Published on February 14th, 2016 | by Christopher Arcus


Battery Charging Explained — Or… Honey, Who Shrunk The Electrons?

February 14th, 2016 by  

LEAF Model SWhich charges faster, a Tesla Model S or a Nissan Leaf? Get out your calculators. Ready, set, go.

All batteries are described by their ability to deliver current and voltage. Open circuit voltage is the voltage batteries produce when not connected to a load or to a really small load like a tiny flashlight bulb. All batteries have internal resistance that limits their ability to deliver current. If the current is too high, the voltage drop across the resistance becomes so high that current cannot increase. The resistance makes batteries heat up and become inefficient, and their life is reduced. The internal resistance is determined by battery characteristics and often by the chemistry and type of battery.

Mom Tesla & LEAFBatteries are also described by their amp-hour rating. The amp-hour rating describes how may amps of current they can carry for how long. Multiply by the voltage and you get amp volt hours, also known as watt-hours (Wh). That’s a unit of energy familiar to many of us, as the typical electric utility bill is measured in kilowatt-hours, kWh. A 1 amp hour battery could deliver 1 amp (A) for an hour. But only if its charge rate, C, allowed.

What’s C?

The guy who invented this charge rate concept and created C was Charles Coulomb. He seemed to like the letter C. Let’s take that 1A-hour (Ahr) battery. If it had a C rate of 1, it could charge at 1A for an hour, the same as its amp-hour rating. If it had a C rating of 2, then it could deliver 2A in a half hour to deliver 1Ahr. But really, it means it could deliver 2A for any time up to a half hour. So, I can combine the information from C and Ahr ratings to determine the maximum current.

What Does that Mean for Charging?

Let’s look at some cases to try to understand this. Suppose I have a tiny AA battery and it’s rated at 1C. Its amp-hour rating is probably reduced to milliamp-hours or less. But it’s still milliamp-hours. So time is built into the idea. A C rate of 1 for a 1milliamp-hour battery is still 1 hour. So, the current might be a lot lower, but the charge rate for any 1C battery, regardless of capacity, is the same, one hour.

But How Does this Apply to Different EVs?

If it didn’t depend on charge rate, we could just divide the battery capacity (in kWh) by the charger power (in kW) and wind up with hours. With C rate, we have to look at charger power, battery capacity, and C rate to see which limits charge rate.

Let’s say we have a 10kWh battery pack and a 20kWh battery pack. Both have the same C rate, one hour. As long as my charger can deliver it, they both charge in the same time. But what if my charger is only 10kW? Then the 20kWh battery pack can only be charged at 10kW, so it gets 10kW in the first hour and takes 2 hours to charge to 20kWh. In that case, the charger limits the battery to less than its full C rate, 1 hour. So, charge rate is determined by the charger or the battery, depending on which limits charge rate first.

So, we really can’t tell how fast a pack charges until we know three things; one, the charger rating in kW; two, the battery charge rate; and three, the battery capacity in kWh. But if the charger is always big enough, the charge rate is just limited by the C rate. If two batteries have the same charge rate, and you have a big enough charger, they charge at the same rate. The bigger battery will just charge at a higher current, but take the same amount of time to charge.

But, actually, there’s more. The actual time to charge a battery depends on how much it’s discharged. We just assumed we had a fully discharged battery and we charged it from fully discharged to fully charged. But that’s a horse of a different Coulomb.

Images © Cynthia Shahan

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About the Author

has studied wind, electric vehicles, and environmental issues. An electrical engineer familiar with power and electronics, he has participated in the Automotive X Prize contest. He is an avid writer, specializing in electric vehicles, batteries, and wind energy.

  • Jenny Sommer

    Is the C rating still a bottleneck? A supercharger delivers ~115-120kW. Is that 2-1.3C?
    My LiPo packs I use for flying can deliver 20-50C and are usually safe to charge up to 5C (says so on the label.)
    I’d expect a full charge in 20min…that’s 3C.

  • Ivor O’Connor

    Real data, empirical data, shows just how different charging times are from theory and marketing hype. For instance regardless of what Tesla says the following is what the superchargers really do:

    Supercharge times for each 5%. This applies to 60,70, 85 and 90 kWh batteries. They all supercharge to same percentage at the same time.

    | 2.8 | 0%-5% | 02 min 48 sec |
    | 2.0 | 5%-10% | 02 min 00 sec |
    | 2.1 | 10%-15% | 02 min 06 sec |
    | 2.3 | 15%-20% | 02 min 18 sec |
    | 2.4 | 20%-25% | 02 min 24 sec |
    | 2.6 | 25%-30% | 02 min 36 sec |
    | 2.7 | 30%-35% | 02 min 42 sec |
    | 3.0 | 35%-40% | 03 min 00 sec |
    | 3.1 | 40%-45% | 03 min 06 sec |
    | 3.3 | 45%-50% | 03 min 18 sec |
    | 3.6 | 50%-55% | 03 min 36 sec |
    | 3.9 | 55%-60% | 03 min 54 sec |
    | 4.1 | 60%-65% | 04 min 06 sec |
    | 4.5 | 65%-70% | 04 min 30 sec |
    | 4.9 | 70%-75% | 04 min 54 sec |
    | 5.4 | 75%-80% | 05 min 24 sec |
    | 6.2 | 80%-85% | 06 min 12 sec |
    | 7.3 | 85%-90% | 07 min 18 sec |
    | 9.9 | 90%-95% | 09 min 54 sec |
    | 32.9 | 95%-100% | 32 min 54 sec |

    • John Norris

      Thanks Ivor. So, 53 mins to charge a Tesla to 80%?

