Discharging with light bulb arrays. - RC Groups
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Feb 22, 2008, 12:27 PM
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everydayflyer's Avatar

Discharging with light bulb arrays.

So simple yet some seem unaware or just like to have more information so here you go.
I most always use #1157 bulbs as they are very common and if one shops a bit can often be purchased at a very reasonable price. Recently I purchased 30 from a local auto parts store in boxes of ten each and was able to get a bulk price of $0.45 each or $13.50 for the lot.

These bulbs are not precession resistors and their Resistance / current consumption will vary a little but they get the job done. Use caution as they do get very hot and with a bunch on at once extremely bright also.

Powered by 12 volts they draw(use) approx. 2.7 amps each. If wired in parallel as the two 5 bulbs arrays are the the total is approx. 13.5 amps. . Notice I used APP connects and one has a connector set on each end. this makes it very simple to connect together for a 10 bulb total and approx. 27 amps.

The APP connectors also make it very simple to connect the two 5 bulb arrays in series Red (pos) of one to Blk.( neg.) of the other . This reduces the current load to 1/2 but doubles the voltage they can handle. This is necessary with packs much over 12 volts as the bulbs have a limited life if over driven a bunch. I have used these 12 volt bulbs at up to 14.5 volts but that is pushing them a bit.

I just happen to have a quantity of approx. 1/16 (0.060") nickle plated copper wire which makes a nice rigid grid for them. Most any wire will do but rigid is nice and makes handling easier. 14 GA. copper household would work well . Electricians use to just throw it away when wiring a new house but with copper prices these days you may have to purchase a foot or two.

Positive and negative have no real meaning here unless you are trying to describe series and parallel. I like color coded connections so I do not get confused as easily.

The bulbs are not difficult to solder but you need to be fast on the bottom contacts. Make sure they are clean(lightly sand) and pre tin. Pre tin the wire and work fast. If you leave the stiff wires long they make a convient handle.

One very convient use for a 2 bulb unit is using it with a Hyperion LBA6 /10 or one of the many other similar balancers to discharge packs which need to be stored. Yes the LBA and clones will over dischrge some before they auto disconnect but it is better than getting disstrated and going to zero and you can put a little back in if needed..
Another use is to load / capacity test a pack/cell etc. Use an line watt meter and the appropriate number of bulbs.

They can alsio be used to discharge LiPolys for disposal.

To hold the bulbs while soldering to the bottom contacts I place a rubber band around the handles osf a regular pair of pliers and grip the base of the bulb in the semi circle section of the pliers jaws.

A check I just conducted ahows that my two 5 bulb arrays draw 14 amps. each at 12.20 volts an connected in series they draw 9 amps.

Last edited by everydayflyer; Feb 22, 2008 at 04:50 PM.
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Feb 22, 2008, 01:56 PM
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ptglass's Avatar
I often thought that would be a great way to completely discharge a lipoly for disposal. Going to make up a single bulb today, just for that purpose! Thanks for reminding me about this.
Feb 23, 2008, 12:01 PM
That's a funny word
This one has removeable bulbs and could be made in any length. It runs from 2-12 bulbs and uses the newer style bulbs (more expensive) and both filaments.

It now has connectors on both ends and is nice to hook a dvm up to one end or even a pair of batts.
Last edited by gulio; Feb 23, 2008 at 02:09 PM.
Feb 23, 2008, 01:43 PM
Restful User
Jacques Flambeau's Avatar
Been using one for years. This array allows the use of 2, 4, 6 or 8 bulbs for approx 3, 6, 9 and 12 amps.

Feb 23, 2008, 05:27 PM
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mmormota's Avatar
In case of high load discharge consider that the cold resistance of a bulb is way lower then the hot resistance.

It means, that the discharge starts with a short but very high current spike.

Suppose a Lipo battery under test, drawing discharge graphs using something like Emeter or a data logger plus the bulbs. If the discharge current is near to the pack's rated maximum, the startup current is way higher and may damage the battery.

A possible workaround is connecting the bulbs one by one.
Feb 24, 2008, 02:21 AM
Registered User
Here is my very basic battery discharging setup. I didn't need anything fancy, just something I could use to discharge my 2s1p and 3s1p lipos at a moderate rate. The lamp receptacle is just a standard household light socket, nothing special. The bulb is 12-volt (the kind used for RV's and boats), 50-watt, and pulls approx 3.5 amps from my 2-cell Lipos, which is well below their maximum continuous rating of 10 amps, so it's a safe discharge.

The total cost for the socket and the bulb was about $4.00. It's very useful for bringing batteries down to storage voltage, or for performing capacity tests without having to run them through a motor. Obviously having an Astroflight Whattmeter (or something similar) is essential to be sure you don't discharge them too far, or at too high a rate.
Last edited by kizzy; Feb 24, 2008 at 06:02 AM.
Feb 24, 2011, 01:38 AM
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EricJ320's Avatar
I know this is an old thread, but I'm wanting to wire up something like this, no sense in starting a new thread when it's covered pretty well here.

