Time is money. With this statement, Benjamin Franklin, the 16th century English writer and philosopher, stressed the importance of using and managing time efficiently, and emphasised the need to use time productively and not waste it. However, time is our currency, and everyone has to assess the value of their time according to their own situation...

Fast battery charging, together with the waiting time at charging stations, is one of the main arguments put forward against electric vehicles compared to traditional refuelling in the case of combustion engines. We are overly anxious about stopping for 30 minutes to rest and recharge the batteries (ours and the car´s) after 400 km of driving, when in reality on most of our long journeys we consume considerably more than this time when we stop, refuel, go to the toilet, have a coffee and carry on. Not to mention the amount of long journeys we make compared to our more everyday journeys where we have the whole night and/or working day to charge our vehicle at the most advisable speeds that will least affect the performance and durability of our batteries. Yes, because in reality, nowadays, we can charge our vehicle batteries at virtually any speed, from a few minutes (2-5) to hours (7-8). So how does that affect batteries, what are the implications, and is it really worth it?

Let´s start at the beginning. Let´s simply remember that Electrical Power (P) is the multiplication of current (I) by voltage (V). P=IV. The voltage of a cell is largely determined by the materials we use to make it, so for all intents and purposes, we can consider it practically invariable. In the same way, the voltage of the battery pack is defined by its design and the series/parallel combination of the different modules and cells that make it up. So, in the same way, we can also consider it invariable. So, the only limitation is the power of the charger we use. The higher the power, the shorter the charging time. However, we have to be aware that if we increase the power and keep the voltage constant, the charging current flowing from the charger to the inside of our battery increases considerably; and this is precisely where the problems start.

Nowadays, we have chargers for all tastes and needs: from domestic chargers below 10 kW, or around 20kW in three-phase; to the ultra-fast 150kW charging that we can find in some stations (and even above). This allows us to charge from a few minutes to hours. We only have to divide the capacity of our battery in kWh by the power of the charger used in kW.

Let´s look at a practical example. A standard battery of around 80 kWh in a mid-range vehicle with a consumption of 20 kWh/100km would allow us to drive around 400 km without refuelling. Suppose we have a choice of three chargers: 10 kW (normal charging); 100 kW (fast charging); and 300 kW (ultra-fast charging). The resulting charging times are 8h, 48 minutes, and 16 minutes for each of the above cases.

To compare charging speeds, a parameter called C-rate is commonly used. It always refers to the generic capacity of a given battery. A C-rate of 1 means that we fully charge the battery in 1h (regardless of the kWh or Ah it is capable of storing); with a C-rate of 2 we would charge the battery in 30 min; and with a C-rate of 0.5 in 2h. If we change the total or nominal capacity of the battery, the times and C-rates remain the same by definition, although logically we are changing the charging current. That is, a 10 kWh battery and a 100 kWh battery, at a charge rate of 1C we charge them in the same time of 1h, which implies that the current we use in the second case is 10 times higher than in the first case.

For our example above, the C-rates corresponding to the different charging speeds chosen would be C/8 (0.12C); 1.25C; and 3.75C. Very important now: What happens as we increase the C-rate or charging current (i.e. the power of the charger we use)?

First:

• The price we pay for charging is NOT the same. For low-power domestic chargers, the supplier´s price is around €0.15/kWh; while for fast-chargers it can be around €0.35/kWh; and around €0.5/kWh for ultra-fast charging. With this, recharging the battery costs €12 in the first case; €28 in the second, and €40 in the third. WARNING! For the defenders of combustion engines: if we consider an average petrol price of €1.5/l, and a consumption of an equivalent vehicle of around 6 l/100 km, we are comparing a price of €3 per 100 km in the first case, compared to €9 per 100 km for our combustion car.

Second:

• Although it is difficult to understand at the user level; due to the working principles of batteries, the intrinsic properties of the materials used and the limits in the dynamics of the charge transfer processes, the capacity that a battery can absorb decreases as we increase the charging speed. This means that if we charge at low speeds we will effectively have the 80 kWh available in our battery for kilometres. If we charge at higher speeds, this kWh decreases considerably. Although it is difficult to quantify exactly, looking in detail at the discharge curves of some cells as an example, it can be seen that we lose approximately 20% of capacity going from the first to the second case (from 0.12C to 1.25C; and around 40% in the third (from 0.12C to 3.75C). So we will no longer be able to travel the initial 400 km, but 320 km in the second case, and 240 km in the third. So, not only is the recharging process more expensive, but also with that recharge we will be able to do much less kms, and we will have to stop and recharge earlier. It should also be noted that this is not too much of a concern from a battery maintenance point of view, as under normal conditions we will recover the nominal capacity of the battery as soon as we recharge again at slow speeds.

Third:

• The number of charge and discharge cycles of a battery, or in other words, the lifetime of the batteries, depends greatly on the charge and discharge rate we use regularly. Closely related to the aspects and limitations mentioned in the previous paragraph, if the charging current used, or in other words, the amount of electrons that we introduce into our battery, is higher than the limit that our materials can manage, we will not only be affecting the battery´s specific capacity, but we will also be damaging, in this case irremediably and irreversibly, the battery´s operation and its real long-term charging capacity. As an analogy that helps us to interpret this fact, we can think of a traffic road totally collapsed by a number of vehicles greater than the number that can circulate in an orderly and efficient manner; or the damage caused by flooding due to not being able to cope with the excessive flow generated by the infrastructure or channels available and the consequent overflowing of riverbeds and rivers.

Again, it is difficult to give exact numbers. In any case, if for well-established chemistries we reach what is considered the 80% load retention limit with respect to the initial one, after around 3000 cycles loading and unloading at 1C; these can decrease to 1500 cycles loading at 2C and 800 cycles at 4C. Although the difference is considerable, again, for lovers of combustion engines, in the worst case we would be talking about a battery life of around 250,000 km; while in an average case we would be talking about 800,000 km...

In short, fast charging is now possible, but it is also more expensive (more €/kWh), forces you to stop and recharge more frequently (more €/km), and negatively affects battery life. So now, is time still money - which is more valuable to you, your time or your money? Clearly, there is no right answer, every situation is different, and needs may change depending on the moment. What is really important is that we have the tools to make the best decision at all times.