It’s true that the arrival of electric cars has forced us to learn a lot of new terms and technologies. Whereas before we had to talk about octane, compression, injection or cubic centimeters, today we have to deal with very different aspects of a car’s electrical system. And once we’ve learned the basics, we have to learn even more complex aspects. For example, do you know what a deep discharge is and how it can affect your electric car?

What is a deep discharge?

Deep discharge refers to the situation in which an electric vehicle battery is discharged to significantly low capacity levels, exposing it to stresses and challenges that go beyond a simple momentary loss of power.

From a technical point of view, deep discharge implies that the battery has been discharged to a very low level, typically less than 20% of its rated capacity. This goes beyond the superficial discharge that occurs during normal use.

When a battery undergoes deep discharge, internal stress and electrochemical processes can trigger negative reactions, such as dendrite formation, loss of capacity and, in extreme cases, irreversible degradation.

Battery fuel loss

A lithium battery never depletes the lithium it contains, so lithium can be removed and recycled almost 100% when a battery has reached the end of its useful life. In reality, the battery’s “fuel” is not so much the lithium it contains as the lithium’s ability to move its ions from one end of the battery to the other.

When this happens, the “counterpart” of the ions inside the battery, the electrons, move into whatever is connected to the battery, powering it electrically.

To oversimplify, the amount of positive ions transferred inside the battery determines the amount of energy supplied to the connected device.

No more battery percentage forever

If a battery doesn’t lose lithium, what does it lose? An inner layer separates the two parts of a battery. This layer allows only positively charged ions to pass through. To oversimplify, it acts as a “filter”.

If this layer deteriorates, it will only “filter” positively-charged ions. And since the amount of energy supplied by the battery depends on the positively-charged ions transferred, the battery’s “capacity” decreases. This is despite the fact that the amount of lithium in the battery has not changed.

The proper functioning of a battery depends entirely on the separation of the two parts of the battery, anode and cathode, by this filter layer. And the dendrites that form when the battery is pushed to the limit are small damages that form in the separating layer.

If the battery has a theoretical capacity of 10 kWh to start with (example), we can lose the maximum capacity and over time we’re left with 8 kWh, 6 kWh or 4 kWh. When we reach almost half this capacity, we can say that the battery’s life is over, at least as far as an electric car battery is concerned.

Deep discharge damages the battery’s chemical structure

The effect of deep discharge is to subject the battery to almost limitless use. This is where the battery’s chemical structure is damaged. The layer separating the two parts of the battery, the anode and the cathode, is damaged.

The more a car battery is subjected to deep discharges, the more the layer that “filters” the passage of positively charged ions from one end of the battery to the other deteriorates.

A single deep discharge won’t have a noticeable effect on battery life, but constantly approaching 0% of the battery has a detrimental effect in the medium to long term. Similar effects can occur with rapid charging, or when the car is 100% charged and still supplying power (this phenomenon is automatically regulated by the car’s chargers and systems, which switch the charging system on and off).

How to avoid deep discharges?

If we compare the battery of an electric car to the fuel tank of a car with a combustion engine, avoiding deep discharge would be very similar to avoiding going into reserve. A good recommendation is not to exceed 20% of your car’s battery capacity.

In general, the electric car manufacturer gives indications of the minimum recommended charge for the vehicle’s battery in each case. This is somewhat related to the so-called depth of discharge, although it’s not strictly the same thing.

Logically, if we have to drive 300 kilometers and this is the vehicle’s maximum range, we shouldn’t stop to recharge the car.

But it’s worth bearing this in mind when we stop to recharge the battery.

If we have to make several stops, we can plan them so that we don’t rush the vehicle’s charge to the maximum, but stop to recharge when we have the last 20% left. Most of the time, we’ll make the same number of stops.

And if we were to use the battery very intensively once, it wouldn’t be a big problem. But it’s a good idea to avoid this to preserve the health of the electric car’s battery.