Why are you supposed to stay within the 20 per cent to 80 per cent range when charging your EV? What really happens if you charge from 5 per cent to 95 per cent? M O’B, Co Meath
There are two simple reasons behind the 20-80 per cent charge rule: time and money.
Time, because most current lithium-ion batteries are designed to be at their most charging-efficient between those two levels. In other words, a 20 to 80 per cent charge level is when you should – all other things being equal – get the quickest charging times, especially on a DC fast charger.
And money? Well, by charging between 20 and 80 per cent, you are – at least in theory – being kinder to your battery and preserving more of its health for the long term.
RM Block
Are you really, though? Let’s take a look.
In some ways, you can think of the 20-80 rule as good old mechanical sympathy updated for the battery age. In general, you don’t wring your car’s engine out to the redline every time you drive it, and in general you neither over-fill your fuel tank, nor run it completely dry (the old “sucking up dirt from the bottom of the tank” myth). So keeping your battery in its happiest zone between 20 and 80 per cent is a good thing, and it should mean that your battery lasts longer and retains a higher percentage of its original charging capacity over time.
Basically, charging a battery to 100 per cent puts stress on its chemical make-up, as well as exposing it to prolonged heat cycles. This is especially true when it comes to DC fast public charging, and it’s why charging speed will generally drop off a cliff once you’re above 80 per cent. Mind you, that’s also a neat trick to get you moving on again and stop you hogging one of the still-too-rare fast-charging points.
Equally, squeezing the last drops out of a battery can also put stress on its internal systems, so it’s best not to, as and when possible.
Does this mean you can never charge above 80 per cent? Of course not; it’s just best practice to only charge to 100 per cent when you know that you’ll actually need to use it, such as when you’re going on a longer journey. For most of us, on most days, especially when you’re doing most of your charging at home, 80 per cent is more than good enough.
There are some exceptions to the rule, though. In some cases, car makers offer a recommendation of charging to no more than 90 per cent unless you really need the last 10 per cent – Volvo and Polestar tend to recommend this for their electric cars.
In other cases, you can be much more flaithiúlach with your charging, especially where lithium-iron phosphate batteries (LFP) are concerned. These use a slightly different chemistry to the currently more common nickel-manganese-cobalt (NMC) batteries, and LFPs are far less sensitive to charging above the 80 per cent mark, so in general you can be a bit more devil-may-care about charging to 100 per cent.
All BYD models, for instance, used LFP batteries, while the latest entry-level versions of the Volkswagen ID.3 Neo and the Cupra Born also use them, and they’re starting to become more common in the industry as they’re both cheaper to make and longer-lived than NMC batteries.
So why don’t all car makers just use LFP and we can abandon the 20-80 rule? Energy density. If you, as a car maker, want to give your car a whopping great WLTP range claim, then NMC is the only way to go, as that battery chemistry can hold more stored energy for a given size and weight than an LFP.
Or does it? Actually, the balance between LFP and NMC is starting to erode as more and more development and research cash is poured into LFP.
And the results are startling, or at least they are so far. BYD has, in its home market in China, a luxury brand called Yangwang, and it makes a large, luxurious, high-performance four-door saloon called the U7. This uses a huge 150kWh lithium-iron phosphate battery, and just last week, BYD ran a test on an apparently randomly selected U7, where it fast-charged it a lot.
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When we say ‘fast-charged’, we mean BYD used its new ‘Flash’ chargers, which use a combination of high-power electricity connections and on-site battery storage to be able to deliver as much as 1,500kW (that’s 1.5 megawatts) at a time. For reference, the fastest and most powerful chargers on the island of Ireland right now can only deliver around 400kW, and most cars will at any rate only usually accept between 150kW and 200kW of DC charging power under ideal circumstances.
Using its Flash system, BYD charged this U7 saloon 350 times in just nine days, as the car ran a series of marathon sessions on a test track in Nanning, in China. These charges took the battery from 10 per cent to 97 per cent each time, allowing the U7 to cover 30,000km in those nine days of running.
How damaged was the battery after this? Hardly at all, with BYD claiming that it still charged up to 98.7 per cent of its original storage capacity.
Now, there’s a few caveats with this. These are all BYD’s claims, and they haven’t been independently repeated. There are no BYD Flash chargers in Ireland yet (although BYD has said that the tech will come to Europe, and it’s installed 10,000 of the things in China already) and the Yangwang brand isn’t even sold here.
But it does at least show the potential of just how good an LFP battery can be and how little degradation can take place (albeit nine days of hard running isn’t necessarily the same as three years of regular driving – time can be a factor in battery life as well as use).
So, the short answer is you don’t absolutely have to stick to the 20-80 per cent rule when charging your battery, but it’s generally a good idea to do so. And, coming soon, a combination of higher-tech LFP batteries and the long-awaited dawn of the solid-state battery (that’s for another day ...) will most likely remove the need to worry about it.
