Nissan Leaf Battery.
Some time ago I wrote my list of ‘Top Ten EV Urban Myths That Deserve To Die’. Even as I wrote it, I knew it wouldn’t be the end of EV urban myths: I might have covered ten of the ‘best’ and put them to bed, but I was sure more would come along to replace them. Or, as I will show here: be recycled in subtly different forms.
The ‘new’ one I am referring to is what I will call here the 80/20 ‘rule’. This myth says that batteries should never be charged beyond 80% or discharged below 20% lest ‘irreversible damage’ occur.
Another slightly different version of this “rule” suggests that if EV batteries can’t be taken safely above or below these limits, then they are really only 60% of their stated size/driving range.
So where has this nonsense come from? Like all good urban myths: it is based loosely on a couple of pieces of information that have been taken out of context, and are borne out of “rules” that might equally apply for an internal combustion engine (ICE) vehicle.
For instance, we have long been told when driving petrol or diesel vehicles that:
- Don’t go below 1/4 in a fuel tank to avoid running out/sucking up dirt or water from the bottom of the tank and;
- Never fill the tank to full if you don’t use the car much to avoid to ending up with months old ‘stale’ fuel. (This particularly applies to 98 octane petrol, but diesel winter/summer formulations can also cause issues).
So what is the significance of the 80 per cent charging limit?
80% is the recommendation for normal day-to-day charging of non-LFP EV batteries, which are still found in most EVs. (More on the other main lithium battery chemistry type, LFP, later).
For longevity of EV batteries, it is considered best not to stress them unnecessarily by charging to 100% every time you plug-in. For today’s EV battery sizes, it is also completely unnecessary to charge to 100% on a regular basis. Even charging my Kona electric to 80% for daily driving, I still only need to charge once every two to three weeks.
It is also worth pointing out that the early EVs with smaller batteries were almost always charged to 100%, and their batteries did not ‘die’ early as a result. Many are still going with those original batteries, albeit with reduced range. To give an example, my 13-year-old iMiEV is still on its original battery with a reliable 70km of its original 110-ish km range left.
The corollary to the above is that you will not ‘irreparably’ damage the battery by occasionally charging to 100% when needed. (For instance, when leaving home for a long trip). So yes, the top 20% of the battery is available for use when needed, it is not ‘lost’.
The other reason for only charging to 80% is when you’re at a DC fast-charger. The physics of battery charging is that the time for an EV battery to charge from 0% to 80% is very roughly the same as it takes to go from 80% to 100%.
(LFP chemistry batteries start slowing at slightly higher percentages, but the effect is much the same: DC charging slows as you near the top of the charge).
This means that if you don’t need that last 20%, don’t waste your time (and potentially delay others waiting for that charger) by staying there. Get moving and, if you need, do one extra stop during the day for 20 minutes or less to get that 20% when the battery is at 40 or 50% and the charging speed is much faster.
(For long trips, 20 minute stops are a must every 2 to 3 hours anyway and even an 80% charged battery will go for well longer than that).
Why no lower than 20%?
As mentioned earlier, this is similar to the recommendation for ICE vehicles. In the case of EVs, it:
- Gives a reserve should you get detoured near the end of your trip or a charger is down. (‘Charge early, charge often’ is also the mantra for long distance EV driving in these days of restricted charger numbers and sites in areas you are not familiar with).
- Is considered ‘kinder’ to the battery to not regularly go to a really low level of charge. Mind-you, whilst it can damage a lithium battery to ‘flatten’ it below its minimum voltage, the one in an EV is virtually impossible to get to that point as the vehicle’s battery management system will: a/ give you ample warning of a low battery, and
b/ eventually shut the car down before the battery is truly ‘flat’. It is worth noting here that below where zero is shown on the dash, manufacturers actually build a floor/reserve into the battery to prevent over-discharging.
Again though, the advice is (just like not always charging to 100% or going below ¼ in a fuel tank): be kind to your battery and charge at a reasonable low point. Like the old ¼ (or 25%) tank, for EVs that point is now generally suggested as 20%, but most EVs won’t even begin to start warning you until around 10%.
