Why do Li-ion Batteries die?

[alohart]

By the way, how did you get the degree symbol (°)?

Under OS X, the key combination Option-0 (Option-zero) produces the º character.

 

[BlueLightning]

Li plating, fast charge and sub ambiant temperature

Live from inside a battery: Researchers observe the phenomenon of 'lithium plating' during the charging process
Read more at: http://phys.org/news/2014-09-battery-phenomenon-lithium-plating.html#jCp
Lithium plating cause short cut
Faster charging would mean more metallic lithium

Li plating at -20 ℃ or - 4 ℉
http://www.sciencedirect.com/science/article/pii/S0378775314012233?via=ihub
Li plating reversable, at rest after charge, Li plating decrease.
After a fast charge, should we give the battery a rest?

Lithium profiling video with neutron depth profiling ( NDP )
http://phys.org/news/2014-09-first-ever-lithium-ion-battery-video.html

Sorry giving only links with no resumes, on vacation with no computer, fighting with my EV Android tablet, missing my iOn, but enjoying California.

 

[suradasa]

Temperature dependent ageing mechanisms in Lithium-ion batteries – A Post-Mortem study. Journal of Power Sources, Volume 262, 15 September 2014, Pages 129–135
http://www.sciencedirect.com/science/article/pii/S0378775314004352
Abstract
The effects of temperatures in the range of −20 °C to 70 °C on the ageing behaviour of cycled Lithium-ion batteries are investigated quantitatively by electrochemical methods and Post-Mortem analysis. Commercial 18650-type high-power cells with a LixNi1/3Mn1/3Co1/3O2/LiyMn2O4 blend cathode and graphite/carbon anode were used as test system. The cells were cycled at a rate of 1 C until the discharge capacity falls below 80% of the initial capacity. Interestingly, an Arrhenius plot indicates two different ageing mechanisms for the ranges of −20 °C to 25 °C and 25 °C to 70 °C. Below 25 °C, the ageing rates increase with decreasing temperature, while above 25 °C ageing is accelerated with increasing temperature. The aged 18650 cells are inspected via scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), inductively coupled plasma (ICP), measurements of electrode thickness and X-ray diffraction (XRD) after disassembly to learn more about the chemical reasons of the degradation. The effect of different temperatures on the electrode polarizations are evaluated by assembling electrodes in pouch cells with reference electrode as a model system. We find that the dominating ageing mechanism for T < 25 °C is Lithium plating, while for T > 25 °C the cathodes show degeneration and the anodes will be increasingly covered by Solid Electrolyte Interface SEI layers.

State of Health decrease approximately with the same rate for T = 70 °C and for T = 0 °C

I would take the low-T results of this study with a grain of salt, given that they're charging at a rate of 1C (i.e., battery charges in 1 hour). I certainly wouldn't recommend QC'ing below freezing, but at L1 and L2 rates, this is much less of an issue. See for example here: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/41485/1/08-0012.pdf. They report no plating at -20°C for a charging rate of 0.1C. I guess L1 would be about 0.05C and L2 would be about 0.15C.

However, it does make you think twice about this question of whether L1 is better for the battery than L2. At +20°C, no; but below freezing, it may be worth it to use L1 rather than programming L2 to start at 2 am, and letting the battery temp drop overnight. I just got my OBDLink MX, so I'll be very interested to see what happens to the battery temperature at night in cold weather.

 

[jray3]

Thank you Surdas. I'm glad to see the state of the art advancing with respect to understanding how lithium batteries actually live and die. Charting the pack temperature against ambient temperature, SOC and charging amps would answer the question of whether L1 actually keeps the pack warmer on a cold winter's night (due to a longer balancing period when the BMS resistors are burning). However, losing effective pre-heating in would keep me from using L1 in winter, along with the super-slow charge recovery time.

 

[PV1]

My winter method is to charge overnight on L1, then switch to L2 in the morning if I want to preheat. The longer charge time of L1 would seem to keep the pack warmer (balancing and just the cells being active) through the night.

I'm starting to see colder weather, dipping down into the low 40's at night, so I can charge L1 one night and L2 another and compare the end-of-charge and morning pack temperatures. OVMS remembers end of charge temperatures (though not by design) as the CAN bus goes nearly silent when the car isn't charging. Temperatures refresh if the car wakes up or is started.

This makes me wonder if re-routing the coolant loop to zig-zag the bottom of the pack would be beneficial? That way, waste heat from the electronics and motor can help keep the battery warm. Last winter, I've seen the charger alone bring the loop up to ~45° when it's 20° outside.

