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I am making a LiFePo4/supercapacitor-hybrid.

Spike current draw can reach 300A for a few seconds. The LiFePo4 batteries I am using is rated for 100A max discharge rate. In order to make sure the supercapacitors do the heavy lifting, I need to limit the max current allowed to be drawn from the LiFePo4's.

In my circuit, the polyswitch will "switch off" in milliseconds at that kind of current draw. The power resistor will allow the supercapacitor to be recharged slowly while the polyswitch cools down. An obvious problem here is a waste of energy.

Is my idea completely senseless, or could it work?

schematic

simulate this circuit – Schematic created using CircuitLab

RockPaperLz- Mask it or Casket
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skogs
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  • To me, it would be much more sensible to switch the recharging branch instead of the high-current branch. But: Why switch at all? – Marcus Müller May 15 '17 at 09:19
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    "300 A for a few seconds" is WOAH. Let's say "a few" is 3. thus, C1 needs to supply Q = 900 As = 900 C. That would mean its capacity needs to be Q/U = 900 C / 12 V = 75 F ; your 70F might be slightly underdimensioned. Also, I'd very much like to mention that at these currents, you need a very proper way of designing your power rails. – Marcus Müller May 15 '17 at 09:23
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    Please link to a polyswitch that you think might do the job. – Andy aka May 15 '17 at 09:26
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    To my understanding, polyswitches also degrade with use (slower switching). This could be a problem. – Joren Vaes May 15 '17 at 10:03
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    This should do the trick: http://no.mouser.com/ProductDetail/Littelfuse/AHEF1000/?qs=sGAEpiMZZMsxR%252bBXi4wRUKZc%2ft%2fSxlu4x%2flQ4JWEkN0%3d – skogs May 15 '17 at 12:19
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    Sorry for being a bit vague on the exact numbers. Things will be scaled appropriately as i do tests. 300A for 2-3secs is an absolute worst case scenario that the system has to survive. – skogs May 15 '17 at 12:52
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    70 F is not going to be enough if you want a useful _voltage_ left after such a current draw. – pipe Dec 29 '18 at 05:09
  • Also be aware of the ESR of a supercapacitor, which can be pretty high. At 300A, you're going to need <10mOhm if you want a drop of less than 3V. This supercapacitor is going to be huge (volume-wise) – BeB00 Aug 29 '19 at 14:49
  • The Maxwell BCAP0360 P270 S18 at DigiKey an ESR of 3.2mOhm and is 360F, 2.7V for <$13 USD. But if you read the datasheet, the 220 Amp Maximum Non-repetitive Peak Current in the footnote states that the 220A figure is the "Current needed to discharge cell/module from rated voltage to half-rated voltage in 1 second" which means your supercaps will leave the LiFePo4's holding the bag rather quickly at those currents. While I have used 5 Maxwell's like these to start my car, I don't know how long they would last under those conditions, and the datasheet says usable current is as low as 23 A. – MicroservicesOnDDD Oct 05 '19 at 12:07
  • Even for the 3,000F Maxwell BCAP3000 P270 K04, which yields a short-circuit current of almost 10,000 Amps (which they warn you not to use as an operating current), the Maximum Continuous Current at 15 deg C is only 130 Amps, and at 40 deg C is 210 Amps. Currently $48 USD at 100 qty at DigiKey. Five in series, about $150 -- actually, not too bad. – MicroservicesOnDDD Oct 05 '19 at 12:35
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    I'm guessing the "max 100A" rating is based off a 3C discharge for 30 seconds. Let me know if that's not the case. Where I work, we use LiFePo4 and discharge them at 24C (600A)for 0.5 seconds. We do that about 6 times per minute. We had no idea if it would work, so we just went through a lot of testing. We use the RMS of the pulses and make sure that RMS value is below the recommended 1-2C discharge rating of the battery – bunker89320 Sep 03 '21 at 19:35

2 Answers2

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Theoretically, an RLC circuit should work:

schematic

simulate this circuit – Schematic created using CircuitLab

L1 is supposed to limit the current during the spike, while providing a lossless current transfer on DC, and making use of energy stored in the B field to charge the capacitor after the spike is over. R1 may be needed to keep overshoots limited. You may still want to include some sort of fuse to protect the battery in case your current spike is longer than you have expected.

However, my feeling is that for your parameters you will end up with a prohibitively large L value for the inductor.

Dmitry Grigoryev
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  • Thank your for this! I will try to do the math and see if i can find the correct inductance needed. – skogs May 15 '17 at 12:53
  • At an intuitive level, an iron core that weighs 1kg is good for a few hundred watts at 50Hz, so it’s able to store a few joules of energy. In your application you’ll need to store several hundred joules and so I’d be inclined to say that it’s not practical, but I’d be interested to see what your calculations indicate. I’d suggest that an active circuit with a mahoosive FET might be the way forward. – Frog Jun 06 '21 at 04:34
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It will work but less efficiency because

  1. Poly-switch take time to reset its self. so it require large cap to compensate this.
  2. Charging cap through resister lead to power loss and heat

If you want to use supper cap to deal with high current spike I suggest to use switching current limiter to replace your resistor and poly-switch. so battery will continue supply current and maximum limit. Note: you might use switching regulator that support CC and CV mode for current limiter.

M lab
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