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I want to build a battery for my robot, and I don't understand how to choose components to use after BMS to charge/discharge the battery.
I have 4 pieces of 18650 batteries: 3.7 V, 6.7 A maximum.
This is my scheme:
enter image description here
I think I will buy BMS like this.
In short, the main characteristics of this BMS:

  • 4S, 40 A.
  • Charging voltage: 16.8V - 18.1V
  • Continuous charge current (upper limit): 20A
  • Continuous discharge current (upper limit): 40A

I don't understand what voltage will be in the output of BMS. I guess the maximum current will be 4*6.7 A. I need this information to properly choose step-up or step-down DC-DC converter for charging and discharging. This is the main question. After that, I wrote some additional information that shows my goals globally.

First, discharge:
I need to source:

  • Jetson Nano (USB-C, 5V)
  • Motor (link) which inputs 7-35V.

I'm going to split a wire and then use step-up or step-down DC-DC converter to make appropriate voltage for each part.
Second, charge. From the description of BMS, I think I will use 5V 3A power adapter, then step-up DC-DC converter to make it about 17 V.
I'm going to use USB-C port to both charge and discharge the battery for Jetson Nano. I'm going to use XT60 to discharge it for the motor.
Do I do it right?
Anticipating the question, the normal power bank doesn't work for my case: it doesn't provide enough current for the motor. I think so because the car shuts down sometimes and these shutdowns correlate with the speed I choose. Also, a normal power (Xiaomi Mi Power Bank 3 Ultra compact) bank turns off automatically because the current is not enough for it, even if I use the low-current charging.
Thank you for making it this far.

UPD: Currently, I'm using 2 power banks. First powers Jetson Nano, second powers the motor. Power banks are normal, intended for smartphones and whatnot. If I use one power bank for both, it doesn't work for some reason.
UPD: Also, I just realized that the motor inputs maximum 2A so I'm confused because I don't understand why it shuts down.

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1 Answer 1

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what voltage will be in the output of BMS.

4 * 3.7 V = 14.8 V, nominally. The range is 12 V (nearly empty) to 16.8 V (full).

A BMS doesn't change the voltage. It it either on (same voltage as the string of cells) or off (0 V).

I guess the maximum current will be 4*6.7 A.

No, the maximum current "might" be 6.7 A because the cells are in series. I say "might" because battery vendors lie and exaggerate the capabilities of cells. Yes, you can draw 6.7 A from this battery, but doing so will reduce its cycle life. My recommendation is not to exceed 3 A.

Charging voltage: 16.8V - 18.1V

18.1 V is too high. The maximum voltage of the charger should be 16.8 V. If you use that charger, it will not charge properly: the BMS will shut down before the battery is full.

Motor (link) which inputs 7-35V.

As you know, that's not a motor. That's an ESC (Electronic Speed Control).

the normal power bank doesn't work for my case: it doesn't provide enough current for the motor.

Well, this battery you're designing will be even worse. Based on the kinds of questions you are asking, and how dangerous a poorly designed Li-ion battery can be, my strong recommendation is that you reconsider. Buy a larger power bank instead, one that can power your load.

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