Electric Unicycle Batteries And The Design Flaw That Makes Wheels Dangerous

Figure 1 - An Airwheel X8 in action

Summary

Electric unicycles are mostly safe but an instantaneous loss of power results in rider injury. Most wheels have a design flaw that causes this dangerous situation to occur as they age. A rider is likely to experience a rare and mysterious shut down on a wheel that was for a long time safe.

This study explains the problem and details investigations into two particular models, an unbranded el cheapo and an Airwheel X8. Both these wheels were flawed with the el cheapo failing quickly. It is possible to avoid shutdowns with the X8 by replacing the battery before it gets old and modifying the electronics can completely avoid low voltage shutdowns.

An interesting observation arising from the study was that adding a second battery in parallel should increase range by a factor of about three.

Introduction

Wheels at low speed are generally pretty safe as you can easily step off. The foot platform striking an obstacle is difficult because the riders body continues forward without the wheel underneath. Some quick foot work is required to get the rider's feet back under their centre of gravity but as they can push off the wheel platform it usually ends well. Motor shutdown on the other hand is dangerous and I'm not the first to observe this. I've experienced it four times and it always hurts.

Why Is Motor Shutdown Catastrophic?

Figure 2 -A shutdown looks similar to this

When the motor is driving forward, the riders weight must be forward of the wheel centre so that the torque produced by the rider matches the torque produced from the horizontal ground reaction as shown in Figure 3 .

Figure 3 - Stable forward motion

If that ground reaction is lost the rider will be hanging in space without a force to oppose gravity and will therefore descend face first as shown in figure 4.

Figure 4 - What happens when the motor stops.

It is worse than tripping over while walking as the rider can not push against anything to try and recover or reduce their rate of descent. No horizontal reaction force is possible. An exclusively vertical force through the feet, which is all that is possible, serves only to increase the riders rate of rotation about a centre of gravity which moves further forward of their feet as they approach the ground. Only hands can be used to break the fall. Hands, knees and sometimes the face will hit hard and suffer injury. The only chance is to pull the knees toward the chest faster than the rider is falling and throw the feet forward. I know its been done but I've never pulled it off. I find I'm already on the ground before I've realised what was occurring.

In this video a rider dismounts gracefully from an unpowered wheel by pulling one knee upwards as he is falling, planting his foot in front of his body and relying on forward momentum to lift his centre of mass above his foot. He is undoubtedly a talented rider but his starting point is one of balance rather than hanging in space and he doesn't have to realise during the fall that he needs to do the opposite of what he would normally do when tipping forward, as is required on a powered wheel.

Figure 5 - This minor scrape was from catching the foot plate and stumbling on the dismount. The shutdown injuries were much worse.

My Four Shutdowns

The first was on a wheel only two weeks old that had a cell in the battery pack fail suddenly, dropping the cell voltage below the shutdown voltage. If you buy the cheapest wheel possible, it will almost certainly have low quality batteries as they are a large proportion of the sellers total expense making battery skimping the easiest way to achieve low prices. This is what led to the hover board failures that made the news a while back.

Figure 6 - Low quality batteries can fail without warning and even catch fire. 

Stupidly, I replaced the failed cell with a cell that had protection which tripped out in use and that was the second shutdown. The wheel seller sent me another battery but took the el cheapo wheel back before the battery arrived. Next I went up market with an Airwheel X8 and had 18 months of trouble free wheeling. Oh what joy! I sensed the battery was aging, removed it to test and fitted the generic pack I'd been sent previously. First time I rode with the new battery it lasted well but the ride ended with a shutdown. I put back the aging battery which had tested as worn and had another shutdown early in the ride. This exacerbated the injuries from shutdown three that hadn't yet had time to heal. Shutdowns three and four were a mystery so some investigation was required.

How Are Wheel Batteries Managed?

The batteries are built from lithium 18650 cells connected in series and packaged with a battery management system (BMS) like that shown in figure 7 and listed on Aliexpress with the specifications in the appendix.

