Factors Limiting EV Battery Acceptance Rate on DC Fast Chargers

Question

There is a bunch of, relatively speaking, inexpensive EVs with 200-250 mile (320-400 kM) range -- e.g., the Kia Niro, Subaru Solterra, Volvo C40, and late model Nissan Leafs. All, in my experience have enough range for day-to-day use (40 mile daily commute, 80 miles airport runs) without "range anxiety", they even work well enough for weekend trips out towards the limit of their range with overnight charging on a level 2 charger.

Where they fall short for my use cases is for road trips in the 600-800 mile range where you're going to need two or three charges -- at least for the two we've settled on (the Niro and Solterra) which have relatively low acceptance rates (77 kW for the Niro and 100 kW for the Solterra) which means that those trips involve three or four hours of charging time (especially in the winter).

So this got me wondering, some of Tesla's Superchargers are capable of 250 or even 350 kW (presumably because some models can accept a charge at these kind of rates) adding 200ish miles of range in around 15 minutes (going between a 20 and 80 percent charge) so it seems that if a Niro could accept a charge at a similar rate five to 10 minutes of charging would add a significant amount of range. This would (for me at least) completely change the experience of a road trip in one of these cars -- and would seem to make for a very nice range vs. charge time vs. battery cost in "moderate priced" EVs.

What determines/limits the DC charging rate of EVs? Are there significant hardware costs that increase as you support higher charging rates -- therefore undercutting the cost savings that come from smaller battery packs?

Answer 1

Generically speaking, higher charge rates will heat a pack more quickly than the lower rates. Tesla models are designed and constructed with active cooling as well as appropriate monitoring and charge control, which allows the higher rates. This cooling system is one aspect of the cost. The electronics which permit the faster charge rates are likely another, as it's effectively a second system within the vehicle, the typical Level 2 on-board charging unit being the first system.

If one finds a current model EV with fast-charge capability, it is an indication that the manufacturer has provided for increased cooling protection.

I've noted that for all the EVs on today's market, those with fast-charge capability also have cautions regarding frequency of the practice. Even with protection, cooking your battery by pumping hundreds of amperes into it will reduce its lifespan.

A comment posted below recognizes that Level 2 charging is accomplished by AC/DC converting/charging equipment installed in the EV, while fast charge is done via high voltage DC supplied by an external charging device. It's an important distinction when considering cost versus capability.

Answer 2

One of the issues you face is that the battery capacity is itself one of the limiting factors for the power a battery can take. This is typically referred to as C-rating. That means you can charge a battery with a certain factor of the capacity as power input. The higher that factor is, the more degradation you get from your battery and thus your expected lifetime of the battery drops.

How high you can push that factor depends on multiple factors, some of which are environment dependent (like temperature) but a main contributor is the chemistry of the battery. There are chemistries which are rated for higher C-rates than others. And of course there is always a tradeoff with other design factors - typically battery chemistries with higher C-rates come with a lower energy density, so you need a physically larger battery pack for the same amount of energy.

For example a battery pack with 40 kWh and a rating of 3 C would accept a maximum charging power of 120 kW. If you have the same battery but with 70 kWh capacity, you could charge it with 210 kW.

Then you also run into the issue that the current is not unlimited even if the power limit is not reached, which means you need a high battery voltage to achieve high charging rates. For example if the charger is limited to 400 A, and the battery voltage is 300 V, you cannot get more than 120 kW into the battery, because the charger limits the current. If you have a 800 V battery, you could get 320 kW into the battery (in theory).

In practice the most difficult part seems to be the temperature handling. If the battery pack temperature is too low, you can't charge it very quickly, if it is too high, you can't charge it very quickly. So a good thermal management is key to get consistent high charging rates.

Answer 3

You have to remember that charging curves don't stay at the maximum charging power. The fall off pretty linearly towards 80% state of charge.