If by the way you do reach 5% or below, just recharge it as soon as practicable on arrival and no harm will have been done there either.
As a final note: the only true way to flatten an EV battery below its absolute minimum is to park at a low point and then leave it that way without recharging for weeks to months as it will slowly self-discharge to a dangerously low level. Even then, the car will at a certain point defensively shut all systems down to slow the process.
Summing up: just like the top 20% of the battery, the bottom 20% is also not ‘lost’. It is there if you need it, but just like an ICE car: don’t drive till the low fuel light turns on before refilling!
LFP batteries
Batteries are still an evolving technology – especially lithium chemistry ones. However lithium batteries have reached a certain level of maturity where changes are incremental rather than revolutionary.
After all, a quick look at the hundreds of billions of dollars being spent on lithium battery manufacturing plants is all you need to realise that the EV industry does not expect a revolutionary ‘killer app’ battery to turn up any time soon. (Unless you’re Toyota, but that’s more to do with finding excuses to not build battery electric cars than reality!)
All the advice I’ve referred to above is what is recommended for cars with standard lithium ion battery chemistries like NMC (nickel manganese cobalt) or NCA (nickel cobalt aluminium).
As the chemistries evolve, some of these recommendations have altered. One recent innovation in lithium battery chemistry is the LFP (lithium-iron-phosphate) battery. In LFP batteries, the cathode material is replaced with iron and phosphorus instead of the nickel or cobalt in NMC or NCA formulations.
As these materials are cheaper and in more plentiful supply, LFP batteries are cheaper to produce. However, LFP batteries still only represent an incremental change in lithium batteries.
They still contain lithium and are still considered a ‘wet’ battery chemistry. (As opposed to the much talked about … but yet to be commercialised … ‘dry’ lithium chemistry batteries).
However, being cheaper to produce and not containing cobalt, they are being adopted by some manufacturers in an effort to reduce EV manufacturing costs as well as being one way to avoid the issues surrounding cobalt mining.
LFP also has the advantage that the charge rate slows at a slightly higher charge level than other lithium ion ones to ramp their charge rate down more slowly after 80%.
However, they too after 90-ish percent charge at little more than a 7kW AC charger would offer. It has also been suggested (and recommended by Tesla) to regularly charge LFP batteries to 100%. It would seem therefore they are slightly more robust and capable of this treatment if you so wish to regularly charge to that level.
LFP batteries also have their downsides, the main one being they are less ‘energy dense’ (that is for the same kWh, they weight more/are bigger as compared to other chemistries) meaning LFP is unsuited to large battery packs where space for the battery is at a premium.
Tesla for instance use LFP for their Standard Range Model 3 and Model Y, but still use NMC chemistry for the Long Range packs.
Summing up:
Like all good urban myths, the ‘80/20 rule’ has a real sounding basis that in fact does not stand up to scrutiny. It is in fact an attempt by some EV doubters to increase the FUD (Fear, Uncertainty and Doubt) around EV adoption to slow the transition by restating in other terms the long-dead myths about EV batteries not lasting or not having enough range.
Unfortunately, the public is not yet fully up to speed on EV technology and are therefore susceptible to cleverly designed disinformation campaigns such as this one.
EV batteries in fact do last well and are quite robust, whatever the chemistry used. They also have high quality battery management systems fitted to ensure they are maintained within the manufacturers set voltage, temperature and charge/discharge limits.
The batteries are also well able to deliver the driving ranges stated in the WLTP or US EPA range figures. (If not the NEDC ones – but that’s another story!)
A side issue is whether LFP or other lithium ion chemistries (like NMC or NCA) are a ‘better’ choice. The proponents of each tech will ty to convince you that theirs is better – but in reality, it is a bit like choosing between QLED, ULED, OLED, 4K or 8K TVs. Each battery technology has slight advantages over the others, but not enough to say any of them is the ‘one’.
Keyword: Debunking the 80/20 limits on EV battery charging: More FUD from fossil fuel industry