Welcome to my most popular blog post. Looks like battery health/life is a big concern amongst my readers!
When I bought my Nissan Leaf at 3 years old, it had 88% battery “health” as reported by the car. Battery degradation is inevitable with electric vehicles, it’s just a question of how much. If you’re interested in what affects battery degradation, I recommend Euan McTurk’s “Plug life Television” channel on YouTube.
The Hyundai Ioniq electric also reports battery health. At 36 months and 36,000 miles the car is reporting 100% via the “Torque” app which reports parameters from the battery management system. I also use the “EVNotify” app, which is aimed at EV drivers, and can notify when the battery reaches a certain state of charge.
I don’t believe that the Nissan SOH and Hyundai’s are directly comparable. Nissan’s battery management system (BMS) reports its best guess at battery health, or capacity, based on 100% being the nominal capacity when new. Whereas, Hyundai always had more than 28kWh available, there is some unused spare capacity to allow for “wear”. What I think is happening is that 100% means “capacity is at least 28kWh”. The BMS might not report under 100% for some years, until degradation takes the gross capacity to below 28kWh.
Note 100% health is not the same as 100% charged. Even a very degraded battery can be 100% charged, but like an old phone, won’t run for long. Now we don’t know exactly how either Nissan or Hyundai calculate the “battery health”. And why should we care? What matters is how much charge the battery can accept- and, ultimately, the range.
The battery gets a mixture of rapid (up to 62kW so far) and 7kW charging. I try not to charge to 100% full very often now, and with charging available everywhere I go (mostly work and home, but also local car parks) I tend to top-up with smaller amounts more often. Using the Ohme cable it’s easy to set the charge to add a certain percentage via the app, or do the mental maths (for example at a 7kW post, to add 10% takes 26 minutes).
I calculated roughly how much charge the Ioniq can take by charging from a fairly low state of charge. Naturally all the figures rely on some assumptions but let’s see how many kWh the battery can take-
The car showed 20 miles remaining out of 125 mile range, that’s 16% on the pack or 4.5kWh remaining. The Polar charger reports I used 25.7kWh at 7kW, which I know from a previous measurement is 95% efficient. So I added ~24.4kWh to the car. 24.4+4.5 = 28.9kWh
Autumn 2019- the car is now 33 months old, with 31k miles – time do do a more methodical battery capacity measurement. The battery management system reports kWh charged, so I used this before and after an almost-complete charge cycle, from 11%. I don’t normally take the car this low. Then there’s a choice of net “display capacity” or gross “BMS capacity”, so lets consider both. Conveniently the battery was at the same temperature before and after charge, any heating will have been compensated for by ambient temperatures around 10degC.
BMS, or gross capacity: 11% to 95% state-of-charge used (7866.8-7841.2) = 25.6kWh
25.6/0.84 = 30.4kWh
Display, or net capacity: 10.5 to 100% state-of-charge, also 25.6kWh
25.6/0.895 = 28.6kWh
Both of these numbers are what I’d have expected from a new Ioniq.
As you can see above, the BMS reports two figures for cumulative energy – charged and discharged. This reflects losses in the battery. They differ by 3.2%. We can use this to adjust the above capacity measurements, giving a net available battery capacity of 27.6kWh. Compared to the nominal 28kWh that’s 1.5% different – and probably insignificant given other variables such as battery temperature.
We do know (from Hyundai service info) that the Ioniq charges to only 95% of the battery’s true capacity; to roughly 4.15 volts when it could be charged to 4.3 volts. Likewise at the bottom end of charge, the battery discharges to 5%. This will help battery life. Tesla are said to use only 90%, which is more tolerable day-to-day, given their larger battery packs. (As an aside, the 2020 Ioniq electric uses a far wider range of charge, 0 to 97%).
What can users do to extend battery life further? Three things: keep the battery cool, avoid leaving the battery fully charged for long periods, and avoid unnecessary rapid charging. If you’re really keen, avoid charging to 100% on every charge, unless you need all the range.
Compared to my Leaf 24kWh at a similar age, the car is at least twice as useful to me. I say this because the Leaf only ever managed one commute before needing a charge. It just didn’t quite have enough capacity for even a gentle second commute on a 50 mile B-road route. The Ioniq can do two commutes, even using the motorway.
Conclusions I can make so far: the range is still healthy at 125 miles in winter and 150 miles in spring; the battery seems to be accepting more than the nominal 28kWh.
It’s also worth mentioning that some manufacturers quote gross battery capacity (Nissan, Tesla), others net, or useable capacity (Hyundai). Personally I think gross battery capacity as a nameplate figure is misleading, it’s like quoting fuel tank size. There’s much more to consider with EVs than just the fuel tank size!