How Does A Battery Management System Keep Electric Vehicle From Exploding?

When we’re driving an EV, use our laptop or  phone, we’re actually sitting next to thousands of highly reactive chemical bombs. Lithium-ion batteries are brilliant, but they are pretty much like our indoor plants. Overcharge them? Fire. Drain them too low? Brick. Even look at them wrong? Thermal runaway!

The only thing that’s standing between us and a very expensive fireball is a glorified babysitter called the Bom… I mean … Battery Management System (BMS). It’s an unsung piece of tech and engineering wizardry that constantly monitors voltage, current, and temperature, making microsecond decisions to keep the battery alive and well for a long time. 

In this Geekswipe edition, we’re looking deep into how these battery management systems, the brain of an EV battery system, actually works.

How do battery cells work?

When we buy a massive battery pack, we aren’t buying one big battery. We are buying hundreds or thousands of individual battery cells wired together. Remember that serial and parallel cell from our chemistry hour at school? 

So anyways, like all group projects in our college, some cells work harder, some are lazy, and in some cases, the whole project is bottlenecked by the weakest link.

Because of microscopic manufacturing defects and differences, no two cells charge or discharge at the exact same rate. If we punch in raw power into the pack, the fastest cell obviously hits its maximum voltage before the rest. And if we keep charging to fill the others, that fast cell gets overcharged, grows microscopic spikes called dendrites, eventually shorts out, and triggers a catastrophic fury straight from hell.

That being said, we are actually fast charging our EVs, phones, and laptops all the time, don’t we? Learn more about the effects of fast charging. So why don’t they catch fire? Long story short, BMS at play! It fixes this using something called cell balancing.

Passive cell balancing

Most cheap systems use passive balancing, which is basically an effective hack. The BMS literally connects a resistor to the overachieving cells and burns off their extra energy as heat so thermal runaway is prevented.

“Wait a minute, why not just reroute—” Yeah yeah! I’m going there!

Active cell balancing

High-end systems use active balancing. Instead of burning off the energy, the BMS reroutes the excess juice from the strongest cells directly into the weakest ones. It’s complex, expensive, and absolutely necessary if we want our power grids and high-end EVs to last more than just a few years.

How BMS predicts state of the battery?

The thing about batteries is, we just can’t measure how much charge it has like using a dipstick. Measuring the state of charge is a guesswork.

“Well we can actually measure—” Yeah! Yeah! I know. The voltage! We can actually measure the voltage of the battery and tell if the battery is at full charge or not, right? Well, there’s a problem. We should be measuring when it’s idle. Because when we press the accelerator pedal, the voltage drops due to internal resistance.

There’s actually a smarter way to measure without the above method. Basically count every coulomb (unit of electric charge) entering and leaving the battery. But this one has a problem too. If the error in the counting grows by 1%, for every cycle this compounds and becomes useless too soon.

That’s where we bring in the real ‘smartness’ of tech. We introduce an error correcting algorithm in the mix to self-correct itself. The BMS runs a digital simulation of the battery, compares its simulated voltage with the actual real-world voltage it’s measuring. When they disagree, the algorithm (Extended Kalman Filter) recursively corrects the math on the fly. It fixes bad guesses immediately, even as the battery ages and degrades. 

If a company is selling a massive energy storage system and they aren’t using an EKF, they are basically scamming you.

So how BMS keeps battery thermal runaway?

You see, a thermal runaway happens when the heat of a cell is not dissipated enough or when an internal short circuit (from overcharging). It also happens when there’s a physical puncture in the cell (like in EV crash scenarios), where the chemical reaction produces oxygen. When this happens, the only best option is to run away from the battery as quick as possible.

So how BMS puts out the fire? Well, you guessed it right, we basically combine the above precise measurement with a coolant release mechanism. Even before the thermal runaway happens, if the temperature gets too hot, the BMS literally opens coolant into the heating cell and sacrifices it to save the battery.

So at the end of the day, it’s the BMS the heart of the battery and what matters if you’re looking into a battery system or an EV. It’s what keeps things from exploding, especially when fast charging batteries, be it an EV or a digital device.