Summary of the whole project enter image description here

I have a small project in mind, just want to find out is this a possible way to make a useful power pack rather then use this normal way [![enter image description here][2]][2]

to make a traditional E-bike power pack


  1. I will use twelve QC 3.0 capable power banks to constantly supply 20V with around 0.9A, then I parallel them by joining 12 outputs into a thick copper wire.

  2. Because the DC-DC step up converter can only handle 15 Ampere max input (10 Ampere if no cooling, so I choose limit to 10 amp )

  3. step on converter specification: It can take in 16-60 Volt as input ( with max Current 15 Amp ) output voltage is 18-80 Volt with max current 10 Amp

  4. If need more power, I might make another step up converter group or groups, later join both (or all 3) step up converters output before BMS ( battery management system )

[![enter image description here][3]][3]

  1. 500W motor will get electrical power from BMS :)

ok, now come some questions

  1. How do I ground the circuit? Do I need to ground the circuit? Is it necessary??
  2. If I use 2-3 step up converter groups, how should I deal with the grounding circuit problem?
  3. Do I need to put diode ? Especially just outside output of each power banks . If No need Diode , then I can save more energy :)
  4. Do I need to put diode outside the step up converter ? I means in between steps up converter groups and BMS

  5. Any other points that might stop the project from working ?

  • \$\begingroup\$ was trying to upload more picture, failed \$\endgroup\$
    – Terry Lim
    Jun 29 '16 at 16:28
  • 2
    \$\begingroup\$ Please be aware that if your 20V battery buss is limited to 10 amps, your TOTAL power (including converter inefficiency) is 200 W max. So call your motor power on the order of 160 to 180 watts, or 1/3 of your expected power. Is this acceptable? \$\endgroup\$ Jun 29 '16 at 16:33
  • \$\begingroup\$ Upload them to imgur and add a link in the comments, We will edit them in \$\endgroup\$
    – Passerby
    Jun 29 '16 at 16:36
  • \$\begingroup\$ @WhatRoughBeast that is why I am thinking to use 3 groups of converter let me try, I am still new to this site :) \$\endgroup\$
    – Terry Lim
    Jun 29 '16 at 16:37
  • \$\begingroup\$ imgur.com/a/YGZJD these are the link to the photos that I suppose to upload, one is sample photo of a traditional battery another photo is the way I am intend to add more converter group thanks @Passerby \$\endgroup\$
    – Terry Lim
    Jun 29 '16 at 16:43
  • Your batteries provide a maximum of 216 watts, and likely significantly less in the Real World.
  • Simply connecting batteries in parallel is a tricky thing to do without damage or catastrophe. One method of providing a safe method involves using series diodes and/or resistors to "balance" the current and prevent the batteries from charging/discharging each other. Either series diodes or series resistors will further significantly reduce the power from your batter packs.
  • A DC-DC converter will further severely limit the amount of power you have available. You probably won't find any DC-DC converters in any serious electric vehicle design because they are just not efficient enough. Match the battery voltage to the motor by selecting the right battery and/or the right motor.
  • Grounding is the least of your concerns. Conventionally, you would select the common side of the circuit (typically the negative bus) and call that "ground" and connect it to the chassis. But it doesn't have any great significance connected or floating.

The three main show stoppers in your design are:

  1. Your proposed batteries don't provide even HALF the amount of power that your motor wants. You need a much better match between battery and motor.
  2. Simply connecting batteries in parallel is risky and dangerous. NOT RECOMMENDED without taking proper precautions.
  3. Using DC-DC converter is not efficient enough for anything but a small toy.
  • \$\begingroup\$ thanks for the answer, much appreciated, what If i combined 3 group of converter so that BCM will have close to 600W as input? but then again, I need 36 power banks, which is quite crazy :( by the way,by including diode will eliminate the "dangerous" parallel risk? \$\endgroup\$
    – Terry Lim
    Jun 29 '16 at 18:02
  • \$\begingroup\$ Using isolation diodes on each battery eliminates the dangerous parallel risk, but it doesn't balance the current. So it won't solve the load-balancing problem. The "power budget" in most electric vehicles is so narrow that using diodes or resistors is rarely done because it wastes so much power. Matching the battery and the motor is a MUCH MUCH preferable method. Even using your battery packs in SERIES would be a much better idea. It wouldn't be the first time a 240V DC motor was used in a vehicle. \$\endgroup\$ Jun 29 '16 at 18:15

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