How to increase the safety of a bomb on wheels, which you call an e-scooter
The year 2020 has proven that electric vehicles are the future of transportation, and European Parliament urged to remodel the urban space in favor of active mobility. Paris is already close to a strategy of 15-minute city, which actively involves e-scooters and e-bikes. Barcelona, London and Milan follow the lead. But behind all the fun and convenience behind e-scooters and e-bikes there is fire risk. Remember how Samsung Galaxy Note 7 caught in fire? Now multiply that fire 50–140 times — that’s how many more battery cells typical e-scooter or e-bike has. And while safe battery technology is being developed, it is relatively easy to decrease the fire risk of your own personal e-transport.
Fire hazard of lithium batteries
Samsung manufactured about 3 million of Galaxy Note 7 and only about one hundred of them got on fire. But people became so afraid, that airlines banned bringing that device on board with $180 000 fine or 10 years in jail for those who disobey. Samsung recalled all manufactured galaxy Note 7 and opened in airports worldwide offices where people could return their bomb-phone. One such place was opened in airport “Borispol” in Kiev, Ukraine.
The problem with lithium batteries catching on fire is that it’s almost impossible to put down the fire. That is because fire requires two components — fuel and oxidizer. Lithium-ion battery contains both of them, unlike the petrol, alcohol or other freely available flammables.
The only possibility firemen have in case of lithium-ion batteries fire — is to try to cool them down to a temperature, at which chemical burning reaction doesn’t start yet. For cooling down a burning Tesla firemen once used 11 tons of water and a special container to drown a car in it. And usually big battery doesn’t fully burn at once, so there is possibility of a self-catching fire a day or two later. Tesla officially states that battery fires can take up to 24 hours to extinguish.
Why lithium batteries catch fire
The reason behind batteries get on fire is Ohms law. It’s a school physics formula, which states: any electric element has electric resistance. And when current flows, resistance of this electric element leads to its heating. Greater current leads to greater heating. Home electric heaters work using this principle.
Mathematically generated heat from electrical resistance is described with the formula P=R*I*I. Here P is heat in Watts, R — resistance in Ohms, I — amount of flowing current in Amperes. There some limitations of this formula, but for quick and simple calculation it’s usually enough.
Any battery has its own resistance, which is typically a thousandth part of 1 Ohm (unit of electric resistance). And lithium-ion cells are suited for high currents, so multiplying these two parameters using formula above one can get an estimation of generated heat.
Sometimes current flowing in battery can go way over designed values, like when battery leads are connected with a wire (short circuit) or when battery cell is punctured all its layers become connected internally. In such situations battery gives all the current it is capable of.
Excessive current generates excessive heat, which doesn’t dissipate in surrounding air quickly enough. When temperature inside battery rises up to 66–75 degrees Celsius, it’s electrolyte (most common is lithium hexafluorophosphate, LiPF6) begins to decompose and burns.
That’s why discharged lithium-ion batteries are much safer — they don’t have energy to generate excessive current.
How battery overheating begins
Here is practical demonstration of battery heating due to its internal resistance. It’s a pack of Ni-Mh batteries — they don’t catch on fire, so it is safe to conduct such experiment. If that were lithium batteries — they should be properly discharged and disposed at that very moment.
So, a pack of four Ni-Mh batteries means the cells have as close resistance as it could be. They were used in photo camera — working conditions were the same for all cells. But with time one cell began to degrade, which results in resistance increase.
Bad battery cell starts heating lot more when charging. Thermometer clearly shows that:
Those battery cells are connected in parallel, which means they get the same charging voltage. Such connection excludes possibility of one cell overcharging, which happens in any battery with serial connection of cells (laptops, e-scooters, e-bikes, e-cars, etc.). When battery cell is overcharged, it can no longer take charging current and all of it goes to heat generation.
Good cells were at room temperature, and bad cell was 3.5 degrees hotter. At the moment when photo was taken, the current was small — those batteries were almost fully charged and had 1.42 volts. Fully charged Ni-Mh battery cell has 1.5 volts. But bad cell had 1.39 volts because it used energy to heat itself.
High-current charging is dangerous
The life cycle of battery cell is strongly influenced by the value of charging and discharging currents. High charging current can deteriorate battery life cycle. That is why it is not recommended to use fast charging. The following chart shows battery capacity loss depending on charging and discharging current (C is battery capacity value):
In some cases battery can be ruined in 25 fast charging cycles. In some electric cars after 60 charging cycles using fast industry charging electrodes and electrolytes in battery cells were exposed to the air, and got to temperatures close to 60 degrees Celsius, increasing the risk of fire or explosion.
It’s a reality that electric vehicles catch on fire while charging with high currents. In 2019 in Belgium Tesla Model S got burned down soon after it was connected to Supercharger.
Consequences of electric scooters fire can be considered greater, than from a e-car because they are usually charged in living rooms and bedrooms, where are lots of flamable things. An apartment can burn down in five minutes, and people and pets risk dying earlier than that: smoke and toxic gases kill 3 times more people than flames.
E-scooters are charged dangerously
Most e-scooters have batteries of 36, 48 or 52 volts, top models work at 60 or 72 volts. Charging current usually is 2–5 amperes — that’s not superfast, but is relatively fast charge.
Let’s take an e-scooter with battery of 48 volts (54.6 volts at full charge, 40 volts when fully discharged) and capacity 12 ampere-hours with total battery power near to 600 Watt-hours.
