How do they get 200A-350A from a small Li-Ion cell?

Discussion in 'General Electronics Chat' started by MikeA, Dec 19, 2015.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Saw this little thing on the store shelf. Weighs like a pound.
    Looking at the spec there is probably a few li-ion cells in parallel?

    How do they step this up to 12+ volts at hundreds of amps?

    20151204_162722.jpg
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    They don't step it up. They use 3 or 4 high C rate cells in series to get a bit over 12 volts at high amps out.

    The 5 volt output typically comes from a tiny buck converter similar to what is in most automotive power adapter units.
     
  3. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Why would they rate it at 8000mah @ 3.7v on the box then? That seems illogical if internally there were several cells in series.

    Why not say 2000mah @ 14.8v?

    But let's assume there are 4 li-ions in series, 2000mah each, for a 2000mah @ 14.8v total. Are there really 100C to 175C li-ion batteries?
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Advertizing in rarely logical.

    Amp hour ratings are the same regardless of number of equal sized cells in series. Only the voltages change.

    350 amps out of a 8 Ah cell set is roughly 48C which there are many high output lithium batteries with honest 50C+ output capabilities these days.

    If you really want to know how it works take the thing apart!:rolleyes:
     
  5. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Right.

    8000mah @ 3.7v = four 2000mah lithium ion cells in parallel
    2000mah @ 14.8v = four 2000mah lithium ion cells in series

    The box says 8000mah @ 3.7v, so to get the voltage to boost an engine starter it would need 4 cells in series, which would mean each cell is only 2000mah each. Which is very typical for 18650 cells.

    But the C rating would still be based on the 2000mah @ 14.8v output. So those cells would need to be 100C to 175C? o_O
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    incase you didnt know it, amp hr ratings also work for short time outputs, 100 also means (if the battery is rated to handle the current) 200 amps for 1/2 hr, or 400 amps for 15 minuits. also, some batteries are rated for less current for longer times.it depends on how the battery is made.
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    One cell = 3.7 volts 8 Ah ~ 30 watt hours of energy.

    four cells = 14.8 volts 8 Ah ~ 120 watt hours of energy.

    As I said before if you really want to know how it works take the thing apart and look. :rolleyes:
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I've never opened on up, but based on the specs...

    120 w-h spent in 1 minute = 7200 w-m. At 12 V this is 600 A.

    So if the batteries are capable of enormous discharge currents for a few seconds, then 1000 A cranking current is possible on paper. There are very large inefficiencies when you have such high discharge currents so the available energy will be less than the full 120 w-h.

    ak
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Of the small booster units I am familiar with they typically have either 4 or 5 lithium based high C rated cells in series giving them a nominal ~13 - 17 volt open circuit voltage and primary leads of around 8 ga or less which gives them a more realistic 200 - 300 peak cranking amps due to the load of voltage losses between their internal battery and the vehicle they are boosting.

    Theoretically on paper they may be capable of a huge short burst of power but in practical application they have barely enough to get the job done. ;)
     
  10. marcf

    Member

    Dec 29, 2014
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    How about 0.8 Giga Amps for 1 nano Second?
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Probably like this, but 1 more in series..
    http://www.hobbyking.com/hobbyking/store/__16225__ZIPPY_Flightmax_8000mAh_3S1P_30C_Lipo_Pack.html
     
  12. MikeA

    Thread Starter Member

    Jan 20, 2013
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    That's very impressive! :eek:

    8ah * 30c @ 11.1v = 2.6kW

    A typical 50 pound car battery might be rated 500 cold cranking amps, which would mean 500a @ 7.2v voltage dropped = 3.6kW

    When the discharge C is calculated for lithium (polymer) ion batteries, is there some sort of standard as far as voltage drop under load? When it's providing 30C, as spec'ed, what is the expected voltage drop?
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Don't forget you will only get a couple of minutes out of the little lithium guy. Maybe 4 -5 times longer from the big old lead acid.
    I think they have internal resistance few milli-ohms so maybe 3/4 volt each at 200 amps. You can see why you wouldn't want to do that for long.
     
  14. Berninia

    New Member

    Nov 21, 2015
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    The LiPO class of Li-ion batteries are known to be extremely high C discharge. The internal resistance is in the micro-ohms. LiPO is the LiFePO4 or lithium iron phosphate class of batteries. The nominal voltage of these batteries is 3.2V so I am not sure if this is what is being used in the jumper starter.

    They can provide such a boost that these batteries are used for welding metal.

    The high discharge is due to the use of nano-technology. One or both electrodes contains nano materials and hence provides a huge surface for the lithium ions to bond. Checkout batteries by A123.
     
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