How-To: Building a light bulb discharger

Why build your own discharger?
First off what I will refer to as a "discharger" is really just a resistive load that can be used to discharge packs. For this crowd I will use its function as its name.

The purpose of this how-to is to give you some ideas of how to build your own light bulb battery discharger. So why does the average RC hobbyist need a discharger? It turns out that they have many uses including discharging packs for storage, load testing a new power supply, prepping a lipo disposal, cycling a new lipo, etc. Like many tools, you don't think they will be useful until you own one and then you can't believe you didn't get one sooner!

Light bulbs huh ...
Why use light bulbs? As it turns out in order to make discharger that is useful, it needs to be a fairly stout. This is because we, as RC hobbyists, use some pretty stout batteries in our pursuit of fun. There are many different DC devices that can be used to discharge RC batteries, things like fan motors, thermoelectric coolers, small and large resistors and of course household and automotive light bulbs. The reason for using light bulbs is two part, first they are easy find and because they offer a great wattage per dollar ratio. For example you can buy a 12V 20W bulb for less that a $1 compared to a equivalent power resistor that could cost $10. Put 10 bulbs in parallel to draw 200W you see where I am going with this. Of course like any solution, light bulbs are not perfect but for the money they are pretty hard to beat.

What kind of light bulbs?
There are literally 1000s of light bulbs to choose from. Anything from a standard 60W 120V household bulb (will not draw 60W on 12V though) to an automotive head light can be used. One popular choice is to use auto tail light bulbs like the good old 1156, a ~20W single element tail light bulb. You can either solder to them directly or buy sockets for them. They are an easy to locate solution but are expensive one in comparison to other options. Another choice would be auto head lights. They are usually 45-55W bulbs but they too are expensive and are a little large for their wattage.

Whatever bulb you choose to use in your discharger, make sure you do your homework before you start buying. Be sure to take the sockets, if used, into account, as well as other requirements for the bulb you choose.

Time for a little math
This is where the planning starts. Step one for building a battery discharger is to decide its capabilities. Do you want a single, multi-use discharger that will work with pretty much any rc pack? Or do you want one that is reconfigurable for use with different packs? Or maybe you want a powerful one for a specific pack? Before you go out to the store and buy a bunch of light bulbs, make you get some target numbers in mind so you know what you need. How do you find these target numbers? Read on ...

Example 1: Discharge a 3s lipo at 10A
Lets start with a simple example, lets assume you want to discharge 3s packs (11.1V nominal) at 10A. Here is the math to figure out what we need. Start by calculating the wattage.

Watts = Volts * Amps = (11.1V) * (10A) = 111W

Note: The nominal voltage of the pack is used here because that is approximately what the average voltage will be under load.

Unfortunately a 111W 12V bulb is a little hard to come by but you can use several smaller bulbs to accomplish this task, you just need a working knowledge of parallel and series connections. Lets say you find a good deal on 12V 20W bulbs. How would you get a 111W draw off 20W bulbs? It turns out that placing 2 of the same light bulbs in parallel doubles the amp draw while retaining the original voltage, effectively doubling the wattage load. So if a single 12V 20W bulb draws 20W, placing it in parallel with a second will create a 12V 40W load. Likewise adding in a 3rd will make it a 12V 60W load, a forth increases it to a 12V 80W. Using this concept we can figure out that putting 6 of these 12V 20W in parallel will create a 12V 120W load and that will be just about right for this use. Yes it is a slightly higher load than 111W but it is close enough for our uses. Below is a circuit diagram.

Example 2: Discharge a 12s lipo at 10A
Now lets look at something a little more complicated, say you want to discharge a 12s lipo (44.4V nominal) at 10A. You could try to locate some 48V bulbs and maybe you could find some but they will probably be expensive and your choices will likely be very limited. Instead why not make use of 12v 20W bulbs like the ones I used in the 3s discharger example above? To do this bulbs will have to be placed in series and parallel. First lets find the wattage.

Watts = (44.4V) * (10A) = 444W

If you simply take 444W and divide it by 20W, as that is the bulb used above, you will get 22.2, call it 24 (I will expand on that in a moment). So it will take 24 12V 20W bulbs to draw 10A from a 12s lipo but you can't simply take 24 bulbs and put them in parallel like above, as that would just burn them all out. The bulbs used above are designed to run off 12V and not 44.4V. This is where series connections come in. It turns out that 44.4V is fairly close to 48V or 4 * 12V. By combining 12V bulbs in series, the resultant bulbs can handle more voltage but doing so will retain the wattage of the original bulbs. For example placing 2 bulbs in series will create a 24V 20W load, placing 3 in series will create a 36V 20W load and placing 4 bulbs in series will create a 48V 20W load.

Now you know that you can connect smaller bulbs together in order to create set of bulbs that can both draw more current and accept a higher input voltage. So what will the resultant circuit look like for a 48V 10A discharger using 12V 20W bulbs? Well first you need to make a 12V 10A discharge circuit, as done in example 1, and then you place 4 of these in series to increase the voltage to 48V. Below is a circuit diagram.

A simple design for most people
Now let me make a suggestion. I believe the following circuit is the cheapest, simplest, best design for most people and that is (12) 12V 20W bulbs arranged in 2 rows of 6, and connected together. This allows for 4 common loads, 12V 5A, 10A & 20A, and 24V 10A. Here is the circuit diagram.

Different bulbs can be used but the math will have to be done to estimate loads and such.

Putting it all together
Hopefully at this point you should grasp why using light bulbs are preferable, how to figure out how many light bulbs and how to theoretically wire them together in the proper way achieve the load you desire. From here it will be your job to put it all together and make something useful. Check out my builds, "My light bulb discharger build(s)", for ideas.