Coupling battery packs of different chemistries

Discussion in 'The Projects Forum' started by feldspar, Jul 1, 2013.

  1. feldspar

    Thread Starter New Member

    Jul 1, 2013
    3
    0
    Hello everyone, this is my first post here, seems like an excellent forum.

    I am hoping to get some pointers to help design a (hopefully) simple circuit to couple a small Ni-Cd (or Ni-MH) battery pack to a larger Li-ion battery pack. I need the Ni-Cds to be switched in as necessary to limit the stress of high current peaks on the main battery


    I have this electric bicycle (an ‘Ezee Torq’) with a 36v 10A/H Lithium manganese battery. The problem is that the battery is unable to cope with the peaks in current caused by steep hills and hard acceleration. Like most lithium batteries it has a battery management system and whenever it senses voltage sag it cuts power to the controller to protect the fragile cells. You can reset it by turning off waiting a few seconds then turning back on.

    I have learned that this is a known problem with this battery type however the battery is still capable of an excellent range if I can manage the current drain to avoid cut outs.

    To make the bike more useable and to get some decent service from the expensive lithium battery, the plan is to add a small auxiliary ni-cad battery (maybe two 18v power tool batteries) to manage the load on the main pack.

    Ni-Cd has a much poorer energy density compared to Li but it is the best chemistry for high current drains. Therefore the challenge is to find a way of coupling the Ni-Cd pack into the circuit only under high drain so it doesn't get used up too quickly. I hope that around 2AH of Ni-Cd may last around as long as the 10AH Li-Mn pack if it is only being used when the current demand is high.

    I have the idea that it ought to be possible separate the packs with diodes and to switch in the extra battery when the main pack voltage sags below a defined limit. Unfortunately this exceeds the limit of my knowledge and I am not sure what type of circuit is required to achieve this. I would be very grateful for any tips and possible solutions.

    Thanks in advance
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    You're right that diodes are likely part of the solution to "OR"ing two power sources. (That's "or", as in either or.) A diode prevents reverse current from one source to the other.

    But your issue is maybe more complicated by the need to precisely align the voltages. What is the nominal voltage of your NiCd pack, and do you know what voltage you need to avoid that triggers the shutdown?
     
  3. feldspar

    Thread Starter New Member

    Jul 1, 2013
    3
    0
    Hello

    Thanks for the reply

    The nominal voltage of the ni-cd pack would be 36v (15 x 1.2v)

    The nominal voltage of the Li-ion pack is 37v (10 x 3.7)

    Both packs measure higher off load

    Cut outs occur when the Li-ion pack sags below 34.5 volts.

    What would the current flows be if I just connected both batteries to the load in parallel separated by diodes? Would the 1v difference in voltage prevent the smaller battery discharging until the main pack dropped below this?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Yes, the lower voltage pack will supply nothing until it becomes equal or higher in voltage itself. There's probably some range where both packs will supply current in inverse proportion to their internal resistance.

    Schottky diodes have a lower voltage drop than regular silicon diodes, something like 0.4V versus 0.7V.
     
  5. feldspar

    Thread Starter New Member

    Jul 1, 2013
    3
    0
    Excellent, thanks.

    I can see the way forward now, i'll post back whether the project is a success
     
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