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Thread: Inexpensive load bank for battery and LED testing

  1. #1

    Default Inexpensive load bank for battery and LED testing

    Howdy All,

    I just finished a simple project. I had the need of a resistor load bank to discharge cells and batteries and test power supplies. I bought six power resistors from an eBay seller. They are rated at 1Ohm and 25 watts each. I mounted them to a piece of plywood using the supplied stand-off clips, and made some 16-ga. jumper wires with spade connectors. With the resistors connected in different serial and/or parallel ways, I can set up impedances of .17, .2, .25, .33, .5, .67, 1, 1.5, 2, 3, 4, 5 and 6 Ohms. The power dissipation capacity ranges from 50 to 150 watts, depending on how many resistors are used in the bank.



    The cheap yellow Chinese DMM to the right of the resistors was partially fried by my Dad. Some of the modes gave innaccurate readings after that happened. Upon opening it up, I found a few fried components, but the 10A current mode still worked well, so I removed the selector dial and soldered the dial contacts on the PCB to lock it in 10A current mode. The original test leads have very small-gauge wires, so I soldered the 16-gauge wires from the resistor bank directly to the DMM's PCB. Using Ohm's law, I used the known impedance of the resistor bank and the measured voltage under load to adjust the DMM's only tuning pot. to get the Amp reading spot-on. It seems to track well at other loads, too. I hate to throw anything out, so this crappy DMM will serve some useful purpose now.

    So far, I've used it to find bad cells in an old 12V DeWalt cordless tool NiCd pack. I first charged the pack fully. I then discharged at about 2A until the voltmeter read .9 volt per cell (9V). Then I checked each cell's voltage. One was showing 0V, and had shorted during discharge. I then re-connected the load, and continued discharging until reaching .9 volts per cell, now with only 9 "good" cells (8.1V). Again, I discovered another cell that had dropped to 0V. The last discharge to .9 volts per cell (now 7.2V) left all remaining 8 cells showing good unloaded voltage. Now I can remove the two bad cells and install two good ones from another pack of similar age. The other pack also had some bad cells, so I salvaged the good ones for making one decent pack from two bad ones. These packs are $50 new, so they're worth saving when possible.

    I've also used the load bank to put a 9 amp load on a 12VDC switching power supply I recently acquired. Those six resistors at 127 watts total sure threw off some heat! I'm glad I used the stand-offs to keep the plywood from getting too hot.

    It's important to keep the unused wires in the bank away from each other, as this could lead to a dead short. I'll probably make some sort of non-conductive clip to hold them apart when not in use.

    Calculation also reveals that the load bank's impedance range will be useful for testing Luxeons LEDs, if the input voltage is not much higher than the LED's Vf. Obviously, these resistors are way oversized for any LED, but until I get a nice assortment of small-value resistors, this will do nicely.

    I made up an Excel spreadsheet that lists various combinations of impedance and current at voltages between 1.2 and 13.8. If you'd like to try the sheet out, go to my web site, then download this .xls file.

    Darron
    Last edited by Gadget_Guru; 08-29-2008 at 09:32 AM.
    Darron
    In the beautiful Adirondack Park, northern NY state

  2. #2

    Default Re: Inexpensive load bank for battery and LED testing

    Another use I forgot to mention: testing various cell types for voltage under load. Thinking of upgrading to a 1.2 Amp bulb? Watch that new Energizer D-cell alkaline drop instantly to 1.3V under a 1.2A load.

    This test can be useful when trying to determine which of those special Carley or WA bulbs to run in your favorite light with a particular battery type.

    Darron

  3. #3

    Default Re: Inexpensive load bank for battery and LED testing

    Very nice.

    I think you're selling your rig short, though. Consider series and series/parallel configurations. You can get a nearly endless list of values that way.

    FWIW you can often slip the right size lug into the end of the ceramic core on the resistors to make it 'safe' in rigs like this, you might want to check that out?

    You might also want to be on the look out for a similar resistor with exposed windings on one side and a clamp to provide a variable tap.

    Doug Owen

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