We have an EV (241 Hyundai Ioniq, 77kW). We have a home charger. With our electricity provider we get a cheap EV rate between the hours of 2am and 5am. I’ve read that best practice in terms of caring for the battery is to not let the battery charge drop below 20 per cent and not to charge above 80 per cent and also not to charge daily. So, what we do is wait for the battery to drop to around 20 per cent or just below, and then we charge it overnight to 80 per cent, ensuring the charging period includes the 2am to 5am period. Based on our usual usage, we charge about once a week. However, we would obviously save money if we charged only between the hours of 2am and 5am. So, does it make most sense for us to charge the battery to 80 per cent over a number of days, charging only during the cheap hours, or in the long run are we better sticking with what we do now? – JM from Galway
You’re doing exactly the right thing if you want to keep your Hyundai’s battery in absolutely tip-top condition. Every battery manufacturer says that keeping the battery’s charge between 20 and 80 per cent is the best way to achieve the longest possible battery life, and that you should only waver outside of those lines when you really need to.
However, you’re possibly overthinking this a bit too. You see, you’re already doing the number one thing you can do to protect your battery and that’s charging almost exclusively at home. Slow charging such as this, on a 7.4kW AC current, is the best way to keep a battery happy in the long run.
According to vehicle telematics experts Geotab, which actively checked the performance and battery health of more than 22,000 electric cars last year, the average battery degradation – and there’s always going to be some – is 2.3 per cent per year. That means a small EV, such as a Renault 5, will lose about 10km of overall range each year. A car with a larger battery, such as Volkswagen ID4, would lose 14km, but that’s from a total claimed range of 565km.
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However, there’s more detail in those numbers when you look closer. The average figure of 2.3 per cent per year is pushed higher by those who regularly use more powerful DC fast chargers.
Geotab’s numbers show that regularly fast-charging above 100kW of DC power can accelerate battery degradation to about 3 per cent per year. If you stick to primarily slower AC charging – just as you are doing – then average degradation falls to just 1.5 per cent per year. Your Hyundai Ioniq 5 (I’m assuming it’s a 5, you haven’t specified in your email) has a claimed maximum range of 507km, so at 1.5 per cent degradation, you’re losing a mere 7km of range per year.
In fact, you may not even be losing that much. ADAC, or the Allgemeiner Deutscher Automobil-Club, is basically like Germany’s version of the AA, and it’s been independently testing the mechanical solidity of cars for decades now.
ADAC’s engineers decided that it was high time to put an EV battery to a proper test. A Volkswagen ID3 S Pro was procured, and so the ADAC guys set about giving its 77kWh battery – the same capacity as the one in your Hyundai, but a slightly different design – a serious kicking.
This wasn’t a lab test – over the course of four years, ADAC’s team drove the wheels off this ID3, putting 160,000km on its clock, and not coincidentally, that’s the mileage limit of VW’s original battery warranty. The car was driven up to Alpine glaciers, along autobahns at high cruising speeds, and in shatteringly low temperatures.
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It was also charged roughly. ADAC’s engineers didn’t have time to hang around, so fast-charging was used for roughly 40 per cent of the ID3’s total charging time, and often the car was left with a charger connected even when 100 per cent charge had been reached – normally said to be a serious EV battery no-no.
According to ADAC’s test – and independently confirmed by battery testing experts at Aviloo – the ID3’s battery has indeed lost some of its original charging capacity: a whole 9 per cent. That means that after some serious mileage (at the time of writing, the ADAC ID3 had reached 172,000km) it still retains 91 per cent of its original charging capacity. Given a claimed range of 557km on the WLTP test, it means that this VW can still cover 500km between charges.
So how has ADAC’s VW ID3 lost only 9.0 per cent after four years, when Geotab’s research says that it should have lost between 10-12 per cent?
Well, there’s a little bit of clever trickery here. When it comes to EVs, you’ll usually see a figure quoted which is the battery’s usable capacity. That’s a measure of how much energy is available for the driver to use when the battery is fully charged, but it’s not the actual total energy storage capacity of the battery. That gross figure may well be 5-7kWh higher than the net usable figure, and this has been deliberately built in so that batteries have a little wiggle room as they age.
Thanks to software updates, carmakers – such as VW – can see how well or how poorly batteries perform over time, and can remotely instruct the car’s control systems to allow a little more of the battery’s total capacity to be used for the net figure. So as the battery ages, and the boffins back at HQ can see that they don’t need as much of a safety buffer, the software can allow more of the battery to be used. So in the case of ADAC’s Volkswagen, the battery was degrading over time and use, but because the software was able to compensate, the amount of energy actually available to the driver was improved, slowing down the apparent degradation.
Is that a cheat? A little bit, yes – you could say that it’s disguising battery degradation with clever software, but if the net effect is a longer-lasting battery, does that really matter?
It’s worth noting that your Hyundai’s battery is rated to accept as much as 350kW of DC fast-charging power, so there should be a certain level of robustness built into the system.
So, as I said, you’re doing everything right, but don’t be afraid to charge to 100 per cent, or drain lower than 20 per cent, when you need to. A bit of variation in how you charge the battery is a good idea too, as according to several research papers an occasional charge to 100 per cent is healthy as it helps to balance the available charge across all of the battery’s cells. Once a month to 100 per cent should be fine.
