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Discharging a large 2V lead acid battery.

Discussion in 'General Electronics Chat' started by Mosaic, Dec 12, 2017.

  1. Mosaic

    Mosaic Well-Known Member

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    Hello:

    I am interested in electronically cycling down a large 800Ah 2.2V Lead acid forklift cell to about 1.9V.
    With such a low voltage, ohm's law indicates a C/10 discharge @ 80A => an avg 25 milliohm load.
    This is quite low....and has to dissipate 160W for some hours.

    So it seems I need some kind of device that sits on top of the battery to provide a very low ohm access to perhaps a nichrome load with fan cooling?
    Perhaps using slotted brass buss bars to accomplish the connection to the screw terminals.

    Any thoughts?
     
  2. Diver300

    Diver300 Well-Known Member

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    Four of these:-
    http://uk.farnell.com/welwyn/wh50-0r1ji/resistor-50w-5-0r1/dp/9508031
    in parallel on a big bit of metal.

    You could go to a higher resistance and more of them to keep them cooler, but the heatsink will be the limitation.

    If you boil water, you will boil about 2 * 800 * 3600 / 2257000 = 2.5 kg of water.

    You could just bolt the resistors to the bottom of a metal can that holds a lot more than 2.5 litres of water. That will keep the resistors at not much more than 100 °C while the battery is discharging.

    With 2 V and 800 A, you could have the resistors in the water. There won't be much leakage current compared to your load current.
     
  3. schmitt trigger

    schmitt trigger Active Member

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    The difficulty will be to find a commercially available resistor of such low value and high power capability.

    Perhaps it will be easier to parallel 40 resistors of 1 ohm, 5watt each.
     
  4. dave

    Dave New Member

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  5. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    If you are looking for a quick,cheap and dirty low resistance load take the metal spiral out of a common notebook and cut into several pieces and wire them across two metal strips or 1/4" - 3/8" all-thread bolts.

    It's most often made from chrome plated steel so it can take running at dull/medium red temperatures for long periods of time with ease.

    The metal spiral out of a notebook works very well as a ~10 - 15 amp load @12 volts so if cut into 4 - 6 pieces you should be able to build a fairly effective 2 volt 150 - 200+ watt resistive load for next to nothing.

    Just put the spiral wire across the power source and adjust the length until you find a length that glows dull red and then cut the rest to similar lengths to increase the current draw.
     
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  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    upload_2017-12-12_17-25-28.jpeg
    I would get 4 or more of these resistors and run them in parallel.
    You will need heavy wire.
    Look at the data sheet. There is a wattage raging for with/without a heat sink. (use a sheet of aluminum)
    They work well in a metal bucket of oil. (transformer oil best but I have used car oil) With this low of a voltage water will also work. But water will cause rest.
     

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  7. Mosaic

    Mosaic Well-Known Member

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    I am looking at nichrome wire as a load across copper buss bars to bolt onto the battery as connections.
     
  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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  9. Mosaic

    Mosaic Well-Known Member

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    That looks like a stable solution, a bit pricey though but prob worth it. More like 200W without additional heat sinking but that seems about right as it will be fan cooled and prob bolted to a 6 x 6" x 3/8" Aluminum heat spreader.
     
  10. Mosaic

    Mosaic Well-Known Member

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  11. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I saw that one. I have never used it. My guess is that you need to bolt it to a good piece of Al. (use thermal compound)
    Only 100 watts, keep the case temperature below 70C.
    Probably need two! if you are going to burn 160 watts.
     
  12. Mosaic

    Mosaic Well-Known Member

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    Running at 70A is just under 100W. Bolted to the 6" x 6" x 3/8" thick alum heat spreader and then fan cooled. Only one way to know ...try it out.

    Since is 20 mΩ its better as I'd see some mΩ losses due to MOSFET and bussbar losses.
     
    Last edited: Dec 13, 2017
  13. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I have a large collection of resistors mounted on Al.
    I use "computer" fans. Some at 110VAC but most are 12Vdc from old computers and powered from a wall-wort.
    I use 4 one inch stand offs to hold the fan in place.
     
  14. Mosaic

    Mosaic Well-Known Member

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    How do u handle the resistance losses from all the connections?
     
  15. dknguyen

    dknguyen Well-Known Member

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    What do you mean? Use bigger connections so they don't heat up.
     
  16. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    I thin you guys are over complicating things AFAIK the battery doesn't need an exact discharge current but one that is simply high enough to not take unreasonable long to do it job while being simple in design and likely cheap.

    A set of jumper cables, two all thread rods with some nuts, plus the spiral wire back out of cheap notebook and 10 or less hours later the battery cell is discharged. ;)
     
  17. Ratchit

    Ratchit Well-Known Member

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    For variable resistance and ability to really dissipate a lot of heat, Google for "carbon pile resistors".

    Ratch
     
  18. kinarfi

    kinarfi Well-Known Member

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    I agree with tcmtech for the most part, I use .030 stainless tie wire when I need a low ohm, high wattage resistor, single strand or several twisted together, not as stable as nichrome but for this project, stable enough and not very expensive.

    Why are you draining you battery from 2.2 to 1.9??
    Jeff
     
  19. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    That's my basic thoughts. If it's just a simple power draining load that only has to work once or a few times it doesn't have to be fancy or pretty or use application specific materials.

    When you only have a few nails to drive a rock works just fine but if you have hundreds or more then get a proper hammer. So far no one's said this has to work more than once or a few times and there is no specified criteria, that's the rough design concepts I am working with. ;)
     
  20. Mosaic

    Mosaic Well-Known Member

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    No, its for 100's of cycles...so a proper spec is required. Nichrome isn't bad as it's compact but.....the advantage of the current sense 1% stability (http://www.ohmite.com/assets/docs/res_tghg.pdf?r=false) via perhaps a differential OPA interface is useful to me.
    I am assessing forklift battery capacity and developing a single cell automated battery analysis tool.
     
  21. gary350

    gary350 Well-Known Member

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    Electric hot water heater heating elements make excellent high power 4500 watt resistors the one in the photo costs only $8. I have 4 heating elements connected together to my 12 KW project I originally built 20 years ago. If I only use it 20 seconds each time water circulation & water cooling is not required. If I use it 2 minutes each time 6 to 8 times in 30 minutes 5 gallons of water gets hot quick. These can not be cooled with a fan.

    [​IMG]
     
    Last edited: Dec 15, 2017

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