Figure 7 - A Battery Management System (BMS) for a wheel is packaged together with the 18650 cells. Note the 4 FET switches in the top left. Three of these are used to switch off power to the wheel when over current or under voltage is detected and the fourth switches off the charging current when the cells are fully charged.

The BMS manages cell balancing, charging and supplies power to the wheel through switches (FETs) that are switched off in the case of over current or under voltage.

On an X8 wheel there are four battery indicator lights. The number lit reduce as the battery discharges until, at the minimum acceptable voltage the wheel beeps a lot, flashes all four lights and goes into tilt back making it hard to ride.

The battery charger is not smart. It supplies a constant voltage up to its maximum current and shows a red light to indicate charging. The light turns green when the current falls low enough to indicate a full charge. The actual charging current is controlled by the BMS, not the charger. I hadn't realised this at first and, when the wheel was disassembled, ended up damaging the original X8 battery and the generic wheel replacement battery by charging through the discharge cable. This bypassed the BMS charge control and caused overcharging.

What Went Wrong?

It is under voltage that caused all my shutdowns and most other people's as well. The primary problem is a design flaw. Sure, we want to protect batteries from under voltage that will wreck them but it shouldn't be by injuring a rider. This flaw lurks always but will not be triggered by most usage scenarios. Normally the wheel goes into tilt back and the rider dismounts before low voltage shutdown occurs, however under certain circumstances shutdown will occur without tilt back. How this can happen is investigated in the following testing.

Method

The batteries were discharged at a low 0.2 amps rate and the voltage periodically measured. This data was overlaid on published test data for the Panasonic NCR18650PF cells, these being the type fitted to the  Airwheel X8. The voltage at which each battery indicator light changed state was measured and the voltage at tilt back and shutdown was measured.

Results

Figure 8 shows battery discharge data for the X8 battery before overcharging, after over charging and for the generic battery after overcharging. It also shows the per cell voltages for shutdown etc.

Figure 8 - Published test data overlaid with testing data of an aging cell X (black), aging cell after overcharging X (grey) and generic cell after overcharging X (blue). The discharge rate for the X tests was 0.2 amps.

The aged X8 battery is shown to be, as suspected, of significantly reduced capacity and was made worse by a single overcharge. The generic battery after one overcharge is shown to be of much lower capacity than the X8 battery after over charging. The shutdown voltage of the generic BMS was 3.56 volts which was higher than the tilt back voltage of 3.39 volts per cell.

Discussion

When the generic battery was fitted, the long first ride suggests the unbranded cells were quite reasonable. However that first ride was, from the outset, doomed to end in pain as the shutdown voltage was above the tilt back voltage. There would never be a warning before shutdown three. The generic pack had a shutdown voltage of 3.56 volts, the X8 pack was 3.22 volts and the AliExpress BMS advertises 2.1 volts which shows BMS specifications vary, so that even a pack of the correct voltage can't be safely fitted on any wheel. They must be matched. Also, the generic cells were ruined by a single overcharge.

Shutdown four is more problematical because the implication is that all X8s and probably most other wheels will become dangerous as the battery ages. The immediate cause of shutdown four is the damage sustained by overcharging, leading, under heavy load, to an instantaneous voltage drop from above tilt back voltage to below shutdown voltage. While this battery was prematurely aged by overcharging, all batteries will eventually suffer similar deterioration and battery capacity is also reduced when cold e.g in winter. In use this deterioration will happen slowly so shutdowns will be rare, occurring at first when climbing a bump late in a ride on a cold day, as the battery approaches tilt back voltage and becoming more common as the battery ages further. A rider is only likely to be aware of a rare and mysterious catastrophic failure on a wheel that was for a long time safe. All wheels with this design will eventually become dangerous.

