Recharging a battery pack is kind of like pouring water into a bottle. The time required depends on how strong is the stream of water filling the bottle. A weak dribble fills the bottle more slowly than does a torrent.

For battery recharging we can equate these as:

• The size of the water bottle equates to battery pack capacity, and the capacity is measured in kiloWatt-hours
• The strength of the stream equates to the charging rate, which is measured in kiloWatts

We talked about these units in How can we reduce the charging time of electric cars?.

A typical charging rate is 6 kiloWatts. An hour of charging at 6 kiloWatts is therefore 6 kiloWatt-hours.

At the risk of oversimplification, the time to recharge is calculated by dividing the number of kiloWatt-hours required by the charging rate. More specifically:

kiloWatt-hours-required = battery capacity - current capacity
time-required = kWh-required / charge rate (kW)

If the car required 30 kiloWatt-hours, and was charging at the 6 kiloWatt charging rate I mentioned earlier, the charging time is (oversimplified) five hours.

Checking into the details you find the charging rate varies throughout the charging process. What follows is a few ways that will lengthen the charging time.

The car might throttle the charging rate to avoid damaging the battery pack

Some electric cars purposely throttle (reduce) the charging rate in certain conditions. Fast charging tends to heat up the battery pack, and heat is known to cause damage to battery packs. Some car makers design the battery management system to reduce the charging rate if the battery pack temperature is too high. That reduction is meant to bring the temperature down.

Unfortunately the road trip scenario can easily trigger this situation.

On a road trip you're driving for as long as possible, then charging as quickly as possible, and get back on the road as quickly as possible. The goal is covering as many miles as possible. But - driving the car causes the battery pack to heat up, and fast charging (in most cases) causes the battery pack to heat up, and getting back on the road as quickly as possible does not give the battery pack any time to cool down.

Try not to get frustrated or angry if your car throttles the charging rate. The manufacturer designed-in that feature to lengthen the usable life of the battery pack.

The charging rate naturally tapers off as the battery pack fills up

Even if the car does not throttle the charging rate, the charging rate naturally decreases as the battery pack nears full.

For an analogy consider an empty movie theater that's about to show a movie. As long as there are plenty of empty seats, people can readily find the seat they want. But as the theater fills up they have to start searching around to find any seat. That search takes some time, and might even require asking folks who've already sat down to shift positions to accommodate the new people.

The electrons flooding into the battery pack need a place - an atom - to sit down. As the pack fills up, the electrons have a harder time finding an atom to sit in. Electrons are then jostling around, causing heat and inefficiency.

As the car is charging try to find a charging rate indicator. Watch the number and see that it starts falling when the pack nears full.

This issue is important when trying to optimize travel time on a road trip. That last 5% can take a very long time to charge, and most of us say it's not worth the time to wait. Instead it's more practical to stop charging when you have enough charge to make it to the next station (PLUS A SAFETY CUSHION).

Centralized control systems might adjust the charging rate depending on the condition of the electrical grid

As more electric cars get on the road, the electric grid operators will start to get nervous about the impact electric car charging has on the grid. So far there haven't been enough electric cars to make any impact. But if (when) EV's become a significant part of the car fleet, they'll make more and more of an impact.

Plans are underway for electric car charging -- and other types of electrical consumption -- to be centrally managed. If this makes your libertarian tendencies to raise their hackles, calm down, take a deep breath, and read the coming bit carefully.

The impetus behind this has to do with increasing deployment of solar and wind energy resources. These are intermittent resources, and for renewable energy to live up to the role we all want it to play there must be large scale energy storage systems.

In some areas, like California, there is so much solar on the grid that it's almost too much during the day.

Energy system regulators are looking at ways to orchestrate energy production, energy storage, and energy consumption. The goal is to time shift excess solar energy so it can be consumed at night, and to time shift some energy consumption to times when there is excess energy supply.

Electric cars, and their battery packs, can play a big role in this picture. As can stationary grid storage battery packs.

Also, electric car charging will become a significant load on the grid. Just as for every other significant load, the grid operators want a way to throttle (at times) or increase (at other times) the consumption by that significant load.

None of this has been implemented currently, except for pilot projects. But in several years expect this to become A Thing, not just for electric cars, but for pool pumps, air conditioning systems, and more.

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