Plugging your electric vehicle into a DC fast charger feels like magic – the charging speed is incredibly fast, adding dozens of miles of range in just minutes. But there’s a catch. At some point the speed starts to taper off significantly. What gives?
It’s not the charger. Most modern fast chargers can sustain full power output throughout the charging session. The lag lies within the battery itself. As an EV battery approaches full capacity, its internal resistance rises dramatically. This causes heat build-up inside the battery cells.
The speed reduction above 80% capacity is so dramatic that the last 20% of charging can take as long as the previous 80%. For example, charging an EV from 10% to 80% may take just 20-30 minutes at maximum speed. But tapering effects can prolong the 80% to 100% portion to another 20-30 minutes.
We explain more below.
Low and high resistance
When EV batteries are low on charge, its internal resistance is also low. This allows it to accept a fast incoming charge without overheating. However, as the battery fills up, its resistance increases. Continuing to charge at high speeds would generate excessive heat which can damage battery cells. Therefore, the battery management system puts on the brakes and restricts the charging rate as the battery level gets closer to full.
The optimal daily charge level for lithium-ion batteries in most current EVs is around 80%. Charging to 100% is fine for occasional long trips, but frequent full charges add strain and accelerate capacity loss over time. That’s why many experts recommend plugging in more often but avoiding full top-offs.
Home and public Level 2 chargers operate at lower speeds that are safe for daily full charging if needed. But DC fast charging is a different story – the extreme power levels require throttling as the battery fills and heats.
Charge rate tapering
How quickly does fast charging taper off? It depends on the vehicle model and its battery thermal management capabilities. In older EVs, the slowdown was quite severe, with speeds dropping to a trickle above 80%. Newer models with improved liquid cooling can maintain 100 kW or more to around 80%, before tapering to protect the final 20%.
Tesla’s Supercharger network is the benchmark for taper management. Their proprietary charging algorithm keeps speeds high for as long as possible, often charging at near peak rates to around 90% before ramping down gently. This minimises total charging time while still protecting battery health.
The newest 350 kW fast chargers face a limiting factor – most current EV models aren’t yet designed to accept over 250 kW. So while the charger could output full power, the vehicle throttles the input. We’re still in the early days of ultra-fast charging; battery tech and charge acceptance rates will catch up over the next few years.
In summary, every electric vehicle must balance charging speed with battery longevity. Faster charging generates more internal heat which takes a toll over time.
Of course, if your electric car is charging slowly, it might not be the charger – it’s probably because your battery is cold, or because you have selected the wrong power output on the charger.
So, while modern cooling methods allow sustained speeds to around 80-90%, all EVs must taper power at near full capacity when battery resistance peaks. Smart tapering minimises charge times while protecting battery health, keeping your EV running optimally for many years.