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I'm building a Li-Ion power bank to charge a Microsoft Surface Pro 4 and for general purpose USB charging.

The Surface AC power adapter output is 12v 2.58A.

Depending on my battery configuration, I have considered using a voltage step-up or step-down converter. I would like the circuit to supply to safe voltage to the surface pro 4 even if the battery bank is currently charging. I will be using a DC power cable to connect the power bank to the Surface Pro 4.

(1a) What would be the best circuit to supply a safe voltage to the surface pro 4?

(1b) Is the frequency produced by the step up and step down voltage converters safe to use on a Surface Pro 4?

I have 8 x 26650 Li-Ion cells to work with and I'm considering the following setup:

  • 1 series - working voltage range 3.0 V - 4.2 V (28 AH)

  • 3 series - working voltage range 9.0 V - 12.6 V (7 AH)

(2a) Which battery configuration do you think would be best?

(2b) What Charge/BMS (Battery Management System) would you recommend?

(3c) What circuit do you recommend to check the battery charge status?

Please leave your suggestions and input.

Thank you for your help!

Surface DC Power Cable

Voltage Step Down Converter

Voltage Step Up Converter

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1 Answer 1

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(1a) What would be the best circuit to supply a safe voltage to the surface pro 4?

For this amount of power you need some kind of switching regulator. The "voltage step up converter" you linked would be suitable as a module for this task. If you want the best circuit for some job, that's a research and consulting project.

As for USB charging, nothing in your proposed design is a dedicated USB charger, so some devices will only consume 100 mA or 500 mA without a device to tell them it is safe to draw more current.

(1b) Is the frequency produced by the step up and step down voltage converters safe to use on a Surface Pro 4?

These parts are all DC so the output frequency is fine. As to whether noise produced by the regulator would interfere with the operation of your device, only testing will tell you.

(2a) Which battery configuration do you think would be best?

The voltage step up converter you selected uses a LM2577 regulator and the step down converter uses a XL4015 buck converter. The LM2577 will work for all configurations, but the XL4015 will only work in one. I suggest two LM2577's for flexibility.

  • 1 series, +5V -> LM2577 okay, XL4015 will not work (output voltage > input)
  • 1 series, +12V -> LM2577 okay, XL4015 will not work (output voltage > input)
  • 3 series, +5V -> LM2577 okay, XL4015 okay
  • 3 series, +12V -> LM2577 okay, XL4015 will not work (output voltage > input voltage for part of the working range)

(2b) What Charge/BMS (Battery Management System) would you recommend?

For a hobby project you are probably best off buying a charger module for your cells. They are cheap.

You could implement the basic TP4056 single cell charger circuit, which can charge a single cell at a time, and use a circuit like this one to bypass the charger if an AC adapter is providing power. The exact solution would depend on the battery configuration and this can get quite complex.

(3c) What circuit do you recommend to check the battery charge status?

Most battery charger chips provide signals to indicate charge status. If you want to get something like the percentage of charge, you will need to measure the battery's current and voltage over time to determine the amount of energy stored in the battery.

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