JoeS
Site Moderator
Posts: 3715
Joined: Thu Dec 15, 2011 5:47 am
Location: Silicon Valley, California

Re: Battery Capacity Testing

Tue Dec 04, 2018 10:17 am

CZeroOwner wrote:...There are other reasons to believe that a natural log model best describes battery aging so with a bit of luck I will have a useable battery for some years to come.
CZeroOwner, thank you for your graphs and analysis. Agree that the log line is perhaps a better approximation, which bodes well for our future.

For myself, I've given up closely monitoring the car and am content with taking occasional samples such as last weekend's 50-mile highway excursion to the San Francisco Auto Show where I plugged in and had RR=84 before the drive home. Good enough, although this replacement battery only has about 30,000 miles on it with CaniOn reading 38.3Ah.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

datom
Posts: 9
Joined: Wed Apr 11, 2018 8:01 am

Re: Battery Capacity Testing

Tue Dec 04, 2018 11:04 am

Guys,
have a look here: https://docs.google.com/spreadsheets/d/1tHPZWJDgSr8zTlYx8h3Vzkc9iAvu376P0HnR_9_IHlc/edit?usp=sharing
I collected capacity and km values from the german C-Zero/In/I-Miev forum and put them into a spread sheet.

The top left figure shows Ah versus age in days, the top right shows Ah vs. km.
The bottom left shows Ah vs (km/age), the bottom right shows a histogram of daily driven km.

Best,
Tom

CZeroOwner
Gold Member
Posts: 25
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Tue Dec 04, 2018 1:04 pm

datom's German data shows a much more drastic decline in capacity than I'm predicting. I seem to remember reading that a Li-ion battery can have a long period with constant capacity followed by a sudden decline. The German data supports this.

CZeroOwner
Gold Member
Posts: 25
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Battery Capacity Testing

Sun Dec 09, 2018 12:47 pm

Image

The black dots in the graph above are battery capacities for 44 German owned iMiev/CZero/iOns collected by datom. There is a link in his post above. The green curve is my model (Ah = 50.4 -1.14*ln(km)) for battery aging discussed in an earlier post. It looks like my car’s capacity deteriorated more rapidly during the first kms than was normal for the German cars. On the other hand, my present mileage is 80,000 km and my battery capacity is now average or above average.

I suggested based on my own data that the natural log model was probably the best description of battery aging, but the German data doesn’t support this conclusion. In fact it looks linear more than anything else.

Of the 44 cars 16 owners reported more than one capacity measurement. These are shown as lines in the graph below.

Image

Some show a sharp decline in capacity early in the car’s life and others a slower decline in capacity later in the car’s life. However there is no statistical difference in the rate of decline between low mileage and high mileage cars. What is interesting is the wide range of the rates of decline. This may be due to variations in the manufacture of the battery cells but it could also indicate that there are things that we can do or avoid doing that will extend battery life.

In my case I bought my 2012 CZero in 2014 from a dealer. The car had no previous owner and less than 2000 km. Other than that, I don’t know how it was treated. When I first saw the car, it was in the dealer’s showroom with the charger connected. From 2000 km to 40,000 km we quick (Chademo) charged the car. Since 40,000km we have slow charged 70% of the time. I usually slow charge between the hours of 10 in the evening and 7 in the morning and the car isn’t charged up every day but on the average every second day.

Thanks to datom for the use of his data.

coulomb
Posts: 167
Joined: Sun Jun 10, 2018 8:32 pm
Location: Brisbane, Australia

Re: Battery Capacity Testing

Sun Dec 09, 2018 3:28 pm

CZeroOwner wrote: The black dots in the graph above are battery capacities for 44 German owned iMiev/CZero/iOns collected by datom.

Let's remember that this is the capacity as reported by the on-board BMS. The fact that range can miraculously be restored (real-world, achieved range) after the "battery smoothing" process suggests to me that perhaps the BMS is overly pessimistic about degradation. Perhaps Mitsubishi are happy with this; mostly this should be invisible to the user, and it causes less real depth of discharge, saving the battery (at the expense of potentially a lot of customer inconvenience).

This is one of the reasons that I'm attempting to reverse engineer the firmware. Progress is slow, unfortunately, mainly due to time availability.

