Power management innovator Chris Hale, managing director of battery technology company Chimera Energy, examines the risks presented by the varying quality of battery management systems (BMS) and the cells they are designed to protect.
Lithium-ion remains the fastest growing battery sector, finding its footing in automotive, energy storage and many portable applications. The global growth of lithium-ion EV batteries has reached more than 750GWh in 2023, up 40% on 2022, with a projected 40% year-on-year continued growth rate as just one example.
The list of applications as we know is ever growing and, each day, we can see the increased number of e-scooters, segways, e-bikes, hover boards, drones, just to scratch the surface. The point though is what we can expect from the batteries in these applications and in many ways the importance of manufacturers or users understanding the role of the BMS.
At a minimum, a lithium battery needs a BMS to protect the cells from overcharge, overdischarge, overcurrent, temperature extremes and to keep the cells in balance. The misconception is the belief that as long as we have electronics, the battery is protected and reliable.
So, taking statistics into account, lithium iron phosphate (LFP) accounted for 40% of global EV demand in 2023 with low- and high-nickel cathodes dominating the remainder. Although LFP is reportedly a ‘safer’ chemistry than lithium nickel manganese cobalt oxide (NMC) and other nickel-based cells, EVs using NMC still maintain a very good overall safety record.
It is important to consider that statistically, EVs are 20 times less likely to catch fire than internal combustion engine (ICE) vehicles, according to autocar.co.uk. It is also reported, according to a study by the Swedish Civil Contingencies Agency and an American insurer, that just 25 out of 100,000 EVs suffer fire damage in contrast to 1,530 per 100,000 ICE cars. Hybrid vehicles on the other hand suffer a greater portion, at 3,475 per 100,000.
The majority of EVs use low- and high-nickel cathode batteries, however, safety is largely down to good design and more significantly through good battery management.
Context of e-bike fires
Where passenger EV battery systems are deemed to be comparatively safe there are many other light/micro-EVs which are not – such as low-cost e-scooters and e-unicycles.
However, there are many factors that can significantly reduce (or increase) the risks of catastrophic failure,and in my view, it is the BMS that is the most important. A good BMS on a bad cell can reduce the risk of the bad cell failing. A bad one on a good cell will eventually reduce the good cell to a bad cell, while a bad BMS on a bad cell is recipe for trouble.
Going online, it is not hard to find a plethora of cheap BMS options for power-banks, e-bikes and DIY batteries. The worrying thing is that there is little knowledge about what using these cheap BMSs really means.
Examination of typical low-cost online BMS
So, for the above BMS, maximum overcharge cutoff would be 4.275V. If you consider that most lithium-ion NMC/NCA specification datasheets have a maximum charge of 4.2+/-0.05V, there is already potential to exceed the maximum safe charging limit.
Given the balancing tolerance of +/-0.025V, the potential cell voltage spread could be anywhere between 4.175V and 4.225V, which may represent around 4-5% capacity difference.
There is no temperature protection, which means there is nothing to prevent charging at temperatures below 0°C or greater than 50°C. As temperature is a critical factor for degradation (and safety), not having temperature monitoring is a bad idea.
Analysis over time of charging impact
As stated, the BMS is intended to provide cell protection. But what about cell degradation? Using the above BMS, the battery charger is being relied on to maintain a safe charging voltage (54.6V [13 x 4.2]) and prevent the BMS overcharge voltage threshold being reached.
Another quick search online finds plenty of cheap e-bike chargers. The trouble is, many of these cheap chargers will not provide a decent specification, quoting a 54.6V, 2A output say, but with no tolerance values. It is difficult to guarantee the charger you buy does not have an output voltage with a tolerance that exceeds 54.6V, or by how much.
In the case above though, if we assume the charger does maintain 54.6V, the mean voltage for each cell is 4.2V. However, there is a BMS balancing tolerance of +/-0.025V/cell, which means that one cell could be at 4.225V, while all the other 12 cells could be down at 4.175V. The pack voltage in this case would be 54.325V with one cell that is fully charged.
The pack is still charging, with 54.6V applied, so an overcharge voltage would be seen on the lead cell, potentially reaching 4.245V. This is still okay, because the lead cell is still below the maximum 4.25V stipulated by the cell manufacturer, and still below the BMS overcharge cutoff of 4.275V. But what if we add temperature and degradation to the mix (or charger voltage tolerance)?
