400A adjustable current limiter for battery testing

Discussion in 'Power Electronics' started by c10yas, Dec 8, 2017.

  1. c10yas

    Thread Starter New Member

    Dec 8, 2017
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    INTRO: (PS, if you're not interested, skip to the last paragraph)
    I'm building a battery pack for my university electric racecar team. We plan to use 18650 cells, the Samsung 30Q which are only rated for 15A continuous. However, Mooch over at e-cigarette-forum tested them and shows that they are capable of 20A continuous and 25A peaks -
    https://www.e-cigarette-forum.com/t...t-results-a-great-20a-3000mah-battery.727190/
    However, in his testing, he pushes them to 50A pulses for 5 seconds and they still work fine. Our problem is that due to weight and size constraints, we're forced to use the batteries in 8 parallel circuits. According to Samsung's datasheet, that's only 120A continuous, but going by Mooch that's 160A continuous with a 200A peak. We can, however, draw up to 270-300A in very short bursts of less than 5 seconds, that's a burst from the battery of about 37A.
    Our problem is that we need to fuse the batteries at the rated max current of the cell which isn't mentioned in the datasheet, and results posted on a forum don't count as scientific evidence of max allowable currents. Moreover, we have a custom cooling solution which should help increase our peak currents even higher than the tests mentioned above.
    Our only option then is to test the batteries ourselves and show sufficient proof of the max current, which brings me to the problem I'm having. A single cell, going by the aforementioned tests should be able to output 50A+ and thus a pack of 8 of them can easily exceed 400A given proper cooling.
    Why do we need to test the whole pack of 8 you ask? If we blow the fuse on one cell, if its defective or, even just badly welded on, the remaining 7 cells will have to push the same current through them. We need to ensure that it won't cause a chain-reaction of fusing and completely break the circuit.

    To test the batteries, we need some sort of current limiter and load. All the circuits I found on the forum and elsewhere are rated for around 50-100A. How would I go about building a battery testing rig at 400A? Could I do a multiple parallel BJTs in a current mirror circuit configuration? Are there any caveats to that?
    Also, what safety precautions do I need to take considering I'm pushing these cells to their limits and to failure?
    I appreciate all the help and suggestions I can get
     
  2. tcmtech

    Distinguished Member

    Nov 4, 2013
    2,808
    2,704
    If you just need a simple high current low voltage load bank common paper notebooks are your answer.

    The chrome plated steel spirals are surprisingly good stock material for making high current low ohmic value power resistors. ;)

    Depending on the gauge of wire they can easily take 20 - 50 amps continuous duty with basic fan forced air cooling and way more than that in a submerged oil bath which can be made from something as simple as paint can and a few quarts of the cheapest lightweight motor oil you can find. :cool:
     
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  3. c10yas

    Thread Starter New Member

    Dec 8, 2017
    2
    0
    It's more than just a low ohmic load. As the current increases, voltage decreases which would reduce current in the outer circuit.

    The minimum voltage the battery can drop to is 2.5V. At an approximately 40A test per cell (the max we'll ever test) that's a load of 62.5mOhm. Any additional load at higher voltages or lower current tests would require an active, adjustable load that limits the current.
    Testing the full pack of 8 parallel cells needs a 7.8mOhm load and the remaining actively limiting the current.

    Thanks for advice on building a low ohmic load, we need one since my only way of measuring current is to measure voltage drop across a known load
     
  4. dendad

    Distinguished Member

    Feb 20, 2016
    2,670
    711
    One place I worked in during training had a large battery test setup. It was steel tube (I think like car brake lines) with water flowing through for cooling. Moveable clamps were attached to allow adjustment. Fencing wire can be used too.
     
  5. tsan

    Member

    Sep 6, 2014
    87
    15
    Perhaps a circuit using mosfets for current adjustment would work. Current is measured for example with 100 micro ohm shunt resistor and measured current is used to control gate voltage. The mosfets shown below are just an example and I used them because I have a model for them.
    18650 battery tester.PNG

    A few volt gate voltage change adjusts current up to 400 A.
    18650 battery tester_simulation.PNG

    Power resistor could be made from serial and parallel combination of 10 mohm or 20 mohm 50 watt power resistors like

    http://www.arcolresistors.com/resistors/hs50-aluminium-housed-resistor/

    About 20 resistors are required.
    View attachment 18650_battery_tester.asc
     
  6. tcmtech

    Distinguished Member

    Nov 4, 2013
    2,808
    2,704

    Just add more load resistors in parallel as the voltage drops or put a sliding tap on the resistance wire sets.

    You can make it fancy and complicated or as simple as you want. What matters is how often its going to be used and how much time and money you want to invest into the design.
     
  7. dendad

    Distinguished Member

    Feb 20, 2016
    2,670
    711
    One application we did was for a hydraulic valve system to control a motor. In stead of using a proportional valve as they are very expensive, a number of on/off valves were used with varying port sizes. This ended up as a digital to analog converter. The same can be used here. FETs or even relays,(or switches) to switch in 1,2,4,8,16,32.... current steps of load resistors so for 8 switching devices you get 256 steps. And that way is easy to extend the current range. This gets away from running the FET in analog mode if that is a problem.
    Just a bit different option that could work.
     
  8. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Another option is to go with a industrial electronics applications size of semiconductor device like a IGBT power brick module. I have worked with such devices for years and the bigger 600+ amp units typically have power dissipation capacities in the 2 - 3+ KW range.

    Just use one of them with a simple current control circuit and a large power resistor ahead of it to do the bulk of the energy dissipating work and you could conceivably make a adjustable constant current power bank without much investment.
     
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