What is interesting is the wide range of the rates of decline. This may be due to variations in the manufacture of the battery cells but it could also indicate that there are things that we can do or avoid doing that will extend battery life.

Indeed. Or it could be the BMS being overly cautious / pessimistic. It would be really good to know which it is.

[ Edit: removed graph from quote. ]

jray3
Posts: 1623
Joined: Tue Dec 06, 2011 1:05 am
Location: Tacoma area, WA
Contact: Website

Re: Battery Capacity Testing

Fri Mar 08, 2019 7:02 pm

Coulomb, thank you for your ongoing efforts. I'll throw in another data point. I'm planning to replace MR BEAN's battery with a junkyard salvage unit shortly after 100k miles, currently at 99,220 on the odometer.
Current stats according to my MUT3 clone:
Battery current capacity: 27.3 Ah
Battery maximum input power: 45.75 kW
Battery maximum output power: 63.50 kW
Voltage spread between high and low cells is now as high as 0.19 V.

DCFC sessions are running significantly longer these days with a degraded battery plus temperatures near freezing. The pack warms up nicely, perhaps too warm at 39 Celsius (102 F) on module 3 and ten degrees C cooler elsewhere in the pack.

I haven't noticed any significant reduction in power/acceleration, and the max output capacity of 63.5kW at low SOC certainly confirms that there's plenty of power to spare versus our 49 kW traction drive.

That means the old pack still has promise for mobile reuse. I plan to first try it as a trailer-mounted range extender, and then consider packing the cells into one of my collector EVs, where 30 miles of range would be easier to live with. MR BEAN is displaying 41 miles RR most frosty mornings, which is pretty much spot-on, as I often head for a DCFC 38 miles away and arrive with 1 or 2 bars.
2012 i-SE "MR BEAN" 102,600 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet

DBMandrake
Posts: 135
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Battery Capacity Testing

Sun Mar 10, 2019 1:49 pm

jray3 wrote:Coulomb, thank you for your ongoing efforts. I'll throw in another data point. I'm planning to replace MR BEAN's battery with a junkyard salvage unit shortly after 100k miles, currently at 99,220 on the odometer.
Current stats according to my MUT3 clone:
Battery current capacity: 27.3 Ah
Battery maximum input power: 45.75 kW
Battery maximum output power: 63.50 kW
Voltage spread between high and low cells is now as high as 0.19 V.

Just so you know, the maximum input power and maximum output power figures are not a measure of the health of the battery with respect to degradation, so you won't see any difference here between an old battery and a new one.

They're actually real-time figures used by the BMS to inform the MCU and other peripherals what the maximum amount of power the battery can supply or receive at any given time so that charging/regen/discharging are limited to remain within these safe limits. These limits are based on the current SoC and cell temperatures.

For maximum input power (charging, regeneration) this is severely limited at 100% SoC (only about 8kW) and gradually increases to 45.75kW by the time you get down to (approx) 80% SoC. It is also affected dramatically by temperature. Below 12C it is limited to no more than about 24kW, and below 0C it is down to around 12kW. (approximate figures off the top of my head)

This limit affects both regeneration and rapid charging, although I have noticed that at high SoC it still doesn't charge as fast as this maximum input power limit, however that may be due to me having a couple of cells with higher than normal internal resistance. (Which limits rapid charging speed)

High cell temperatures would also limit maximum input power however I think the cells have to be well past 40C before it will start to limit charge rate and it's rare to get the cells on these cars that hot. (At least in the UK!)

Maximum output power limits maximum acceleration, however this figure doesn't vary nearly as much. It does drop a little when you get down to about 10% SoC, (which is why turtle mode can limit acceleration) and it may also drop slightly at below freezing temperatures or cell temperatures above 40C, but only a little.

So looking at these two figures at a random SoC or cell temperature doesn't really tell you much of anything.

At cell temperatures between 12C and 40C and SoC between about 20% and 80% I would expect these two figures to be at their maximum.
DCFC sessions are running significantly longer these days with a degraded battery plus temperatures near freezing. The pack warms up nicely, perhaps too warm at 39 Celsius (102 F) on module 3 and ten degrees C cooler elsewhere in the pack.

