Battery Temperature and Cooling During Charging

[Admin]

“Why Does Charging Slow Down at 60–70%, and What Does Thermal Management Do?”​

Among EV drivers, one of the most common mysteries is why the charging process starts out fast and then suddenly slows around 60–70%. Many assume the charger is throttling back or malfunctioning, but the truth lies in battery chemistry, internal resistance, and temperature management. In this article, we’ll break down what really happens during charging — and why thermal management systems are absolutely essential to battery life and safety.

1. Battery charging is not linear​

A common misconception is that if your EV has a 100 kWh battery and a 200 kW charger, it should take roughly half an hour to charge from 0 to 100%.
In reality, charging speed is never constant. Lithium-ion batteries behave differently from a fuel tank — they resist charge more and more as they fill up.

To understand this, let’s look at the three main charging stages:

1. Bulk (0–50%) – Fast phase
   At low state of charge (SOC), cell voltage is relatively low and internal resistance minimal. The charger can deliver maximum current, so charging is rapid and efficient.
2. Taper (50–80%) – Controlled phase
   As SOC rises, the voltage of each cell approaches its nominal maximum. The battery management system (BMS) begins to reduce current flow to prevent overheating and overvoltage.
3. Balancing (80–100%) – Finishing phase
   The final portion is slow and gentle. The system carefully balances the voltage of thousands of cells to within millivolts, ensuring full capacity without damage.

That noticeable drop in charging power around 60–70% is the transition between phase 1 and phase 2 — a deliberate safety and longevity feature, not a flaw.

2. Temperature: the invisible factor behind charging speed​

Every chemical process inside a lithium-ion cell generates heat. The higher the charging current, the more resistance converts energy into thermal load.
The optimal temperature window for most EV cells is around 20 – 40 °C (68 – 104 °F).

When the pack temperature rises above this range:

• The electrolyte begins to degrade faster.
• Lithium plating may occur on the anode surface, permanently reducing capacity.
• Internal pressure increases, which can lead to venting or thermal runaway in extreme cases.

On the other hand, cold batteries also charge slowly — the ions move sluggishly, and the BMS will intentionally limit charging power to avoid metal deposition.

That’s why temperature control is a two-way problem: batteries hate being too hot or too cold.

3. What thermal management actually does​

Modern EVs use sophisticated thermal management systems (TMS) to regulate pack temperature during both driving and charging. Depending on the design, this can include:

• Liquid cooling loops that circulate coolant through channels around or between battery modules.
• Refrigerant-based chillers that use the same compressor as the cabin A/C to extract heat.
• PTC heaters or heat pumps to warm up a cold pack before fast charging.
• Temperature sensors embedded throughout the pack to allow precise control via the BMS.

When you plug in to a DC fast charger, the vehicle immediately measures the pack temperature and adjusts the charge rate accordingly.
If the cells heat up too fast, cooling pumps and fans ramp up, and the current is reduced.
You might hear the car’s fans running loudly — that’s the thermal management system working hard to keep the chemistry in its comfort zone.

4. Why charging slows down — the chemistry explained​

As the battery approaches about 60–70% SOC, internal cell voltage is nearing its limit.
The BMS faces two constraints at once:

1. Voltage ceiling — Each cell can only reach a safe maximum (usually around 4.2 V). To avoid overshooting, the BMS must taper current.
2. Thermal ceiling — The higher the current, the more heat generated. Even with active cooling, temperature rise can outpace what’s safe.

Therefore, the system reduces charging power not because the charger “can’t handle it,” but because the battery shouldn’t.
Maintaining high current beyond this point would exponentially increase stress, heat, and long-term degradation.

This is why some EVs can charge from 10% to 60% in 15 minutes — but take another 25 minutes to reach 100%. The slowdown protects the cells and ensures they retain capacity over many years.

5. Preconditioning and intelligent charging strategies​

Many modern EVs now include battery preconditioning: before arriving at a DC fast charger, the car automatically warms or cools the pack to its ideal temperature.
Tesla, BMW, Hyundai, and others integrate this feature through navigation — if you select a Supercharger or DC fast charger as your destination, the car begins optimizing the pack temperature en route.

This ensures the pack is “ready” for peak charging performance the moment you plug in, reducing the time spent in the low-power phase.

As EV technology matures, these systems are getting smarter:

• Predictive algorithms estimate heat buildup based on charge rate and ambient temperature.
• Some vehicles share data with the charging station to coordinate current flow.
• Future designs may use immersive liquid cooling directly in contact with cells for even tighter thermal control.

6. What happens without proper cooling​

Poorly managed heat can shorten battery life dramatically. Studies show that continuous exposure to temperatures above 50 °C (122 °F) can halve a lithium-ion cell’s usable lifespan.
Without active cooling:

• Capacity fades faster due to accelerated electrolyte breakdown.
• Resistance increases, causing even more heat under load — a vicious cycle.
• Extreme cases can lead to swelling or thermal runaway.

That’s why smaller plug-in hybrids (PHEVs) often limit charging power compared to full EVs: their compact battery packs have less room for complex cooling systems.
Similarly, early EVs with passive air cooling (like the first-generation Nissan Leaf) suffered from faster degradation in hot climates such as Arizona or California.

7. What this means for drivers​

When your EV’s charging power suddenly drops from 150 kW to 60 kW around 70%, it’s not an error — it’s battery preservation in action.
Here’s what you can do to help:

• Plan around 80% for long trips — beyond that, you gain little range for the extra time.
• Precondition before fast charging, especially in cold weather.
• Avoid repeated DC fast charging in very hot conditions.
• Keep your EV’s cooling system serviced — coolant levels and filters matter just as much as in an engine car.

By respecting these limits, you’ll not only charge more efficiently but also prolong your battery’s lifespan and performance.

8. The future: smarter thermal management​

Next-generation EV platforms are taking thermal control to a new level.
Expect innovations such as:

• Integrated heat pumps that manage cabin and battery heat simultaneously.
• Phase-change materials (PCM) that absorb excess heat during rapid charging.
• AI-driven BMS algorithms predicting optimal current flow per cell in real time.

The goal is to make the charging curve flatter — maintaining higher speeds for longer without compromising safety.
As these technologies evolve, the “slowdown” at 70% may become less noticeable, though it will never disappear completely, because physics and chemistry still set the boundaries.

Final Thoughts​

Charging an EV is as much about thermal management as it is about electricity.
Every curve on your car’s charging graph reflects a delicate balance between voltage, current, and temperature.
When the system slows down, it’s not wasting time — it’s protecting your investment.

So next time you watch your charging rate drop, remember: that’s your EV working smart, not weak.
It’s cooling, balancing, and safeguarding thousands of tiny cells so that you can enjoy reliable electric driving for years to come.