Charles mentions in the first post using them up to 14.5v, is it possible to wire these up in such a way that would allow them accept a 4S pack at 16.8v? Maybe a bank in series and then parallel a couple of banks? Ideally I'd like to be able to discharge 2S, 3S, and 4S with the same array. I just don't know if that's possible, or how to wire them up if it is.

This is mainly for discharging packs to storage voltage, but also for disposal when needed. Thanks for any help!

Edit: Ok it helps if you read Charles post carefully. After doing that, and studying the pictures a bit more, am I correct in that the top two in the picture are parallel arrays, each usable separately for <12v batteries, or can be plugged together in series for >12v packs?
Last edited by EricJ320; Feb 24, 2011 at 01:45 AM. Reason: The light bulb went on!
Feb 24, 2011, 07:34 AM
Southern Pride
everydayflyer's Avatar
Bulbs in parallel just like batteries equals less resistance which equals more amps.

Bulbs in series equals more resistence which equals less amps.

Thread here has some nice bulb arrays and a great LVC device to prevent over discharge.


Feb 24, 2011, 11:12 AM
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EricJ320's Avatar
Not sure why I stumbled across this thread and not that one in my searches, but that is exactly what I needed! Thanks Charles!
Nov 19, 2012, 03:04 PM
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Wild Bill's Avatar
Originally Posted by Bill Harris
Been using one for years. This array allows the use of 2, 4, 6 or 8 bulbs for approx 3, 6, 9 and 12 amps.

Bill or whoever can answer this question for me.

What are the specs. on the switches that you used for your bulb discharger? And where did you purchase them? Radio Shack, I hope. Small town, not a lot of choices.

I have the bulbs, I just need to buy the proper sized switches.

Thanks in advance,

Bill (also) (It's a great name, isn't it ).
Nov 20, 2012, 09:42 AM
Southern Pride
everydayflyer's Avatar
The switches only have to be rated for the current of the bulbs they control, that is if they switch two 1157 tail light bulbs then say 6A. Auto parts stors,Wal Mart and many other stores carry suitable 12V automotive switches. I use Anderson Power Poles on min so they are east to combine arrays in series,parallel etc..
Nov 21, 2012, 09:00 PM
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Wild Bill's Avatar
Any reason for larger packs I can't run T3 bulbs in 12volt for 3 and 4 cell and T3 bulbs in 24 volt for 5 and 6 cells.


Nov 21, 2012, 10:05 PM
Registered User
No reason at all. Provided you're operating the bulbs within their prescribed voltage limits and the resultant current draw meets your desire, have at it.

Understand that at 50 watts/bulb, they will be extremely bright and extremely hot. You will most assuredly need to have some means of dissipating heat in order to get decent life from the bulbs.

Most of us use 1157's or similar as they're inexpensive, ubiquitous, and a known entity.

Nov 21, 2012, 11:25 PM
Mach 1
pathfinder's Avatar
I have been using these made buy Deans for years.One pic. has some bulbs added to it.And it has the Deans Black Box added to it.The Black box stops discharge at 5.4 volts.
Jan 18, 2014, 09:34 AM
Registered User
Count me in!

New to this hobby since the past 3 weeks. Unfortunately terrible rain season came, and I didn't had any chance to run my cars. Hence the battery needs to be stored in storage voltage.

As a newbie, my charger is a cheap compact iMax B6 LiPO without any discharging feature. Did took a look at discharging feature offered by more expensive charger, but they seems to stick at mere 600mAh-1A. So even if I would spend some 3x more on the expensive chargers, looks like discharging feature won't be satisfactory as it would take too much time.

After some learning on the forums, seems that DC bulbs are the perfect load to discharge LiPOs.

Hence the birth of Alien LiPO Discharger!

It is made using a couple of Osram motorcycle bulbs, less than $1 each. Each bulb has two filaments, 12V 35W each, so total filaments of 140W at 12V. Stand is made from some pipe clamps I had from my canceled hobby project almost 2 years ago. A lipo buzzer alarm is attached and set to beep at 3.8V per cell.

After some runs, I can confirm the initial difference of resistance when the bulb is cold. With initial 8.4V and cold bulb, it seems to spike at around 10A on first couple of seconds, then went steadily at 8A.

Discharging the Turnigy 5600mAh 50-100C from full charge to 7.6V took 19 minutes.

No cool automatic cutoff stuff here, but I wouldn't mind unplugging it manually as it's just a short 19 minutes of waiting
Last edited by watch_mania; Jan 18, 2014 at 09:41 AM.

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