If it was only the cell capacity that has degraded then rapid charge times would actually get faster. The thing that slows down rapid charging time is increased cell internal resistance, which is usually a sign of cell(s) that are starting to fail prematurely.

I know this first hand as in the last year two of my cells have dropped significantly in capacity relative to the others and their internal resistance is higher as well. This high resistance causes them to reach the peak cell voltage of 4.105 volts too early during a rapid charge (at a lower than usual SoC) and as a result the BMS is forced to throttle the rapid charge rate back sooner than it should have to, even though all the other cells could take a higher charging rate.

This can clearly be seen in the Canion voltage graph - the two high resistance cells get to 4.105v very quickly during the rapid charge while the rest are around 4.075v at the same charge rate - those other cells could take a much higher rate before hitting 4.105v.

When i first got the car it would maintain the full 43kW up to about 55% SoC and stay above 22kW up to about 70% in warm weather. (Cells 20-30C)

Now with two high resistance cells even if I start at 20% SoC in warm weather it will only remain at the full 43kW until about 30% SoC, and is down to about 22kW by 50% SoC, 11kW by about 70% SoC and 7kW by 80% SoC.

Charge time from 20% to 80% used to be about 18 minutes, now it's about 28 minutes with the last 10% being really, really slow! (And sometimes it shuts itself off at 70% ish instead of waiting until 82% like it used to, so I have to restart it to get to 82%)

If you have Canion it's fairly easy to test for individual cells that have high internal resistance - first you have to get the battery quite warm - all cells between about 20-35C, ideally 25-30C as that gives minimum cell internal resistance and maximum rapid charging speeds. Not easy in winter but possible if you do a couple of speedy motorway sessions with a rapid charge in between.

Before charging, take a note on Canion of any cells that have unusually low voltage compared to the rest at 20% SoC. These cells are ones with lower capacity and potentially (but not necessarily) have high internal resistance as well.

Put it on a rapid charge and watch the cell voltage graph - if you have a weak cell that also has high internal resistance what you'll see is that before the charge it had a lower voltage than most other cells but during the charge it's voltage will almost immediately go higher than most other cells - if it goes from lower than the others to significantly higher than the others quite quickly on a rapid charge this is a sure sign that the cell has higher internal resistance as it will cause the voltage to be excessive under charge.

If you continue to monitor it until the charge rate starts to drop below 43kW you would see the high resistance cells sitting pegged at 4.105 volts, while other cells may be 20-30mV lower. That's what I see on mine anyway.

If I could get my hands on a couple of good cells I'd be tempted to replace them, because while I can put up with some capacity loss (now down to 35.2Ah after a series of large drops from the 39.9Ah I had two years ago) the much slower rapid charging speed is a real pain in the behind when I do actually want to rapid charge.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

Gary12345
Posts: 27
Joined: Tue Feb 12, 2019 4:15 am
Location: Essex, UK

Re: Battery Capacity Testing

Mon Mar 18, 2019 7:28 am

Hi Simon,

I'm awaiting delivery of an 8-cell module, as long as the cells are good then i'll be selling most of them as I dont need them.

There is also a guy on ebay UK who sells them and someone in europe on facebook who strips down EV batteries and sells the cells (not on the sea shore...).

Thanks,

Gary.

krox
Posts: 4
Joined: Mon Apr 08, 2019 6:44 am

Re: Battery Capacity Testing

Mon Apr 08, 2019 7:34 am

Is there a software for checking the HV battery capacity for Android?

DBMandrake
Posts: 135
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Battery Capacity Testing

Mon Apr 08, 2019 8:26 am

krox wrote:Is there a software for checking the HV battery capacity for Android?

There are several.

Canion, Hobdrive, and Evbatmon will all give the HV battery Ah capacity.

Canion is free but requires a more expensive STN11xx adaptor (such as OBDLink LX) and will not work with a cheap ELM327 adaptor.

Hobdrive has a free version and will work with a cheap ELM327 adaptor so is probably the cheapest outlay to read the battery capacity if you already have an Android device.

I have used both apps and they are both useful tools to have besides just giving the Ah reading.

EVbatmon is expensive and won't do anything else other than give battery capacity so I can't recommend it.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

Return to “Batteries and Battery Management”