Dangers of high charge and temperature
To put into context, I recall an instance a few years back of a battery system being designed and tested according to the cell manufacturer’s data sheet. The cells had a maximum allowable charge voltage of 4.3V and maximum storage temperature of 60°C.
The pack was charged to 4.3V/cell and left in an environment chamber at 60°C for two days. On the second day the pack went into thermal runaway. Those were new cells being tested, rated to 4.3V.
Taking accelerated degradation as a function of charge voltage and temperature, the lead cell will most likely degrade at an accelerated rate, compared with other cells in the pack. As the lead cell degrades more than the others, it will tend to have greater capacity loss per cycle and increased cell impedance. Initially the lead cell will have (in this example) a capacity 4–5% higher than the other cells, so on discharge, end of discharge will be based on one of the lower-voltage balanced cells.
At some point though, after a number of cycles, the lead cell would have lost the 4–5% capacity margin over the other cells and will now hit end of discharge first, as well as being the highest charged cell. The lead cell becomes the weakest cell.
At this point, degradation will accelerate in relation to all other cells, as capacity loss during cycling is also a function of both depth-of-discharge and state-of-charge.
As the cell grows weaker, the ability to maintain balance during charge with 42mA on what may be a 3,000mA cell (1.4% balancing current vs cell capacity) on a 2A charge, becomes less likely. If we also consider the increased cell resistance for the degraded lead cell, the effectiveness of the balancing current at the point where a pack voltage of 54.6V is reached, (while still drawing 2A), will be minimal.
This means it is very likely the balancing tolerance will be exceeded and, while the majority of cells are still below a 4.2V threshold, the lead cell will start to increase above 4.245V from balancing tolerance.
The higher the voltage, the more stressed and weaker the cell gets. That in turn means a higher voltage on charge – harder for balancing to manage. Eventually, the upper BMS overcharge voltage threshold will be reached (which may be as high as 4.275V for the BMS given above). An excessively high voltage on the cell is not good and over time could lead to catastrophic failure.
Degradation and failure
If the pack continues to be used, it may be worth understanding what is degrading in the cell and what may eventually lead to the risk of thermal runaway.
The above example may take the extremes of the tolerances in the BMS to prove a point, but even without, the principle is the same. It just may take longer for a lead or weaker cell to reach the point described.
It is important to understand degradation within the battery cells to appreciate where and how they are subject to failure. Fig 2 offers a good representation of key degradation mechanisms. However, it is typically the lithium plating and dendrite growth that presents the greatest risk to thermal runaway (Fig 3).
Loss of active material, electrolyte decomposition, gas formation and structural decomposition will all likely occur during normal cycling.
Stress cracking is generally through fast charge/high-rate discharge, along with pore blockage and gas formation.
Lithium plating, more critical from a safety perspective, is generally a function of top-of-charge, over-discharge and cold temperature charging. For cold temperature, as intercalation rates are slowed due to sluggish reaction rates with temperatures below freezing, some lithium ions will deposit as lithium metal (depending on the charging rate and temperature).
At top-of-charge, charge voltage and temperature play an increasing role. As reaction rates are a function of temperature, the higher the temperature, the greater the degradation. Voltages at top-of-charge, especially around 4.2V, will risk the onset of plating, accelerated as the voltage increases above 4.2V – and more so in higher ambient temperature.
Using the e-bike battery example above, overcharge from a poor charging system or bad BMS is where the plating concerns arise. In contrast, EV batteries tend to ‘top and tail’ to extend battery life, typically an 80% depth-of-discharge and 80% state-of-charge. Topping and tailing to extend battery life is a great way of significantly reducing the risk of lithium plating.
Although heaters are also occasionally employed for cold charging, cold charging below 0°C at the very least is inhibited or the charge current significantly reduced. Either way, EV battery management will significantly reduce the probability of lithium plating and will certainly account for the significantly lower instances of fires.
The bottom line
Getting the most out of a battery by fully charging it comes with the knowledge that, over time, if the cells do not maintain proper balance and especially if the ambient temperatures are elevated, increased calendar ageing is likely. If the BMS allows cells to exceed 4.2V, the rate of degradation will increase, along with the increased risk of dendrites from plating. This is why care should be taken when selecting a BMS and charger. Cost over quality may ultimately pay the price in the long run.
