiCad or NiMH battery packs.
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