One of the challenges of buying a used electric vehicle is knowing the health of the battery, which is usually the single most expensive component of the car. What is the typical battery degradation with age, and is this particular car any worse than typical? I know of no quick way to test it before buying, when you typically don’t have the opportunity to drive it from full to empty.
I recently bought a used 2017 Hyundai Ioniq Electric (28 kWh), and wanted to know how much battery capacity remained.
This is my attempt at measuring the battery capacity, by measuring the amount of energy delivered by the battery when driving from fully-charged to near-empty.
Methodology
- Drive from 100% to a very low state of charge
- Using an OBD reader, look at the “Cumulative energy charged” and “Cumulative energy discharged” numbers at the beginning and end of the drive.
Multiplying the efficiency reading on the dashboard (kWh/100 km) by the trip distance doesn’t seem particularly accurate, even when the counter is configured to reset after a recharge. I suspect the efficiency counter is some kind of weighted average (preferring more recent observations) rather than strictly an average since the last reset.
Results
| Charged (kWh) | Discharged (kWh) | Cell voltage | |
|---|---|---|---|
| 100% (95% BMS) | 32735.2 | 31585.0 | 4.12 V |
| 4% (5.5% BMS) | 32736.4 | 31612.2 | 3.18 V – 3.24 V |
| Difference | 1.2 | 27.2 | – |
Regenerative braking is considered charging, which increases the “cumulative energy charged” counter during the trip. This complicates the calculation somewhat because I need to subtract the extra discharge energy available due to being charged by regenerative braking to get the total energy available solely from the battery. The efficiency of the battery (lifetime energy discharged divided by lifetime energy charged) is apparently around 96.6% (31612/32736 = 0.966).
The total energy discharged from the battery minus the energy gained from recharging gives the total energy delivered by the battery on my trip: 27.2 kWh − (0.966 × 1.2 kWh) = 26.0 kWh.
If I trust the state of charge numbers (100% start, 4% end), then scaling to the full capacity of the battery (from 100% to 0%) would be 26.0 kWh / 0.96 = 27.1 kWh.
It appears my nine-year-old battery can still deliver 27.1 kWh, or 97% of the rated capacity. The car manual says the battery has a capacity of 28 kWh, but I don’t have a measurement of the actual capacity when it was new (which could have been higher).
Although the Hyundai Ioniq Electric is known to have low battery degradation, losing only 3% in 9 years seems unreasonably good. Unfortunately, without a measurement from when the battery was new, I can’t distinguish between very low degradation vs. the battery originally exceeding its rated capacity. I will have to repeat the experiment to see how it degrades in future years.
Battery stats (July 2026)
- Age: ~9 years (Manufactured June 2017), 150,000 km
- State of health: 96.9%
- Fully charged (100% display, 95% BMS): 398V total, 4.14V per cell, no deviation (20mV resolution)
- Fully discharged (0% display, 2% BMS): 295V total, 3.02V – 3.10V per cell
More Measurements
A summary of results from repeating this experiment over time.
| Date | State of charge | Energy Discharged | Energy Charged | Cell Voltages (end of test) | Capacity (100%→0%) |
|---|---|---|---|---|---|
| 2026-06-20 | 100% → 4% (5.5% BMS) | 27.2 kWh | 1.2 kWh | 3.18 – 3.24 V | 27.14 kWh |
| 2026-07-07 | 99.5% → 0% (2% BMS) | 31.0 kWh | 4.1 kWh | 3.02 – 3.10 V | 27.18 kWh |
After doing this twice, the experiment seems repeatable even with different driving styles. The first experiment was mostly 100 km/h over several hours. The second experiment had mainly city driving (and much more AC use) over three days. The calculated battery capacity differed by under 1%.
Leave a Reply