Everything they’ve told you about EV battery simplicity & reliability is a lie

10 Feb

If EV batteries are always in the news, how come power tool batteries are - seemingly, at least - so damn robust and reliable? Like, dude, they use the same fundamental technology. You never see Milwaukee demanding you don’t charge your M18 over 70 per cent.

Here’s a question I got from a viewer, recently:

You're a tool bloke John, why don't our tool batteries do the same as EV car batteries?

Don't get me wrong, ffs I've got a shite load of Li power tools, just want to understand why my/your/our Li tool batteries don't do the same as car EV batteries, i.e. spontaneously combust.

For reference, I still (dunno why, sentiment maybe or I just can't throw out useful tools..) have a Metabo SBZ 18 Impuls drill, charger and a couple of 18V 2.2 Ah batteries made in or around 2012 that still work fine.

Genuinely curious because we don't (as far as I know) hear about tool batteries behaving like car EV batteries.

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A simple question, with a surprisingly complex answer.

Power tool battery Vs EV battery (on fundamentals)

A typical 18 V power-tool pack has:

~10 cells
passive cooling
very simple BMS (sometimes none at cell level)
few sensors
short duty cycles
18 volts output
90Wh of stored energy
Slow charging at low power

A typical EV battery has:

hundreds to thousands of cells (thousands of 18650s ~500 if ‘pouch’ or ‘prismatic’)
computer-controlled active liquid cooling
dozens of sensors
active software control
long duty cycles
tight temperature uniformity requirements
400-800 volts output
75kWh of stored energy
Charging at 150kW (or more)

Complexity scales non-linearly.
Going from 10 cells to 500 cells is not “50× harder” — it’s thousands of times harder to manage safely.
COMPLEXITY IS THE ENEMY OF RELIABILITY
Complex systems require the highest levels of quality assurance, and the greatest number of systematic safeguards. And EV batteries are complex. Whereas power-tool batteries are allowed to fail

A power-tool battery:

costs a few hundred dollars
is user-removable
lives outside the cabin
failure usually means “tool stops working”
fire risk is localised and rare

An EV battery:
costs tens of thousands
is structurally integrated
sits under occupants
failure can mean vehicle loss, house fire, or worse
attracts regulators, insurers, lawyers and media

So EVs are engineered to an extraordinary safety standard, and manufacturers try to act act long before risk becomes visible. This really is a herculean task, in terms of engineering and widespread public deployment.

Duty cycle and design intent are completely different

Power-tool batteries are:

designed for short, brutal bursts
expected to run hot
expected to degrade faster
expected to be replaced

EV batteries are:

expected to last 10–15 years
expected to tolerate daily charging
expected to work in all climates
expected to fail gracefully, not dramatically

Not only that, EV batteries are a textbook tightly coupled system with a great deal of stored energy. ‘Tightly coupled’ meaning ‘nothing fails casually, in isolation’. The whole thing can go from ‘just another day at the office’ to ‘Chernobyl’ in a heartbeat.

The Truth About Complexity

You’ve been handling batteries all your life. They look so simple, and seem so inert. As I see it, they can fail in (at least six) entertaining ways:

Cathode failure
Anode failure
Separator failure
Cell container / pouch breach
Positive terminal / tab
Negative terminal / tab

In a power tool battery, that’s 60 things that can fail. In a pouch/prismatic cell EV battery, that’s 3000 things. (Because: 500 cells.) And that’s before we even leave the cell.

Each of those numbered domains above has multiple failure mechanisms.

Lithium plating
Dendrite growth
SEI breakdown
Gas generation
Internal shorts
Loss of electrolyte wetting
Thermal runaway initiation
Mechanical fatigue
Manufacturing contamination

So in formal FMEA terms (Failure Mode & Effects Analysis) each “failure domain” typically explodes into several distinct failure modes. Like going up a tree.

There’s also interconnection- and module-level failures (often underestimated)

Per cell:

Weld / bond to positive tab
Weld / bond to negative tab

That’s another 2 per cell (= 1000 interconnection failure points)

And those are:

vibration-sensitive
thermally cycled
corrosion-prone
current-dense

And then there are the vital peripheral systems, like the Cooling system, with its umpteen failure modes.

Coolant leaks
Blockages
Pump failure
Flow imbalance
Loss of thermal contact
Cold-plate delamination

One cooling failure can affect dozens of cells simultaneously.

And then there’s the BMS & related sensing systems:

Voltage sense drift
Temperature sensor misplacement
ADC calibration error
Firmware logic errors
SOC estimation error
CAN comms faults

Protection systems can fail too:

Contactors can fuse
Fuses can nuisance blow
Fuses failure to ‘open circuit’
Precharge systems can fail
HV isolators can degrade over time and fail when needed
We’ll see more of these failures as the EV fleet ages disgracefully. That’s a mathematical certainty, not an opinion.

Many of these systems are:

rarely exercised
safety-critical
time-dependent (aging)

The second law of thermodynamics does not give EVs a free pass, and it’s ridiculous to believe it does.

Even the structural enclosure is vulnerable:

Mechanical deformation
Crash load paths
Seal degradation
Corrosion

These failure modes are not independent. They cascade together rapidly in unpredictable ways. Not every failure mechanism can be known in R&D.

EVs are far simpler than ICE: the great EV lie.

They’re actually more complex, but the complexity is different. In many ways, more insidious and harder to grasp for non-technical people.

ICE powertrains have thousands of parts
Many are mechanically forgiving
Many failures are progressive, noisy, or gradual
Catastrophic failure is rare and localised
ICE is not as tightly coupled
EV battery packs have fewer mechanical parts
Far more tightly coupled, safety-critical elements
Far less tolerance for hidden defects

What this means is: Your engine gets noisy or smokey over time. You get to a point where visiting the mechanic is necessary. He looks at the symptoms, does diagnosis and performs some remedial actions (component replacement).

Your EV runs like a dream … until there’s a fire hot enough to melt steel, just under your arse, generating a toxic shitstorm. You’ve got to get out, and you're stopped in the right lane of the freeway.

Waste heat management

Waste heat management of the battery is a huge issue with EVs, that does not exits with power tool batteries.

Power tool batteries charge at maybe 100W. If waste heat is 10%, there’s a 10W heater (effectively) inside the enclosure, and passive convection can take care of that.

But EVs are different.

If you are charging or discharging at 150kW (accelerating hard), EV batteries have a low surface area to volume ratio, so they are poor at dissipating heat passively.

They need active cooling systems.

Power tools batteries are the opposite (high surface area to volume = good passive convectors).

Also EV batteries are big in relation to individual cell size, so a great many individual many cells - hundreds to thousands - are a long way from the outside of the 'box'.

This means heat they generate must pass through other cells, if passively cooled.

Power tools are the exact opposite - there's only 10 cells in the pack.

Finally, there's a lot of waste heat to manage.

An EV battery charging at a 150kW DC fast charger probably generates 5-10% waste heat (because the charging process is not 100% efficient).

This means 7.5kW-15kW of waste heat. (That's 3-6 domestic power points running flat out, inside a small box, generating heat - enough heat to heat a house in winter.)

This can all be managed in engineering terms, but if something goes wrong - sensor failure, some automatic cutoff relay fails, some other safeguard falls over - you're on the news.

John Cadogan