Usually e-scooter batteries use lithium-ion cells (3.0–4,2 volts) of 18650 format with capacity close to 2.2 ampere-hours. So such battery will have 13 cells connected in series, which makes one line of 48 volts with capacity 2.2 ampere-hours. And there is 5 such lines connected in parallel to achieve capacity 12 ampere-hours.
Assuming battery is charged with current 2 amperes and all cells have equal resistance, than each cell will be charged with current 0.4 amperes. That is totally inside safe range for 18650, which is from 1 to 2 amperes, yet it is capable of generating lots of heat.
Cells in battery do not have equal resistance when new, and difference gets greater with time and usage. Let’s say the most degraded element converts all the current from charger directly into heat. Multiplying 2 amperes by 4.2 volts we will get almost 8.4 Watts of heat.
How much is 8.4 Watts of heat? Try touching a soldering iron or a heater with the same power rating, but only with a thermometer — a burned skin guaranteed if it is touched with bare hand. Notebook and smartphone processors (processors convert almost all consumed energy into heat) with same power consumprion require cooling or they will melt down.
Why battery protections don’t work
Lithium-ion batteries have a number of protections, but often they don’t work. Measurement of battery parameters is done by Battery Management System (BMS). It is a special board, which is connected to the battery, and has a thermometer to prevent general battery overheating. That is why usually touches only one or two battery cells. What about temperature of the rest battery cells? The answer is: who knows.
So, when one element somewhere inside the battery degrades and becomes a heater, BMS doesn’t cut off the battery — it just doesn’t know that fire is about to happen.
Charging device of e-scooter has a limit of maximal current, but faulty battery can stay below that limit when it cuts off charging.
There is also cell overcharging problem, which happens in any battery with cells connected in series. Fully charged element should be cut off from charging, trickle or float charge at cell full charge is not suitable for a Li-ion battery, since it would cause plating of metallic lithium and compromise safety.
BMS controls the voltage on each cell, but it doesn’t cut off a cell at precisely 4.2 volts — it cuts off at a little higher value. Cutting off at exactly 4.2 volts would mean that cell stays undercharged. Some elements in battery will be overcharged, and excessive charge will be dissipated as heat.
Realistic scenario
So, a real situation: hot day with 30 degrees Celsius in shadow. Owner of e-scooter goes full throttle for quite some time to get a nice cool wind or he goes up a long steep hill. This requires lots of energy, so battery will give all it capable of, and will heat up from the currents flowing thru it.
After a fun ride owner immediately connects e-scooter to a charger. Some elements get overcharged quickly and convert rest of the energy into heat. Some bad elements don’t fully charge and also heat up.
Enclosed case of the battery doesn’t allow heat to escape, temperature of bad elements rises to critical 66–75 degrees. BMS considers everything is fine — its thermometer located on a good cell with safe temperature.
The following moment something like this happens:
How to safeguard an e-scooter
The main threat of Li-ion battery is heat, so to decrease overheating the best practice is to significantly lower its charging current. This can be done by simple change in e-scooter charger.
Here is a charger for Kugoo M4 Pro with battery of 48 volts 12 ampere-hours. Charger is loaded with resistor of 22.4 Ohms, and if charger had no current limitation, the current should be 2.4 amperes. But it supplies exactly 2 amperes of charging current:
The value of supplied current is determined using special resistor called shunt. It has very low resistance so it would not influence current flow in a great manner. But the resistance is significant to create a smal voltage drop on the shut. This voltage drop is compared to the etalon value — the grater the drop, the greater current flows thru shunt.
It is easy to find a shunt on a board — it’s a big green resistor near the brown and blue output wires:
By the color of its stripes it is possible to determine its resistance value using online calculator. The colors are brown, black and silver — resistance is 0.1 Ohm. Gold ring shows that this value has 5% accuracy. The shunt is big because it is rated at 1 Watt of heat dissipation — it generates 0.4 Watts when current is 2 amperes (P=0.1*2*2=0.4).
If 0.1 Ohm gives 2 amperes output, then 0.2 Ohm will give current of 1 ampere, 0.5 Ohm will result in current of 0.4 ampere.
In scraped electronics two shunts were found: one 0.24 Ohm and 0.499 Ohm. That’s for easy charge current changing in future, if it would be necessary.
One end of original 0.1 Ohm shunt was disconnected, and shunt of 0.499 Ohm was soldered into its place.
Measurements showed the charging current was 0.41 amperes.
Current of 0.41 amperes on the ampermeter means that each cell is charged with current 0.08 amperes. It’s 0.34 Watts if directly converted by the bad cell into heat while charging.
How much heat is that? It is slightly warm. NASA research shows that Li-ion elements of 18650 format have thermal conductivity of up to 0.43 W/m*K.
Battery pack of the e-scooter will be capable to dissipate the heat and stay below dangerous temperature theshold.
Outcomes
The downside of such conversion is much longer charging time. It now requires about 30 hours, when previously only 6 hours were needed. But safety is more important.
It is possible to incorporate a switch between shunts to easily return to max charging current. If e-scooter is charged outdors where it can do no damage when it catches on fire — it can be charged with current of 2 amperes. If e-scooter is charged at home, a slow charge will be selected and no worries about overheating the battery.
Written by: Eugen Barilyuk