      • Ivor O’Connor

        Yes it takes 53 minutes to charge to 80% if you start at 0%. The Tesla website claims it takes 40 minutes to get to 80% but they assume the battery started at 10%. Maybe they used special laboratory batteries because as you can see it actually takes 48 minutes. They probably used those same special laboratory batteries to get the 170 miles in as little as 30 minutes number too.

        • Carl Raymond S

          Thanks Ivor, I had accepted the published figures as fact, and it appears that real world isn’t so fabulous.

          I’m a little disappointed, as it means the ICE brigade will still be able to throw corner cases at us where FF rules the day.

          On the other hand, we have to remember that most road trips will begin with a full charge and end with just a little charge. So that fraction of time spent charging 1/7, 1/5, 1/4 depends on whether it’s a one, two or three charge journey. For most, EV still rules on those 363 days where we don’t travel more than 500km.

          • Ivor O’Connor

            There is always a little hype with everything. Being a Tesla fanboi though I like to keep things as accurate as possible so I’m never caught unaware by the FF crowd.

        • neroden

          It actually depends on which Supercharger you’re at. I’m betting those numbers are from the 90 kw Supercharger. That was advertised as 40 minutes 10%->80% but does run a bit slower so more like 48 minutes.

          The 120 kw Supercharger (which are being installed in most new installations where the local power grid is up to it) is, of course, faster.

          • Ivor O’Connor

            You’ll notice they discovered the superchargers charge, regardless of pack size, at the same speeds. So regardless of whether it is a 60, 70, 85, or 90 kWh pack the speeds are determined by the cell chemistry making up the pack. When they get, hopefully soon, 150 kWh packs they’ll need that 120 kW to get the pack from 5 to 10% in 2 minutes but it’s not likely to make the charging a second faster.

            On the other hand with larger packs they’ll be able to push in more mileage faster. So a 150 kW pack might get in 150% more power, mileage, in the same amount of time. However every 5% increment shown above will probably take exactly the same amount of time as it had before.

          • eveee

            There is a reason for that. C rate. As long as the charger is powerful enough and allows it, the battery pack determines the charge rate.
            The 150kwhr pack couldn’t charge faster than the 130kw fast charger allows. A full charge would take longer than an hour.

            Teslas have a charge rate of of about 0.8 to 1.2, and it varies.

            If they all charged to 80% in 40 minutes, their charge rate would be limited to 1/(40/60 x 1/0.8) = 1.2

            Notice, I said limited to. The battery pack and battery management system (BMS) talk to the charger. If the battery was overheated, the charger and BMS are supposed to say stop charging.

            The 60kwhr battery limits the charger to charge it at a lower current than than the 90wkhr battery.


            Charge rate slows way down at over 95%, in part because the BMS will be doing cell balancing.

          • Ivor O’Connor

            To paraphrase and simplify what you just said: The SCs are more than enough for the current packs Tesla uses. The BMS, Battery Monitoring System, which tells the SC how much energy is to be sent is the limiting factor. For instance it always tells the SC to deliver 5% regardless of pack size in 2 minutes when going from 5 to 10% charge. Then tells, regardless of pack size, to deliver the same energy but at half speed between the 60 to 65% level. The BMS ensures the SC delivers based on the current pack charge.

          • eveee

            Yep. Exactly. It doesn’t just happen. Its designed to control the charge rate.
            Heres another tidbit. With Teslas solid battery thermal management system and cooling system, the battery can be cooled or heated every time it charges. That really helps battery life and performance tremendously.
            I knew a guy with one of the original S-10 pickups built with the EV-1 drivetrain. When it charged, it would turn on the air conditioning system to maintain the battery temperature, if it required it. You can do that with a well designed system. And it really helps a lot. The S-10 battery was NiMH.
            Those were the days.

          • Bob_Wallace

            That’s my question. Ivor has time data. Times based on which SC version?

            A charger that puts out 33% more juice could cut times significantly.

        • Bob_Wallace

          This is copied from the Tesla Supercharger web page.

          “Model S is currently the only EV capable of charging at up to 120 kW, which equates to 170 miles of range in about 30 minutes. There are many factors that affect the actual charge rate, including ambient temperature, utility grid restrictions and charging traffic, amongst others.”

          “Charging from 10% to 80% is quick and typically provides ample range to travel between most Superchargers. Charging from 80% to 100% doubles the charge time because the car must reduce current to top off cells. Actual charge times may vary.”