So, consider for example Toyota bZ4X which is available in United States with a Panasonic Japanese battery FWD model (150 kW rate) or with a CATL Chinese battery AWD model (100 kW rate), but in luckier countries such as Finland even the AWD model has a Panasonic battery.

100 kW doesn't sound so bad, right? Typically you charge from 10% to 80% when using fast charging, so the approximately 70 kWh battery pack would need 49 kWh of energy. With 100 kW, that's half an hour, right?

Not so fast! The 100 kW rate is only for a practically empty battery with around 10% of capacity remaining. It reduces pretty linearly from 100 kW to nearly 0 kW towards reaching 80% state of charge.

So the charging time you thought to be half an hour is actually about an hour instead!

There are several factors affecting the acceptance rate:

• Maximum charging rate of the battery specified by the battery manufacturer or automaker
• State of charge, the charging rate drops pretty linearly between 10% and 80%, and above 80% it stays on such a low level that you really want to charge above 80% only overnight or during your work day.
• Battery temperature: when too cold, energy is first mainly used to heat the battery and charge at very slow rate, after the battery has heated up, energy is fully used to charge the battery. Some cars have a battery priming mode: once you set the destination charger at satnav, the car is intelligent enough to start using battery energy to heat up the battery at the optimal moment before reaching the charger, which slightly reduces your range but can cut charging time in half if it's really cold outdoors.
• Battery cooling, which can limit charging time but can also extend battery lifetime if it's often fast charged. Typically new cars have liquid cooling. Some old cars like Nissan Leaf don't have liquid cooling, and they have turned out to have worn-out batteries really often. So Nissan Leafs are cheap second-hand but that doesn't mean you should buy one. It probably needs a new battery.

Oh, and the obvious: the charger also may have its power limitations, and in some cases the power is shared between different ports. So a two-outlet charger with one car connected may have twice the power than with two cars connected.

Answer 4

To answer your question

The battery size (kWH storage) and intake rate of charge (kWH) are intertwined.

Because when you are injecting 100kW (340,000 BTU/hr) into anything, the limiting factor is the ability to cool it.

The capacity of coolant isn't the issue. It is the surface area you can create between the coolant and the battery. For instance, in Tesla's packs, they have a cooling pipe that slithers between pairs of cell rows in the pack, giving each cell 1 side in contact with coolant. You could do that differently giving 2 sides of contact, but the cooling pipe would be longer and the battery bigger and heavier (weight of coolant).

Given a basically similar cooling method, the cooling capacity ties to battery pack size. Joe builds a homebrew EV with eight Tesla Model S modules. Kim builds a homebrew EV with sixteen Model S modules. Kim can get twice as much heat out of that pack because the pack is twice the size. As such, Kim can charge twice as fast (in kWH) at the level 3 fast charger. However Kim's car is a Chevy Suburban, whereas Joe's is a Geo Metro. So kWH doesn't necessarily map to miles of range.

Even more, the higher the rate of charge for a given battery, the hotter the pack gets - and the less efficient the charging is. And the harder that beats on the battery pack's longevity. Your automaker has to make such design decisions, and may administratively limit charge rate to ensure a long pack life.

Since charge intake rate can vary so much by design of pack, size of pack and risk calculus of management, there really is no substitute except the published data from the manufacturer or competent testers.

Answer 5

The acceptance rate is completely within the charging unit of the vehicle itself. On board there is a converter which changes the voltage from AC to DC so the battery can be recharged. It is this converter which determines the rate at which a battery can be recharged. This article by Powerlink Systems fairly much says it:

Electric vehicles have a charger on-board the vehicle that converts the alternating current (AC) delivered by the EV charging station to direct current (DC) so that the battery pack can be recharged. On-board chargers have a maximum power acceptance rate, measured in kilowatts (kW), that determines how fast the battery can accept electricity while recharging.

Clipper Creek further divides it into three areas:

1. Vehicle Acceptance Rate (aka the car’s charger, in kW)
2. Vehicle Battery Capacity (in kWh)
3. Charging Station Delivery Rate (aka the stations’ max output capacity, in kW)