Battery Life Span
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- Last Updated: Wednesday, 26 February 2025 16:05
The good and bad treatment of EV batteries
Types
We are all pretty familiar with the batteries offered at our local supermarket. From Zinc-Carbon, Alkaline, Lithium, Nickel Cadmium, Nickel Metal Hydride, Zinc Air and others. On reading the notes one finds each is recommended for different types of use. In the rechargeable range some suffer from memory effect and some don't.
Memory effect is when the capacity reduces to match the regular charge discharge cycle. Nickel Cadmium were the worst. Use 10% and recharge it and very quickly it forgot the remaining 90%. Sometimes a few complete discharge and recharge cycles could recover much of it. They also tended to self discharge just sitting around. More recent versions with additives have improved this somewhat.
Lithium don't suffer from this effect, but as you might guess they have other issues, such as being flammable when too hot. The point is the type of chemistry used has both advantages and disadvantages, each result in a compromise between energy, weight, cost and lifespan.
The confusion between chemistry
In the EV world all tend to be a composition between lithium and materials, such as Nickel, Cobalt, and the nature of the electrolyte. Depending on the type fitted to a particular version of the same model the recommended charging process varies. However, they all suffer deterioration over time, but the amount depends on how charging is managed by the owner. The manufacturer may specify a recommended percentage, typically 70 to 80% for best life for a performance version and 100% for a standard version.
The reason for this is basically cost. In the Tesla range both model 3 and Y each have about 3 model versions. Basic Rear wheel drive (RWD) the cheapest, Long range, and then Performance. Long range can have one or two motors as do all performance models. The base model has a LFP battery because they are cheaper, have a better charge and fire risk tolerance, but lower capacity. LFP also have a lower max load capacity, meaning they cannot provide enough grunt to support two motors. So the battery chemistry is varied to suit each version. Initially Tesla's all used the same type, but even earlier performance models are different to the latest versions. A lot has changed in five years, particularly for China manufactured batteries with the promise of sold state with a dry electrolyte. These allow faster charging and much greater range, but cannot yet achieve a higher number of re-charge cycles and potentially life span. MG are predicting their models will have these in 2027.
How to be Kind to your EV battery
The first thing to do is determine the type fitted to your vehicle. The software should show this somewhere on a menu. LFP just show 100% but NMC also show daily drive settings between 60 and 80% on their charge settings.
A recent independent study decided to calculate lifespans by running charge/discharge tests on groups of single lithium cells of various types until they reached a given percentage loss, like to 70% of new. Some were fully charged and discharged and others for varying ranges like 80 to 20%. Others were tested using many varied ranges, from 80 to 60, 60 to 40, 70 to 30 etc. The best lifespan was for a 65% average with a +/- 10% max/min. (75 and 55) The difference in charging cycles was quite dramatic with a usable number of lifespan charges ranging from 600 to 6000 cycles. This was calculated based on total energy extracted with each cycle the equivalent of a full charge. i.e the small charge range of 20% (+/- 10) needed 5 times more events to equal one full cycle, so basically 10 times longer.
As an engineer I follow of the U-Tube channel Engineering Explained and there are two videos of analysis for both NMC and LFP types. It covered the reasons for the different charge limits for each type and the manufactures recommendation of 100% for LFP is not the whole story. The reason is the min/max voltage range of each type. The NMC cell voltage typically ranges from 3.1 to 4.2 and LFP from 3.3 to 3.5, and this makes calculating the % of charge inaccurate by measuring the voltage as the slope is almost a flat line. At full charge it rises higher very quickly with a similar thing happens at the lower minimum, thus making the 0 and 100% limits easy to determine, but not in between. The much higher slope of NMC allows a pretty accurate % of charge state using the current voltage. Bare in mind that when scaled up 100 times for an actual 400 volt pack the min/max difference becomes some 120 volts. For LFP the curve from 70 to 100% is almost flat and the state of charge cannot be determined by voltage. Therefore state of charge is determined by measuring the energy consumed since the last 100% state and scaling this from when flat. Thus it is important to regularly charge an LFP to 100% to re-calibrate range. The key word is regularly and the E.E video suggested monthly.
LFP cells still suffer similar losses as NMC when kept fully charged or flat, so the recommendation was to maintain around a 60% charge for day-to-day usage. The monthly 100% charge should be done when the full range is actually needed and not perform a full charge every time. The independent battery tests pointed out that an occasional full to flat cycle, when needed, had little effect on the maximum possible life span cycles. They calculated the lifespan of an EV battery when maintained at 65% for the average 14,000 Km per year in N.Z, should out-live the life of the car at some 20 years. Indeed many Tesla's in USA have original batteries that have covered some 300,000 Km with the record being near 800,000 km. Not likely many cars in N.Z are ever going to reach this.
Actually maintaining an average charge of around 65% is very easy to do, and only a little extra planning is needed when a long range is needed, which for most owners is rarely an unexpected event. Personally I do a long range trip over 400km about every three months. I mostly plug-in to a free mall 7Kw charger while having lunch, which gives about a 10% top-up and enough to safely return home again with about 15% or a 100Km buffer remaining. For that night I set the charge limit to 50% (the minimum possible) and this covers my on-going daily use indefinitely. Over time as the opportunity arises, I connect to the free mall chargers when shopping and over a few weeks it gradually climbs to 75%. Over a 12 month period, according to the Tesla app, this costs around $200, of which 30% is free mall charging, so actual cost of charging is near $140. Of course, its not actually free due to the $12 of coffee and cake one consumes each week while at the mall!