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I plan to build a DIY electric bicycle using a standard geared BLDC hub motor (36V and 250W rated, 500W peak). To get 36V rated battery, at least 10 Li-ION 18650 cells are to be connected in series (that's how it is usually done). In this case, the battery at maximum charge will give out 42V and will discharge gradually till 30V (limited by a BMS).

Assume that I need to build 9s2p battery. This means that the battery will operate in a voltage range between 37.8V - 27V. Assume that BMS and controller are all good with that.

What will happen to a motor if it's connected to such a battery (with lower voltage compared to 10s2p)? How the maximum speed, torque, efficiency and the lifespan of the motor will be affected? Will power output change or will it be compensated by the increased current?

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    \$\begingroup\$ Suppose I say: the output of the motor will be unaffected by the lower battery voltage. Would you believe me? If not, would you ask why? In case my statement is nonsense, could you provide an argument to prove that it is nonsense? You appear unconcerned by the amount of current which could be flowing, are you sure the batteries can handle that current? What makes you think you can just "design" an E-bike drive train without understanding the basics which you ask? \$\endgroup\$ – Bimpelrekkie Jul 29 at 12:45
  • \$\begingroup\$ I'm now asking just a theoretical question regarding motor performance only. The 2p battery will be rated for 20A, hence I'm not worried about current handling capability. \$\endgroup\$ – viwack Jul 30 at 7:17
  • \$\begingroup\$ And I'm encouraging you to think so that you get a better understanding of how things work. I fail to see the point of asking / answering "when I do this, what happens?" questions when you only want a yes/no answer. \$\endgroup\$ – Bimpelrekkie Jul 30 at 7:28
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The maximum possible speed varies linearly with the battery voltage.

With a typical ESC that uses PWM to drop the voltage, and assuming the batteries are good for the current, if you want a low speed, there will be no difference in maximum torque and power. Efficiency will only be affected slightly, as a function of the ESC.

Lifetime might be improved as you won't be able to reach higher speeds.

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