battery chemistry

EV charging times, what is happening to my battery?

This article looks at what is going on inside the battery when you charge it, and what that means for you.

Its well known that allot of EV drivers worry about range, these tend to be those that have not owned an EV before. 

Most journeys are only local and the vast majority of EV charging will be done at home. You will plug your car in overnight, just like your phone, laptop etc.  

If you want to do longer trips, charging faster will enable you to travel further in a day or save you  money from the initial purchase by installing  a smaller battery pack.

Charging speeds depend on the type and amount of electricity available at the point of delivery, what charging kit you have in your car and the size of your battery.  There are three main tyoes at the point-of-delivery:

1: Rapid chargers that deliver 43kW AC and 50, 100, 150 or even 350kW DC

2: Fast chargers that deliver 7 or 22kW AC

3: Slow chargers of 3-7kW, includes charging at home (2.3kW) and plugging into street lights (5.5kW)

Your batteries store Direct Current (DC), while the electricity delivered from the national grid is Alternating Current (AC).  This means that before the electricity from the grid can charge up your batteries, it means it needs to be transformed into DC.  AC-DC transformers can be very efficient (>95%), but the large currents and power involved makes even a 5% loss a lot of heat that has to be dealt with. 

In an AC charger, this happens in your car, where the ability to get rid of large amounts of waste heat is limited.  In DC chargers it happens in the charge point, which is better able to reject waste heat, which is why DC chargers can provide more power.  However just because power can be delivered does not mean it can be used.

The speed at which each cell in your battery pack can charge is limited by your batteries chemistry.   For each battery chemistry there is a maximum charge (and discharge) current beyond it will heat up too much and reduce its life.  EV manufacturers have two basic strategies for this:

1: Remove the heat with a cooling system in the batteries, so you can dance closer to the edge

2: Increase the car’s operating voltage: power = volts times amps, so increasing the volts means that

    over a given period you can pour in more power for the same maximum current.

Charging Lithium (Li) batteries is not linear either.  If you fully discharge a Li battery, you will destroy it.  So most cars won’t let you use the last 5-10%.  For most of the charging period – up to about 80% of charge – you are limited by current, so for any given charge current, charging strategies focus on the current/heat balance (current generates heat).  

For the last 10-20%, the battery chemistry starts limiting the current it can accept and as you get closer to the maximum battery voltage (batteries gain voltage as they charge) charging slows down dramatically.  

You can’t force it, to try and maintain the pace, because over-voltage will destroy the battery.   That’s why you often see rapid charging marketed as e.g. “40 minutes up to 80% of full charge”.  The part that is not stated, is that the last 20% can take several hours.

Depending on model, EVs can be recharged to 80% in as little as 20 minutes, though an average new EV would take around an hour on a standard 50 kW rapid charge point. Power from a unit represents the maximum charging speed available, though the car will reduce charging speed as the battery gets closer to full charge. As such, times are quoted for a charge to 80%, after which the charging speed tails off significantly. This maximises charging efficiency and helps protect the battery.

Rapid DC chargers provide power at 50 kW (125A), use either the CHAdeMO or CCS charging standards. These are the most common type of rapid EV charge points currently, having been the standard for the best part of a decade. Both connectors typically charge an EV to 80% in 20 minutes to an hour depending on battery capacity and starting state of charge.

Ultra-Rapid DC chargers provide power at 100 kW or more. These are typically either 100 kW, 150 kW, or 350 kW – though other maximum speeds between these figures are possible. These are the next-generation of rapid charge point, able to keep recharging times down despite battery capacities increasing in newer EVs.

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