Public Charging Without Guesswork

What you will be able to do

After this lesson, you will be able to choose the right kind of public charger for the stop you are already making, instead of treating every charger as interchangeable. You will also know why battery level matters to charging speed, so you can avoid the common mistake of arriving too full and then wondering why a rapid charger feels slow.

The idea explained plainly

Public charging gets easier once you stop thinking in terms of fastest charger available and start thinking in terms of best charger for this stop. The useful question is not just, “How powerful is the charger?” It is, “How long am I going to be here, and how much energy do I actually need?”

There are two broad public-charging situations to keep in mind from Lesson 1. AC charging is usually slower and is often found where you already plan to stay for a while, such as shops, car parks, workplaces, or hotels. DC charging is usually used when the stop itself is mainly about getting energy into the car quickly, such as on a motorway or on a longer trip.

That means a slow or moderate charger is not automatically a bad charger. If you will be parked for two hours while shopping, eating, or working, a lower-power charger can be exactly right. If you are stopping only to continue a long journey, a rapid DC charger usually makes more sense.

The second idea is that cars do not charge at one flat speed from empty to full. Charging usually slows as the battery gets fuller. This is often called the charging curve. You do not need to memorise a graph. The practical rule is simpler: the first part of a rapid charge is usually the quickest, and the last part is usually slower.

That is why a car that can accept up to 100 kW DC will not necessarily sit at 100 kW all the way to 100%. It may charge briskly at lower state of charge, then taper off as the battery fills. So if you arrive at a rapid charger with 70% or 80% already in the battery, you may get much less benefit from that expensive, high-power charger than you expected.

For everyday public charging, three plain rules will carry you a long way:

• Match the charger to the stop. If the car will be parked anyway, destination charging can be efficient and low-friction.
• Use rapid charging when time matters. It is most useful on through-journeys or when you need a meaningful top-up quickly.
• Do not chase 100% on a rapid charger unless you truly need it. Often it is quicker overall to add the energy you need and move on.

This fits the approach from Lesson 2. A good charging plan is the one that reliably replaces your driving with the least friction. Public charging works best when it supports the stop you were already making.

Worked example

We will stay with Maya from the earlier lessons. Her car has 60 kWh usable battery capacity, 7 kW AC charging, 100 kW DC charging, and recent efficiency of 4.0 miles per kWh. That means each 10 kWh is worth about 40 miles in similar conditions.

This week, Maya faces three different public-charging situations: a shopping stop, a motorway stop, and a hotel stay. The point is not to find one perfect charger type. The point is to see why each stop calls for a different choice.

Stop 1: shopping for 90 minutes

Maya starts with 55% state of charge. She does not need urgent energy. She is doing a supermarket shop and a few errands and expects to be parked for about an hour and a half. In the car park there are AC chargers and, a short drive away, a rapid DC charger.

Which should she use? The AC charger is the better fit. Her car can take up to 7 kW on AC, so in 90 minutes she could add roughly 10 kWh if the charger delivers at that rate for most of the session. Using her usual efficiency, that is about 40 miles of driving added while she shops.

That may not sound dramatic, but it is useful because the car is parked anyway. She avoids waiting in the car, avoids detouring to the rapid charger, and comes back to a meaningful top-up. The rapid charger would only make sense here if she were unexpectedly low and needed a lot of energy quickly.

Stop 2: motorway break on a longer drive

Two days later, Maya is on a longer motorway journey. She arrives at a service area with 18% state of charge and wants enough energy to continue comfortably to her destination, with a little margin. This is the situation rapid charging is for.

She could stay until 100%, but that is usually not the best use of time. Because charging tends to slow as the battery fills, the time spent going from, say, 80% to 100% may buy fewer practical miles per minute than the earlier part of the session.

Suppose Maya looks at the next leg and decides she only needs to leave with about 70% to finish the journey comfortably. Going from 18% to 70% is a gain of 52% of her usable battery. On a 60 kWh usable battery, that is about 31 kWh. At her recent efficiency, 31 kWh is roughly 124 miles of real-world driving in similar conditions.

