Electric car with lead acid & lithium batteries

Discussion in 'The Projects Forum' started by Arlynn52, Jun 15, 2010.

  1. Arlynn52

    Thread Starter New Member

    Jun 15, 2010
    1
    0
    I'm building an electric car with a 72 volt traction pack: 40 amp hrs of lithium batteries and 115 amp hrs of lead acids in parallel. Each pack has its own charger and will be charged seperately.

    I isolated the leads with 10 paralleled 120v/50 amp diodes to provide 500 amps of semi-conduction. The general plan was/is that the leads would backfill the lithiums but not have to absorb the regen surge (I understand that li can handle it much better than lead).

    The issue is the difference in ampacity - will the leads damage (stuff?) the lithiums due to the difference in capacity even though the 2 pack voltage is the same? I don't think so, but am afraid to make the last connection until I'm sure, since the lithiums cost just over $2,000.

    Any thoughts would be greatly appreciated.
     
  2. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    The voltage can't be the same, their discharge characteristics are different, as soon as you start drawing power the packs unbalance, and they'll do nothing by fight each other from then on.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Agreed.

    Mixing battery types is begging for trouble.

    Using diodes wasn't a great idea either. You'll have a built-in power loss.

    Instead, use MOSFETs as "ideal diodes". They don't have a relatively fixed voltage drop.
     
  4. Solcar

    Member

    Jun 8, 2007
    21
    3
    Maybe you could keep them separated at all times, switching between the two types as needed. The diode losses in the original idea vary between about 5 and 15%, and you would be able to avoid them. LiFePO4 has a charged state of about 14.6 while lead acid has about 13.8. That is a pretty substantial type of voltage difference that you could reduce problems from, since the operating voltage of the lead acid will be much lower than the lithium.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    It seems the OP needs them connected in series to hit the 72v mark. Keeping them separated via relay or switch would just not work.

    And using them together would not work either.

    The Lithium will most likely explode as they try to draw a ton of current from the lead-acid batteries. This leads to UN-controlled charging. For lithium, thats a super NO-NO.

    You WILL burn this sucker down.
     
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