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Joined: Tue Jan 21, 2020 12:27 pm
Location: Seattle, USA

Does the efficiency of charging degrade over long term?

Tue Jan 21, 2020 1:05 pm

First, pardon me if I'm demonstrating a fundamental ignorance of battery electrics. That is very likely!

I'm very aware that the max storage capacity of Leaf batts degrades over time and use.

However, I'm wondering if the efficiency of charging stays roughly the same, or worsens, over the life of the battery. To be more specific, say 10kwh is used from my home source for a charge. Will this provide me about the same driving distance when the batt is new AND when the batt is old? In other words, if 10kwh is put INTO the batt, will I always be getting the same energy BACK OUT of the batt, as long as the batt still has fair capacity? If not, then I suppose there would be a somewhat growing inefficiency with the life of the batt, and per-mile cost of operation would grow.

I'm particularly interested in how this has worked in the older 24kwh batts, since they have likely experienced the most degradation. Are drivers still getting a mileage-per-kwh similar to what they got out of the same batt in 2011?

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Delivery Date: 01 Aug 2017
Leaf Number: 311806
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Re: Does the efficiency of charging degrade over long term?

Tue Jan 21, 2020 3:46 pm

The Hx (usually referred to as the 'internal resistance' of the battery) also degrades as the battery ages. I'm not sure exactly how this physically manifests itself, but I imagine the 'internal resistance' would be part of the battery voltage-source model. If that is the case, then any current going through the battery in either direction (charging or discharging) would pass through this internal resistance and generate waste heat. In short, yes, I would expect the efficiency of charging and discharging the battery to degrade as the overall health of the battery degrades.

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Delivery Date: 12 Jun 2011
Location: Phoenix, AZ

Re: Does the efficiency of charging degrade over long term?

Tue Jan 21, 2020 7:11 pm

Theoretically the internal resistance increase over time should reduce the overall efficiency, but my experience has been minimal change in charging/discharging efficiency for normal driving and L2 charging. I did not notice any reduced charge/discharge efficiency with the gradual capacity loss of the original and replacement batteries in the 2011.

The 2015 saw gradual capacity loss (much slower than the 2011) until several cell pairs became weaker than average. The few weak cells really reduced range because they reached maximum voltage during charging and minimum voltage during discharging before the other cells due to increased internal resistance. The higher internal resistance caused increased power loss during periods of high power consumption and slower charge rates when using DCQC, but was not noticeable during gentle driving or L2 charging. The increased internal resistance caused dramatically fast drops in SOC during highway driving when the battery was less than about 50% SOC. This was because the higher internal resistance caused lower terminal voltage under load and the motor current increased to maintain power output. The increased current caused higher voltage drop which caused higher current draw so it became a cascading drop in SOC. The only solution was to get off the highway and slow down when SOC was low. The higher internal resistance did not really impact L2 charging, but did increase DCQC charging time so there was clearly some drop in DCQC charging efficiency even though I had no way to measure it. The battery temperature did run a little higher toward the end, but not as much as you might expect because the internal resistance of the whole pack was still relatively low.
Silver LEAF 2011 SL rear ended (totaled) by in-attentive driver 1/4/2015 at 50,422 miles
Silver LEAF 2015 SL purchased 2/7/2015; traded 8/10/2019 at 82,436 miles
White LEAF 2019 SL Plus purchased 8/10/2019

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Joined: Tue Jan 21, 2020 12:27 pm
Location: Seattle, USA

Re: Does the efficiency of charging degrade over long term?

Thu Jan 23, 2020 1:42 pm

Thanks goldbrick and GerryAZ.
So it seems that there may be some loss of charging efficiency, but that it may not be very much.
It seems to me that most people are assuming that the charging efficiency stays the same for the life of the batt when they estimate future cost-per-mile. Perhaps that's not a bad assumption.

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