That is the key decision. She is not charging to a round number because it feels neat. She is charging to the amount that suits the next leg. If she stayed much longer just to reach 100%, the car would likely be charging more slowly by then, and she would spend extra time for energy she may not need immediately.

Stop 3: overnight hotel stay

At the hotel, there are destination AC chargers in the car park. Maya arrives with 28% state of charge and will be there from evening until the next morning. This is another case where a rapid charger would be the wrong tool. She has time, and the car will be stationary for hours.

Her 7 kW AC charging rate means the car can add energy steadily while she sleeps. Even if she does not calculate the exact number of hours, she can see that an overnight stay is plenty of time to recover a large amount of charge. The practical result is simple: she wakes up with the battery in a much better place for the next day, without spending part of the evening sitting at a rapid charger.

Across these three stops, the best charger changes because the stop changes:

• For the shopping stop, AC destination charging fits because the car is parked anyway and a modest top-up is enough.
• For the motorway stop, rapid DC charging fits because time matters and she needs a substantial amount of energy in a shorter stop.
• For the hotel stay, AC destination charging fits because she has many hours available and does not need speed.

Notice what Maya did not do. She did not choose chargers by label alone. She used battery level, time available, and actual energy need. That is the habit to build.

Common mistakes

• Using a rapid charger for a long parked stop when AC is available. If the car will sit for hours, a destination charger is often the simpler choice.
• Arriving at a rapid charger with a high state of charge and expecting maximum speed. Rapid charging is usually most effective when the battery is lower.
• Charging to 100% by default on a trip. Sometimes you need to, but often it adds waiting time for little practical benefit.
• Ignoring the purpose of the stop. A charger at a hotel, cinema, or supermarket is not trying to solve the same problem as a motorway rapid charger.
• Thinking a slower charger is useless. If your stop is long enough, slower charging can be exactly the right tool.
• Confusing charging power with energy added. A higher kW number tells you potential speed, not how much energy you actually gained over the whole stop.

A calm rule of thumb: if you are stopping because you need energy quickly, look for DC rapid charging. If you are stopping for some other reason anyway, see whether AC destination charging can do the job with less fuss.

Exercise

Use the sample journeys below and match each one to the most suitable public charger type: destination AC or rapid DC. Then write one sentence explaining why.

1. You are meeting a friend for lunch and will be parked in town for about 2 hours. You arrive with 48% state of charge.
2. You are halfway through a long motorway drive and want enough energy to continue after a short break. You arrive with 15% state of charge.
3. Your car will be parked at a hotel from 7 pm until 8 am. You arrive with 35% state of charge.
4. You are doing a 25-minute stop near a main road because you unexpectedly need more range to get home. You arrive with 12% state of charge.
5. You are at a retail park for 75 minutes and only need a modest top-up before tomorrow. You arrive with 62% state of charge.

Now repeat the same exercise using your own routine. Pick three real stops you make in a normal month. For each one, write down:

• how long the car is usually parked
• whether the stop is mainly for charging or for something else
• your likely arrival state of charge
• whether destination AC or rapid DC makes more sense

How to check your result

Your answers are probably sound if they follow these patterns:

• Long parked stops such as hotels, work, or extended shopping usually point to destination AC.
• Short stops on a through-journey, especially at low battery levels, usually point to rapid DC.
• If you chose rapid DC for a stop where the car would already be parked for a long time, ask whether AC would have done the same job with less waiting.
• If you chose destination AC for a stop where you urgently needed a lot of range in a short time, ask whether rapid DC would remove that pressure more reliably.

Suggested matches for the sample journeys:

1. Destination AC, because the car is parked for a useful length of time and the battery is not especially low.
2. Rapid DC, because this is a travel stop where time matters and the battery is low.
3. Destination AC, because the car will be parked overnight.
4. Rapid DC, because the stop is short and the battery is very low.
5. Destination AC, because the stop is long enough for a modest top-up and there is no urgent need for rapid charging.

If your own answers feel uncertain, go back to one question: Am I trying to fill time while the car is parked, or am I trying to add energy quickly so I can continue? That one distinction will solve most public-charging decisions.

Next, we will build on this by looking at what range really looks like once weather, speed, terrain, and heating or cooling start changing the numbers.