New Battery – 18 Month Check – Weak Cell?

It’s been 18 months and 22,000 miles since I had the main traction battery replaced in my LEAF. By and large it is fairing well. At 15 months old the battery was showing early signs of degradation.

Battery has increased capacity recently.

Interestingly in the last 2-3 months the battery has “recovered” some of the lost capacity I observed  and is now showing about a 5% reduction of capacity overall rather than the 7% I recorded earlier this year. That means the battery is losing about 3% per year on average which if the rate of degradation continues I will be at 15% capacity reduction at 5 years. A much better outcome than I experienced with the original battery.

That’s the good news. And now for the bad news….

Weak Cell? Click image to enlarge

Early sign of defective cell?

While the battery capacity trend is encouraging, I did experience a rapid loss of range recently near the end of a long journey where the battery was approaching the point where I would receive a low battery warning. I investigated the battery statistics when I got home and found that LEAFSpy Pro was indicating that cell pair #37 was weak. The car was at about 19% state of charge which seems reasonable for the 3.6 volts recorded. The reason the cell was flagged is because it varied from the rest of the pack by over 100 mV.

This is the first time since I got the car that I’ve recorded a weak cell. A single warning of this type does not mean the cell pair is definitely faulty, it could simply be a result of an imbalanced battery pack. However I will keep my eye on the battery cells each time I approach low battery warning.

Should this recur again with the same cell pair, I will ask for Nissan to do a thorough test of the battery.

What this means

The voltage of cell pair #37 had dropped to 3.611 volts. A battery pack is only as strong as its weakest cell. To protect the battery and its cells against damage, a single cell with a low voltage of 3.5V will prompt the car to shutdown. Hence the low battery and very low battery warnings the LEAF to let drivers know that their car needs a charge sooner rather than later.

The voltage of a cell can be used to approximate the charge level of the cell. This varies by the specific chemistry used in a Lithium Ion battery. The table below is an approximation of state of charge and battery cell voltage for Lithium Ion batteries in general.

4.2V – 100%
4.1V – 90% (LEAF full state of charge)
4.0V – 75%
3.9V – 55%
3.8V – 30%
(3.5V – LEAF Turtle/Shutdown)
3.3V – 0%

The LEAF limits state of charge between 17% and 90% of the cells theoretical capacity reserving some capacity to prevent damage at high or low states of charge. More reserve is present at low states of charge. (Source).

The LEAF battery cells have a theoretical minimum voltage of 2.5V below which the cell will become unstable when recharged, with the possibility of thermal runaway. The LEAF keeps well away from this theoretical low voltage state for both the long-term durability of the battery and for thermal safety.




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