          • Ivor O’Connor

            Yes, I’ve been using those numbers from the Tesla site for a very long time. So it was nice to see crowd sourced empirical data showing far more accuracy.

      • omar


    • eveee

      48 minutes 10% to 80%. Shouldn’t really discharge to zero. Bad for the battery. 30 minutes only gets you to 55% from zero.
      170 miles is 64.15% of 265. This depends on Pack size. Smaller sizes could charge faster if not internally limited.
      Taking it from 10% to 74% in 33 minutes.

      • Ivor O’Connor

        Unless the Tesla SC is limited in power that’s not true. (Which happens in many SC stations when another car is charging in the adjacent A/B stall.) The BMS limits the cells equally whether there are 4 or 7,000 cells.

        So actually just the opposite is true. Larger packs allow a 170 mile charge faster than smaller packs.

        • eveee

          Could be. I just don’t know what Tesla sets its limited stock at. I was just going through the scenarios to see how the range fit vs charge.

        • eveee

          Yes. If you keep it at the highest discharges and tesla probably yanks up the controller charge rate. But as packs reach 90kwhr and beyond, the charger power is gonna have to increase to allow it. Think I read somewhere they planned to do exactly that.

  • Carl Raymond S

    All too hard for the average Joe, who just wants to know how long it will take for a 500km road trip.

    The most meaningful number, is km of range added to the vehicle per minute of wait time. Tesla have shown that people are happy to spend 1/7th of a journey in stop mode – and have provided technology which allows that. Anything less (e.g. Bolt) – and you have to ask “Why?”.

    • Jamset

      You can tell the average person “you can charge the battery to 80% in 30 mins and the remaining 20% will take another 30 mins”.

      • Carl Raymond S

        I was thinking about a potential EV buyer who wants to compare EV A with EV B.

        You are right, so long as Joe also knows how far he can safely travel on an 80% charge – so just a little extra math.

        The size of the battery in kWh helps, but doesn’t give the full picture as different cars have different efficiencies in terms of km/kWh.

        Hence my original suggestion – publish the charge rate in number of km of range added per minute – at say, the 50% charged mark.

        We can safely assume that specs for cars with the best charge rate will make this clear, while the remainder will bury it in detail.

        • Jamset

          With LCD screens you can display what you want.

          Km/kWh. kWh/hour. % charge.

          On my computer I can display the date as DD/MM/Year or Year/MM/DD.

          • Carl Raymond S

            If Joe is sitting in the car, he’s either purchased it already, or test driving it. I was thinking about Joe sitting at home, browsing the web, researching which car would be the best fit for his family.
            For this, we need all makers to quote the same charge rate statistics, and it’s already apparent that there’s little hope for that – unless it’s legislated.
            If it is legislated, we want the legislation to insist upon a number that is meaningful i.e. number of km of range added per minute when half charged.

          • Jamset

            We need a law to get batteries to allow a setting to automatically stop charging it beyond 80% as it is bad for cycle life.

            Of course if someone wants to charge it to 100% then there should be an option to change the setting.

          • loaddown

            According to my ScanGauge, my 2012 Prius C shows a full drive battery (on the car display) with only 70% actual State of Charge. You will probably find similar protection in other electrified vehicles.
            I am not sure if there is some hack to strech that factor.

        • Bob_Wallace

          They can bury that information in their publications but there will lots of places online where people can easily compare EVs.

          We should expect to see site where one can enter their average annual mileage. How often they drive >200, >350, >500 miles a day during a year. What they pay for electricity during the hours they typically park. What they expect the price of fuel will be over the next X years.

          Plug in the numbers and see how different EVs compare.

          • loaddown

            When you find that site, please post it; I am looking for the same evaluator.

        • Dennis Mathias

          Well, at this point this point all this is kind of moot since no one has the capacity or charge rate of the latest Teslas. They own that. And with their superchargers those seal the deal.

          • Carl Raymond S

            That is the situation now, comparing say Leaf to Model S, and it’s well known to regular readers of CleanTechnica. I’m still thinking about Joe Average, trying to quickly size up a Bolt versus a Model 3 versus whatever else appears on the market.
            And I’m still trying to figure out exactly what it is that Tesla ‘own’. Is it really a secret sauce, or just the will to go there?

          • neroden

            What Tesla “owns” is mostly the will to do it — and now, the fact that they *have* done it.

            Anyone could have thermally managed their battery. Tesla did from the start, Nissan didn’t. Tesla now has 8 years of debugging of the thermal management and is ahead of GM and the others.

            Anyone could have built a Supercharger network. Tesla did, others didn’t. Anyone could have made their car Supercharger-compatible. Tesla did, others didn’t.

            Anyone could have built a battery-electric car with a 200 mi + range. Tesla did, others didn’t.

            Anyone could have designed an electric car from the ground up, rather than modifying a gas-car design. Tesla did, others didn’t.

            Now, what Tesla has is *reputation*, *brand value*, and a major head start.

          • Carl Raymond S

            I think you’ve nailed it.

    • Ivor O’Connor

      1/7th? Closer to 1/4.

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