With smartphones and laptops, everyone’s always telling us, “Don’t overcharge them”.
But is it the same for an EV? What about all those people who claim that overcharging an EV is impossible? It’s a confusing time to be an EV owner, so as a workshop specialising in EVs, you’ll want to know exactly what to tell them. The true answer is that although it’s extremely unlikely that someone could ever overcharge their EV, it is technically possible. This page explains how.
How an EV’s propulsion battery is made up
Before looking at whether it’s possible to overcharge an EV’s battery, let’s first take a moment to define exactly how that battery works.
An EV battery is made up of hundreds of individual cells. Each of these contains an anode, a cathode, a separator and an electrolyte solution.
The anode and cathode allow ions to flow through the electrolyte during charging and discharging. The separator prevents short circuits and affects ion flow and internal resistance.
With all the investment in EVs over the last decade, cells can now be found in cylindrical, prismatic or pouch formats. Each of these has different charge-holding, space-saving, weight and cooling characteristics.
The cells are grouped into modules, which vary from brand to brand and even from car to car, making maintenance and replacement much easier. Several modules are assembled together to form the complete battery pack. That’s the EV’s propulsion battery.
The role of the battery management system
Of course, the car also includes an onboard battery management system (BMS), cooling systems and structural supports.
The BMS is a highly advanced piece of electronic programming developed by the EV manufacturer.
It monitors voltage, current, temperature and various other parameters to ensure everything operates within safe limits. If any parameter deviates from the norm, the BMS intervenes to prevent damage.
For instance, if you were to charge all the individual cells that make up an EV’s propulsion battery without a BMS, you would vastly overcharge a few cells and undercharge the rest. The BMS helps all the cells’ share’ the charge by ‘bleeding off’ any excess voltage as heat.
Within the BMS, you’ll find sophisticated charging protocols. These manage how and when the battery charges when you plug your car into a charger. They’re responsible for all manner of things, including adjusting the charge rates based on factors like battery temperature and state-of-charge (SoC). For example, during rapid charging, the system may slow down the charge rate to prevent overheating.
The BMS is also responsible for thermal management. It controls all your battery’s cooling mechanisms to maintain optimal battery temperatures. If a battery becomes too hot, these systems activate to cool it down. That keeps everything safe and optimises the charging conditions.
How passive top balancing keeps EV cells safe during charging
Each cell in an EV’s battery pack has its own voltage and SoC. Passive top balancing works near the end of the charging cycle. If one cell charges faster than the others, it reaches the upper voltage limit first, and then overcharging begins. The BMS would then step in to route the excess current around the high-voltage cell using a bypass resistor, which dissipates energy as heat and slows that cell’s charge. Meanwhile, the lower-voltage cells continue charging. The BMS continuously monitors the voltage of every cell throughout the cycle. When all the cells reach the same SoC, the BMS stops charging, even if the car is still plugged in.
Passive balancing sacrifices some efficiency (through heat) to keep the pack safe. It’s a good trade-off.
As long as the EV’s onboard BMS is functioning, all components are in good condition and the fail-safes (cell fuses, thermal cut-offs, etc.) operate correctly, everything should remain safe. For nearly all electric vehicles, this holds true almost all the time.
However, in extremely rare situations, issues do occur.
If there’s a fault in the car, the BMS or the cooling system could theoretically lead to overvoltage in a few cells, but the pack-level protections make this very rare.
Even in cases where the battery begins to overcharge, the BMS should be able to keep on top of the cooling system, reducing heat and keeping everything safe. In very rare instances, though, it may fail. Isolated parts of the battery could become excessively hot.
But here’s the bottom line
While it’s technically possible that an EV could overcharge, it’s highly improbable, especially if an owner keeps up with recommended maintenance. The systems in place are highly advanced, both in the cars and in the chargers themselves.
However, as an independent workshop specialising in EVs, keeping up to date with emerging technologies, such as active balancing, fosters customer trust and ensures you’re prepared for whatever the EV charging market brings next.
At Rotronics, we provide EV-specific chargers and testers tailored to propulsion batteries and 12-volt auxiliary systems. These highly advanced tools give you and your team everything you need to diagnose EV battery problems and get your customers back on the road again.
Contact us today to learn more about our solutions and how we can support your business in this rapidly evolving industry.
