DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Wed Jul 03, 2019 1:33 am

As expected, some time after the battery calibration the BMS has made an upwards adjustment of about 1Ah from 32.7Ah to 33.6Ah.

So it seems that when you do the full battery capacity measurement in Diagbox it initially sets the "usable" reported Ah capacity to about 2Ah below the actual measurement of the worst cell in the pack, (33 vs 34.8) and then some time later it will adjust that so that the usable is about 1Ah below the actual capacity of the worst cell.

Here are my latest figures:

Image

Image

Beside the one off upwards correction it is still counting down 0.1Ah fairly regularly, I've seen it drop 0.4Ah in less than a month, which is a lot higher than expected. In short my weak cells seem to be failing rapidly so I've decided that I am going to attempt a cell swap of the four worst cells.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Thu Aug 01, 2019 1:34 pm

So I finally bit the bullet and committed to replacing four cells in my pack, as the precipitous loss of capacity at what I believe is an accelerated rate continues, and I know that I'm going to really struggle for range this winter if I don't do something about it, not to mention the slow rapid charging the high resistance cells cause.

I ordered them from Second Life EV Batteries in the UK, for £212 including shipping and they arrived in a few days.

I can't say that I'm terribly happy with the packaging they arrived in - basically two layers of cardboard box and some bubble wrap, I was expecting some sort of wooden or otherwise protected box, as one serious drop by the courier would have done significant damage to the cells... :roll:

As it is, they look OK so I have started testing them one by one:

Image

As my charger/discharger is designed for considerably smaller cells and has a maximum charge rate of 6 amps and maximum discharge of 2 amps it's going to take a while, probably about a week to test all 4 cells by the look of it. :lol:

The cells arrived charged to 3.96 volts, so first I have to charge them up to 4.1 volts, which took about 8 hours due to the charge rate tapering off more than it should due to the resistance of my charging leads, (as this prematurely puts the charger into constant voltage mode when the actual cell is not yet to 4.1 volts) and this added about 12Ah of capacity, so from that the cells must have arrived at approximately 75% SoC.

I'm now discharging the first cell to 3.0 volts and after more than 12 hours it's still at 3.88 volts and has discharged about 18Ah so I should know the result for the first cell by tomorrow. After that I will then be doing a storage charge to bring the cell back up to 3.8 volts for storage until it's ready to go into the car, and when I swap the cells I'll discharge the pack in the car until the good cells are also at 3.8 volts (which is approx 30% SoC) as it's far quicker and easier to bring the cells in the car to the same SoC as the replacement cells than doing so with the slow charger.

I plan to use the method discussed over on speak EV of lowering the battery pack down on threaded rods to a small trolley which I will then wheel into the work area in my garage to disassemble the pack out of the weather. This is all conditional on the cells having adequate capacity after testing - otherwise they're going back!
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Fri Aug 02, 2019 9:41 am

Took a while but the first replacement cell has been tested and I'm glad to report it's in good health. :)

I'm actually recording two different Ah capacity figures from the test of each cell - one from 4.1 volts down to 3.63 volts, and another figure discharging all the way to 3.0 volts.

The reason I did this is because while the raw cell capacity of 50Ah claimed for a new cell according to the data sheet is actually based on a discharge to 2.75 volts, (my charger will only discharge to a minimum of 3.0 volts so I used that) the car itself only ever discharges the cells down to about 3.63 volts or 320 volts for an 88 cell pack, and at 3.63v per cell the pack is considered to be at 0% SoC and the car will shut off.

This is probably done to make sure that the voltage under heavy load does not dip below 3.0 volts, as it would if the unloaded voltage was allowed to go much below 3.63 volts.

So while a discharge to 3.0 volts will give an idea of the raw capacity of the cell outside of the car and in relation to Yuasa's specs, a discharge to 3.63 volts should give a more realistic "usable" capacity that probably matches up with that reported by the BMU, and is thus the figure that matters when compared to what the BMU is reporting.

As it turns out there's only about 3Ah of capacity between 3.63v and 3.0v anyway, so effectively the car is leaving about an 8% buffer below "0% SoC", or perhaps a 10% buffer if you compare it to a discharge to 2.75v. The following voltage curve during a 1.3A discharge shows that 3.63 volts is very close to the "knee" in the voltage curve where the voltage drop starts accelerating:

Image

I've highlighted 3.58 volts on the graph to obtain the time marker not 3.63 because there was about a 5mV drop in voltage across the cables at the 1.3A discharge rate so the cell measured at the terminals was actually 3.63 volts at this time. Looking at the same time mark on the capacity graph we can see that the usable capacity down to 3.63 volts is almost exactly 40Ah:

Image

Full capacity of the cell discharging to 3.0 volts is 43.573Ah. As the BMU reset capacity of the cells is 45.8Ah that means this cell has a SoH of 40/45.8*100 = 87.3%.