I just returned from a quick jaunt a few miles up the road. It's roughly 60° outside, pack temp is 64°, motor is 91°, and inverter is 79°. I didn't go on the highway, just main roads at 45 mph.

 

[jray3]

My winter method is to charge overnight on L1, then switch to L2 in the morning if I want to preheat. The longer charge time of L1 would seem to keep the pack warmer (balancing and just the cells being active) through the night.

Thanks PV1, but I guess I'm just too much of a weenie to venture outside in my jammies and switch charging cords in the interest of preheating. (seems to partially defeat the purpose) ;-p

 

[PV1]

There's the difference. I can switch from within the garage, even though the car's outside. I have the EVSEUpgrade and can switch from 120 to 240 with the same J-Plug in the car.

 

[mixmike6]

During the cold months I was planning on charging through the night on 120v/8a to keep the pack active and warm.
I let the car rest about an hour before charging it when I go home and the same when I leave in the morning.
In the winter should I let the car rest after charging it in the morning or would it be better to unplug it while still charging and leave?

 

[HParkEV]

In the winter, I usually time the charging so that it is fully charged just before I leave the house. At below freezing temperatures I usually need the full capacity of the pack to get me through my commute and errands. I noticed that if I don't time the charging and it finishes many hours before I leave the house, the cold battery is not capable of delivering as much range. How much less range is hard to quantify, I never kept very close records, plus the driving conditions vary from day to day so its hard to compare.

 

[PV1]

This makes me wonder if re-routing the coolant loop to zig-zag the bottom of the pack would be beneficial? That way, waste heat from the electronics and motor can help keep the battery warm. Last winter, I've seen the charger alone bring the loop up to ~45° when it's 20° outside.

I just returned from a quick jaunt a few miles up the road. It's roughly 60° outside, pack temp is 64°, motor is 91°, and inverter is 79°. I didn't go on the highway, just main roads at 45 mph.

I got some measurements this morning. It was 40° outside this morning, so I had to preheat. I charged up last night on level 2 from 12 bars to full. I started charging at 11:30 PM.

Event               PEM Temp     Motor Temp     Battery Temp   (°F)

End of charge         52             54               61
Start of preheat      48             50               57
10 min. later         52             54               59
20 min. later         55             57               59
After 20 min. drive   55             81               59

A twin diverter valve could redirect coolant from the radiator to a coil under the battery pack (between the case and the plastic shield) so that this waste heat can be used to keep the cells warmer. The coolant would be kept away from the radiator when heating the pack. The motor temp at the beginning of my drive shot up to 91°F about a mile from my house before the coolant pump kicked in, so the coolant is quite warm when it hits the battery (if redirected. The coolant doesn't touch the pack normally). It settled between 78° and 81° while driving at 48 mph on a level road.

 

[barrylevine]

During the cold months I was planning on charging through the night on 120v/8a to keep the pack active and warm.
I let the car rest about an hour before charging it when I go home and the same when I leave in the morning.
In the winter should I let the car rest after charging it in the morning or would it be better to unplug it while still charging and leave?

Is it definite that leaving the car charging at 120V keeps the battery pack warm, even after charging is complete?

This seems to be be a good thing not only for better range on a cold morning, but may also help battery longevity.

I read that at 32F or below, recharging Li-ion batteries causes lithium plating on the anode, deteriorating the battery.
So if one sets off on a trip in the iMiev with a cold battery that's below 32F, wouldn't regen damage the battery until it warmed up?

 

[PV1]

It's possible, but the car does start reducing regen pretty drastically when the pack falls to near freezing temperatures. The less charging current, the less likely plating is to happen. I'd say that if you're really concerned about it to put the car in Neutral when coasting or slowing down to disable regen and use the friction brakes, but that will take a 10-20% bite out of range.

I agree that slower, level 1 charging keeps the pack warmer overnight. It'll still cool down just as fast once charging is done, but since charging takes longer, the battery doesn't sit idle as long.

 

[Don]

Is it definite that leaving the car charging at 120V keeps the battery pack warm, even after charging is complete?

Yes . . . . and . . . . no. Charging the pack does warm it a bit, but once charging is complete, the 'warming' is done with and it cools off pretty quickly. This is why trickle charging it at 8 amps is recommended - If you time it correctly, it will still be charging (and warming) when it's time for you to leave in the morning . . . . hopefully with a 90% charge

Don