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I’m working on a project, where I want to go from 5 Lithium-ion cells in series, to 5 Lithium-ion cells in parallel, for versatility purposes i.e. in time, I want to be able to change the output voltage from 5-60V, hence the step-up voltage regulator approach.

What I want to achieve is: To connect a 3.7V Lithium-Ion powersource, to a DC motor (from a powertool) rated at 18V. Later I would like to repeat this for other voltage motors as well.

Is it realistic to build this on a print at the size of circa 10x10x1 cm or 4x4x0.4 inches

For now, I have tried to use an off the shelf Pololu U3V70X Step up voltage regulator with some success. I can get the correct output (18V), however at a very small Amperage, not enough to run the motor. I have also tried to put 3 regulators in parallel, to get higher Amps with some success, but this is not optimal as the Voltages need to be perfectly adjusted.

I have tried to put 3 regulators in series with individual cells as a source, this is needed as the voltage regulators share a common GND (GNDin = GNDout), with an output of 6V each = 18V. This works, however, I still find the Amp output to the low side as I would like to reach 20A max discharge, as this is needed when i start this specific motor.

I can se from the answers, that one of the main concerns are the stall currents of 130A.

A few specs for my setup:

Current working setup from the battery manufacturer: 5x LGDBHE21865 2.5Ah in series = 20V (5x4V) * 2.5Ah = 50Wh Std. discharge 500mA Maximum discharge 20A Link to cell specs: https://secondlifestorage.com/showthread.php?tid=1938

Wanted setup: 5x LGDBHE21865 2.5Ah in parallel = 4V * 12.5Ah (5x2.5Ah) = 50Wh With a step-up boost controller taking 4V->18V with approx. 2.5Ah and stil a max discharge of 20A

Motor specs: Mabuchi RS-775WC-8514 Vin = 6-20V Imax 18.7A
Link: https://product.mabuchi-motor.com/detail.html?id=129

Voltage regulator specs: 3x Pololu U3V70X Adjustable Step up voltage regulators Link to Voltage regulator: https://www.pololu.com/category/243/u3v70x-step-up-voltage-regulators

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  • \$\begingroup\$ We are not a design house because you do get what you pay for. We don't do product recommendations because that's more opinion than an answer. We will comment on a design. \$\endgroup\$
    – StainlessSteelRat
    Sep 7, 2020 at 15:22
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    \$\begingroup\$ To get 20A at 18V you'll need to handle 100A + losses, say 110A, at 3.6V. Which is ... not trivial. \$\endgroup\$
    – user16324
    Sep 7, 2020 at 15:25
  • \$\begingroup\$ @StainlessSteelRat Thank you for your comment. It was not my intension to ask for recommendation, I'm looking for answers to my problem. I have changed my original post. \$\endgroup\$
    – Mat
    Sep 8, 2020 at 7:17
  • \$\begingroup\$ @BrianDrummond Thank you for your comment. In your opinion, is it possible to achieve this on a small print (circa 10x10x1cm), with regular components? \$\endgroup\$
    – Mat
    Sep 8, 2020 at 7:21

2 Answers 2

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Ouch.

If the stall current is 130A, the converter now needs to handle over 600A at the input, not merely 100A.

It is time to stop and reflect why you are abandoning a simple solution which works, for a difficult and expensive one which probably won't.

If you want adjustable 5-60V (exceeding 18V only once the motor is running and lightly loaded, otherwise you are only making the stall current problem worse), then start with 60V (15 cells in series) and use a buck converter to reduce voltage (increasing current) instead.

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  • \$\begingroup\$ Thank you for your answer Brian. I have thought about the buck version with 15 cells, however I would have loved to keep the initial battery size down to 5 cells, to keep the weight down for the smaller powertools. My initial idea was to make a battery with flexible voltage, and then interconnect several batteries to increase the Ah for more demanding tools. I also thought the parallel coupling could spare the cells for the high currents, they presumably handle in series. Can buck converters better handle high currents? \$\endgroup\$
    – Mat
    Sep 9, 2020 at 6:44
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I have tried to put 3 regulators in series with individual cells as a source, this is needed as the voltage regulators share a common GND (GNDin = GNDout), with an output of 6V each = 18V. This works, however, I still find the Amp output to the low side as I would like to reach 20A max discharge, as this is needed when i start this specific motor.

First, the stall current on your motor is 130A, not 20A, so getting that motor started is going to be tricky unless you're going to give it a spin before applying power so that the boost converters don't see the full stall load.

Second, those boost converters have a maximum input current of 10A, and so a maximum output current of ~2A given losses. Getting to 20A would require at least ten of them. You've only got 3. I recommend looking for some bigger converters.

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  • \$\begingroup\$ Thank you for your answer. My main concern is, if it is realistic to build a print with existing components that can handle these currents, within a reasonable size (10x10x1cm / 4x4x0.4 inches) \$\endgroup\$
    – Mat
    Sep 8, 2020 at 7:57
  • \$\begingroup\$ The reason I use the Pololu converters is to Proof my concept, i can add more than 3 if that is what it takes, however, if it's not realistic to make a single board that can handle these currents, within a reasonable size, I will look into another way to create a multi voltage battery pack. \$\endgroup\$
    – Mat
    Sep 8, 2020 at 8:11

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