Increasing Wheel Range

Another observation from Figure 8 is how little of the available battery capacity is actually used in the X8. At a likely occasional peak of 7 amps draw the wheel goes into tilt back at 1.25 AH which is less than half of the total energy available. Adding a second battery in parallel would reduce the current draw by half and avoid tilt back until about 1.8 AH. So changing out a battery when flat would double the range, but running a second battery in parallel will increase range by nearly a factor of three. With a single battery, I've always found range insufficient but two batteries in parallel would equate to about three hours riding time, by which time the rider is likely to be fatigued anyway.

Why Are Wheel Designs Flawed?

A BMS is used in most lithium battery installations for battery protection and the same logic is ideal for nearly all scenarios, excepting wheels. A battery control chip is required for every cell and the chips to do it are abundant. Ideally a wheel wouldn't do low voltage shutdown but that is what the available chips do. I guess wheel manufacturers have just gone with a standard BMS design. What they haven't done, in the X8 at least, is try and work around this limitation in the wheel control logic and they should have.

What Can Manufacturers Do?

The goal is to try to always get tilt back before shutdown and shut down slowly enough for a rider to react and step off. If there is to be a shutdown voltage, setting it lower, at the minimum cell voltage would help as it makes tilt back more likely to occur before shutdown. This would be 2.5 volts for the Panasonic NCR18650PF cells in the X8 rather than the current 3.2 volts. The 2.1 volt cutoff in the AliExpress controller meets this requirement but if 2.1 volts ever occurred in use it would damage the cells. What they really should do is, not allow battery cells to reach minimum cell voltage during riding without shutting down.

Avoiding minimum cell voltage can be done by reducing current sufficiently to maintain another voltage level below the tilt back voltage. For the X8 controller with the shutdown voltage already reduced to 2.5 volts, perhaps 2.9 volts would make a good minimum.  This only has to occur for a second or two as tilt back will already have been triggered. What the rider will experience is a reduction in torque, probably when it is most needed climbing an obstacle, followed by tilt back. This is far better than shutting down. The wheel will go from rigid to spongy but loss of power will be gradual enough for the rider to react and they will still be able to push horizontally against the wheel, making it possible to dismount.

What Can A Rider Do?

A careful rider can stay in the safe zone. Use quality cells and test a new pack before fitting as there are a lot of dodgy lithium batteries for sale. Store the wheel inside in winter and don't stop for too long in freezing conditions.Then monitor the cells as they degrade and replace them early. Replacing a battery prematurely is wasteful but better than suffering a shutdown.  Alternatively a rider could modify the BMS to bypass the FET switches so that the battery remains connected when the FETs switch off or connect the gate to the drain on the FET switches so that they can't switch off. This will eliminate shutdowns but also over current and under voltage protection for the batteries.

For the X8 wheel the battery capacity lights can be used to measure battery aging. From figure 8 it can be seen that, when new, the 4-3 indicator light transition occurs at 0.5 AH of discharge and the 3-2 transition occurs at 1.1 AH of discharge so there should be about the same ride time between
the two transitions. As the battery ages the 4-3 transition happens earlier but the next 3-2 transition remains about the same amount of time after the previous. This is what I'd observed in practice, 4-3 battery indicator transition was happening earlier in a ride as the battery aged and on the overcharged battery the 4-3 transition occurred immediately on commencing riding. Using data from the graph and the knowledge that I hadn't yet had a shutdown with the aging battery, it looks like a ratio of up to five in ride time between the 4-3 and 3-2 transitions is safe. At some point beyond that it becomes dangerous.

Buying Replacement Batteries Is Difficult

Replacement batteries are hard to locate so most people probably don't replace them. That would suggest there are a lot of old wheels around with dangerously aged batteries that will hurt their riders.

I've found only one supplier on Aliexpress. They don't specify which cells you will get which is annoying but at least they offer a low shutdown voltage of 2.75 volts per cell which improves on the 3.21 volts of the original Airwheel X8.

Figure 9 - Wheels can traverse fairly rough terrain

Appendix

Specifications of a wheel battery management system (BMS) listed on Aliexpress.