From a pile of batteries and wires, we now have a high-performance power-pack equipped with charging circuits to ensure cells are balanced and that the battery pack performs well for many charges.
We built the power system with parts purchased online (parts list and product links). The battery pack uses 20 cells. We chose that size to be a good compromise between cost, size, performance, and range. This post shows the steps used to assemble the components. If you want to build the battery pack yourself, be careful that you carefully check the wiring and take some time to understand the risks involved with working with batteries. The parts are all chosen to be safe and reliable, but a short-circuit of the batteries can cause fire. Stay safe! Wear protective gear (safety glasses) and only do this kind of work if you understand the risks.
The Battery Pack
Each cell is an 18650 Samsung 30Q and costs about 3.5eur (70eur for 20 cells). Each cell stores 3000mAh, which means that at a 1C discharge rate, 3A will be supplied for 1hr at around 3.7V. Looking around at the different commercial skateboards, and the available motor controllers, we chose to combine 20 cells in a 10s2p configuration. 10s means 10 cells in series (add the voltages), and 2p means 2 cells in parallel (add the capacity). So overall our pack has a peak voltage of 42V, a nominal voltage of 37V, and 6000mAh of capacity. Multiplying Volts x Amps we get Watts, so the total power held in the battery pack is 42 * 6 = 252Wh. The batteries are happy continuously discharging at up to 15A, which in our pack means that the battery is happy outputting 17kW! That’s way more power than we need.
How much power do we need? A slow electric bike has a 250W power system. Looking around at other skateboards, the exciting ones have at least 500W power systems. So we’ve doubled that, and then some, so all of our components are capable of over 1kW of sustained power delivery. It’s good to design electrical systems to have some head-room, since all components will be more efficient, run cooler, and last longer when they run at 50% or less of their peak capacity. But it’s nice to know that we have power to play with. We will tune the motor controller to deliver the power smoothly, and not too fast, to keep the skateboard fun, and not too insane.
How long will the battery last? We estimate that we will need something like 150W to cruise along on flat ground. At 37V, we need the wiring to be happy to carry the current (Amps) of 150/37= 4A… no big deal. But what about peak current? If we are accelerating with 2kW, 2000/37 = 54A. Our 2p battery set up is ok with 2x15A sustained current delivery, which is 30A. Only half of what we need to deliver our crazy peak power of 2kW. If we check the detailed specification for the batteries (cells), we can see that the cells are ok with 25A current drain for short periods, which is near enough in this case. In summary, we can expect the current to vary between 4A and 50A. At 4A, we are delivering 148W of power. Since our motor, motor controller, and belt-drive system should be quite efficient, most of the 148W will be available mechanical power, rather than heat. At 50A, we need to be careful with our wires, connectors etc so that they don’t heat up and melt under peak load. The motor connectors are happy delivering 60A continuously and momentary loads double that. We chose a 10AWG wire for the main power wires from the battery to the motor controller (far thicker than we should really need). We can now estimate the range of the skateboard in a few ways. First, let’s use our 148W estimate: 252Wh / 148W = 1.7hrs. According to the ESK8 Calculator, with our motor, wheels, and gearing, we should have a top cruise speed of 30km/h. Roll that back to 20km/h for our 148W estimate, and we could have an effective range of around 34km. Not bad.
Assembling the battery consists of 3 main steps:
1. Spot Weld the cells together being careful to use the thickest available metal ribbon to ensure that the 30A – 50A momentary current loads do not cause excessive heating due to internal resistance.
2. Attach the discharging wires and motor controller main power connector plug
3. Attach the balance charging module and the individual cell charge balancing wiring
Carefully assembling the battery pack using a spot-welder and short lengths of metal ribbon was fun (and a little nerve-wracking at times knowing that a wrong connection could be a short-circuit!.. which would heat things up very quickly). Working with naked cells means working very carefully to maintain a clear works-space so that no off-cuts can accidentally touch opposite polarity of the cells or pack together. It was very satisfying to have the final battery pack quickly take shape. Each joint is done by pressing the spot-welder firmly onto the pack over the metal strip, and pressing the foot-pedal. To make the weld, over 800Amps are delivered through the welding hand-piece.
We have built 3 battery packs. Over 20 wires were connected to the battery pack. The two thick red wires are the main power discharge connection. The middle-thickness black wires are the main charger power wires, and the thin red and yellow wires are the cell charge-balancing wires. The charger supplies about 80W (42V at 2A) which is perfect for charging the pack in around 4hrs (252Wh/80W= 3.15hrs). We avoid damaging the cells by spot-welding small patches to each pair of cells for the balance wire connectors, and then spot-welding those patches onto the cells. This avoids soldering directly onto the cells which can easily overheat parts of them and cause long-term cell damage.
From left to right: The charging socket connected to the battery, and the discharge wires. By pre-assembling the wires like this we could carefully adjust the wires to the right length and then spot-weld the metal strip onto the battery to make the final connections without risking the excessive heat of a soldering directly onto the battery cells.
The battery charging circuit we used didn’t come with any instructions. Luckily we were able to find a wiring diagram in an online forum from someone else who was working with the same device. The power of the internet!
Taking extra care to trim the wires to exactly the right length ensures that everything will pack neatly into the enclosure. The finished pack is reasonably neat, but it was a bit confusing at times ensuring that everything was connected correctly.
The battery charge controller circuit connects via two main (black) wires to the battery, and 5 red wires + 5 yellow wires to the battery pack. The red and yellow wires are used to ensure that each pair of cells is charged to exactly the right voltage. Without this type of charging circuit the batteries will become unbalanced leading to decreased performance and potentially unsafe situation where some cells may be over-charged. Our charge-control circuit prevents this and keeps our battery safe and healthy.
A piece of plywood protects the metal ribbon that connects the cells from mechanical wear & tear whilst we’re working on the battery pack.
Finally, the charger was tested by closely monitoring the temperatures of wires, cells, and circuits, whilst periodically checking the cell voltages as they were charged. Success! All remained cool and quickly and evenly reached full charge. Leaving the pack on the charger for a couple of hours past when it reached full-charge did not result in over-charging either. We have a workable system with everything performing within the specifications.
With the batteries completed, we turn our attention to the motor controller.
Heat-shrink tubing works best when it covers the ends of the connectors, that way when it is plugged in to the motor wires the plastic of the heat-shrink totally covers the metal.
We are now ready to focus on the mechanical aspects of the build. But can we finish before ice & slush covers the ground?! Winter is coming
In the meantime, enjoy this video we found that shows some of the important parameters (e.g. maximum power used). This is not our skateboard, but is a useful reference that we will compare our performance with when we soon begin testing. (I can’t wait!! 😃)