Battery Pack Primer

FAQs & How-tos
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When I began looking into VESC, I had a lot of questions about batteries and how to find the one that was right for me. This is intended to serve as a primer to new-comers. The information here isn’t exhaustive, but it will help familiarize you with the terminology and start learning more.

  • Onewheel battery packs are usually built from two different sized lithium ion cells: 18650 and 21700. Pint and XR use 18650 and GT uses 21700. With VESC you can use whatever you want.

  • 21700 are larger cells with generally more current and more capacity.

  • More current, measured in amperes, means more torque.

  • Packs are usually described by their configuration. ie 15s3p (CBXR and JWXR), 19s2p (TORquePack), 20s2p (CBCSO).

    • The first number is the amount of cells in a series and the second number is the amount of series in parallel.

    • When measuring the voltage of a pack, you multiply the number of batteries in series with the max voltage of the cell: ie 4.2V * 20 cells = 84V

    • When measuring current, you multiple the max amps and the amount of series in parallel. ie 35A * 2p = 70 amps.

  • Calculating capacity or range can be a little complicated. Sometimes you’ll see batteries measured in amp hours (Ah) and other times in watt hours (Wh). Watt hours is the more useful measurement, because it accounts for the voltage of the pack. The formula for watt hours is:

    • (cell amp hours) * (numbers of series in parallel) * (nominal voltage of pack) = watt hours
    • For example, the Chi TorquePack is ~570 Wh. The formula is: 4.2Ah * 2 * 68V = 571.2 Wh

  • Voltage sag is a thing. Batteries will provide less current the lower the pack voltage gets. You feel this as the nose dips and you get less torque. So higher voltage is generally better.

  • The size and configuration of your pack is limited by a few factors:

    • Physical space - the biggest pack I’ve seen in a onewheel is 20s2p with 21700 cells. This was in a BadgerWheel TORqueBox with a custom built pack and no BMS.
    • VESC specifications - for example, the Little FOCer v3.1 is rated for 100A at 84V.
    • Motor limitations - you can buy a bigger VESC and carry around a backpack full of battery cells, but you will still be limited by the amount of power that the motor can handle before burning out.

You can find the specs for different battery cells here: https://www.18650batterystore.com

I like to use this for quickly calculating battery pack specs: Battery pack calculator : Capacity, C-rating, ampere, charge and discharge run-time calculator of a battery or pack of batteries (energy storage)

There is always more to know, but hopefully this gets you started without being too confusing. Let me know if anything is unclear, confusing, or just plain wrong and I’ll get it updated.

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Thanks. This is a great reference. Your work is appreciated!

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Great primer. The benefits of higher pack voltage is huge, provided you can get enough current out of the pack to get the motor into saturation where more current is just pointless.

  • Doubling the voltage means you can send twice as much power at the same current. Current causes heating and waste energy and requires larger wires.
  • As the motor spins faster the magnets rotate past the coils and will induce a back EMF (essentially back pressure in volts) and you need to have enough drive voltage to keep pushing positive current against this back pressure. This limits the max speed of the motor. If you want to go faster at this point you need to send a huge current through the q-axis to reduce the magnets B-field which reduces the back-emf and allows you to go faster, but you are spending a ton of extra amps on field weakening in addition to normal drive current and your pack is gonna run at absolute max output to do this. Watch battery temps. If you just have more volts then you can go fast without losing as much torque and magnet strength.
  • Higher voltage means you can more quickly change current at the same duty cycle. You can overcome the inductance of the motor more quickly and reach the optimal current at the p-q angle to deliver the requested torque immediately. Effectively your mosfets can change the current faster.

Source: I’m an EE who designs linear motors, actuators, and solenoids and the associated driving circuits.