CHARGING BASICS 107: Reason why some rapid chargers cutoff at 90% SoC | Is it to prevent battery life degradation?

You might have noticed that a few DC rapid chargers cut-off their charging at 95%; some even at 90%. Why do they so? Will the electric vehicles’ battery damage if you let them to rapid charge up to 100%?.

The simple answer is : No, not all! It is just a speculation amongst EV drivers that this limit has been set because it might be bad for the battery to charge it up to 100 percent. 

Then why do some rapid chargers charge only up to 90% of the state of battery and cut-off? This article may help you to understand the reasoning behind.

The reason is common sense & business; not technical!

When you connect an electric vehicle at a rapid charger, it will usually takes on a high charging power up to about 70-80%; after which the charging power starts to drastically taper-off to lower levels, as low as a normal destination AC charging station at home/office.

So, even if you’ve connected to a rapid charger, the EV will charge only at around 20 kW speed after certain percentage of battery SoC (state-of-battery). So some intelligent charge point operators cut-off power at these limits, so that they can free-up the charger for someone else who really need rapid charging.

For example, let’s take a look at the charging curve of Audi e-tron Quattro 50 (P.C: Fastned).

 

In the charge curve above, you can see that the EV charges around 150 kW until it reaches 80% of battery SoC (within 20 minutes); from that point the charging speed drastically reduces and takes approximately another 15 minutes to reach 100% state of charge.

It takes about the same amount of time to charge from empty to 80% SOC than it does to charge from 80 to 100% SOC. With that, the EV driver might as well use a destination charge point after this point. Consequently it doesn’t make sense to continue a charge all the way to a 100% because it's a waste of everyone's time including anyone else waiting to use the rapid charger and crucially yours.

It's also worth highlighting that plug-in hybrids (PHEVs) capable of rapid charging such as Mitsubishi Outlander therefore should only stick to destination charge points, thus freeing up rapid chargers for EV drivers in a hurry.

What happens if you still charge upto 100%? Won’t it affect the battery?

Theoretically speaking, yes,  it is never a good idea to charge any battery up to 100%, as it may impact the life of the battery. Lithium-ion batteries do not like being held at high voltages for long periods of time so this will accelerate their degradation; and I would always recommend 80% as the maximum range.

However, the modern battery management systems (BMS) in the newer electric vehicles do a trick to handle this issue. Most of the modern EVs have SoC buffers at high & lower limits, whereby they keep some of the cell's true capacity in reserve and never use the upper SOC buffer to prevent the cells from being overcharged; and the lower SOC buffer prevents degradation due to over discharge.
 

So, when your vehicle’s dashboard tells you that it’s fully charged as 100%, the reality is that the battery pack is probably somewhere in between only 80-90%; same is the case for lower margins as well, when the dashboard shows 0% battery, the actual SoC of the battery would be around 10-20%. Below pictures may explain it well. 

But it is important to note that these upper & lower SoC buffer are not always the same; it changes with the life of the battery. This is the main reason why should not go upto 100%, because in an old EV, when the dashboard shows 100%, the actual SoC could also be close to full – which is not good for the battery. 

Summary:

Charge point operators who stops charging at around 90% are not to be blamed; but to be appreciated. In fact, I would even go further by adding the below additional restrictions.

• further lower the rapid charge limit to 85% or 90%.
• introduce over stay penalties for anyone who leaves their car plugged in when is finished charging
• ban plug-in hybrids (PHEV) from using rapid chargers since they can't make anywhere near full use of them.
• In addition, I'd have at least one destination post at any site containing a rapid charger which could be used by PHEVs, small BEVs or EVs that genuinely need to charge to 100% SOC.

Charging infrastructure needs are constantly growing; and is impractical to keep adding rapid chargers; If we (EV drivers) adhere to basic rules such as above, everyone can utilize the available rapid charger infrastructure efficiently.