Also 43.573/50*100 = 87.1% - a very close match, which fits my theory that 50Ah is the full capacity of the cell when new and 45.8Ah is the "usable" capacity of a new cell in a car due to only discharging to 3.63 volts in operation.

According to the BMU battery calibration I did recently the very best cells in my pack still have a usable capacity of 40.5Ah and the worst ones that I am replacing are down around 34Ah, so the new cell regardless of whether you take the 40Ah or 43.6Ah figure is at least as good if not better than the best cells in my pack, let alone the bad ones they're replacing. :mrgreen:

I'm now doing a 3.8v storage charge of the cell before moving onto the next one, and fingers crossed the other ones are as good as the first...

By the way if you're wondering why my graphs don't begin at zero capacity and the 4.1 volt starting voltage its because the discharge cycle took so long at 1.4 amps that the time counter on the charger unit actually reached 65535 seconds and overflowed, resetting back to 0 seconds and starting the graphs anew from that point. :lol: Fortunately the Ah capacity figure did not reset or rollover as well.

So while the summary page says the discharge took 54908 seconds, it was actually 54,908 + 65,535 = 120,443 seconds or about 33.5 hours. :twisted:
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Mon Aug 05, 2019 5:33 am

Second cell tested and also good - within 0.1Ah of the first cell - probably within the margin of error for the measurement technique. I'm surprised in fact at how closely the first two cells tested are matched.

It makes you wonder what happened to the cells in my pack to cause the best and worst ones to diverge by more than 6Ah despite being in much the same environment - identical charge and discharge current since its a series string, only the temperature of the different cells would differ, and even then they would only differ by more than a few degrees if the car is being driven hard and/or rapid charged a lot - and I rarely rapid charge.

Manufacturing tolerances/defects that only come to light after a certain amount of cycling ? Different degradation profiles due to uneven temperature distribution around the pack ? Repeated deep discharges causing any initial tiny imbalance to be blown out of proportion over time ? (A cell already weaker will be discharged to a lower SoC than good cells and will suffer incrementally more degradation as a result, increasing the spread between good and bad cells progressively)
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

kiev
Posts: 1012
Joined: Sun May 03, 2015 7:15 am
Location: The Heart o' Dixie
Contact: Website

Re: Is my battery dying ?

Mon Aug 05, 2019 7:11 am

According to user piev, all cell failures are due to a lack of electrolyte. He has studied numerous technical papers looking at cell life factors and boiled it down to this simple conclusion.

These battery cells operate by reversible chemical and electrochemical reactions involving the anode, the cathode, the electrolyte, and a microscopic SEI layer. Every cycle consumes a tiny bit of electrolyte and eventually things get out of balance.
kiev = kenny's innovative electric vehicle

SylphH
Posts: 1
Joined: Tue Sep 25, 2018 7:42 am

Re: Is my battery dying ?

Wed Aug 28, 2019 6:35 am

Hi DBMandrake,

I'm wondering how you have gone with this transplant of the 4 replacement cells?,
I've been keenly following your posts about this, as I have a 2010 i-MiEV which I am considering individual cell replacements for.
I have access to a hoist and ease of removing the battery though, :p, so it's more about purchasing good second hand cells (I'm based in Melbourne, Australia).

Did you finish your cell testing of these 4 cells?, and did you swap them into the existing pack?
Cheers!
:mrgreen:

DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Fri Aug 30, 2019 8:15 am

SylphH wrote:Hi DBMandrake,

I'm wondering how you have gone with this transplant of the 4 replacement cells?,
I've been keenly following your posts about this, as I have a 2010 i-MiEV which I am considering individual cell replacements for.
I have access to a hoist and ease of removing the battery though, :p, so it's more about purchasing good second hand cells (I'm based in Melbourne, Australia).

Did you finish your cell testing of these 4 cells?, and did you swap them into the existing pack?
Cheers!
:mrgreen:

Yes, I finished testing all the cells a few weeks ago - as I could only charge them at 6 amps and discharge at about 1.3 amps it took nearly a week to test all four, and by then I forgot to update this thread... :twisted:

The results were:

USABLE capacity 4.1 to 3.63 volts (3.58 during 1.3A discharge due to cable resistance) and FULL discharge to 3.0 volts.


Cell A - 40.0Ah USABLE, 43.573Ah FULL
Cell B - 39.9Ah USABLE, 43.471Ah FULL
Cell C - 40.8Ah USABLE, 44.543Ah FULL
Cell D - 40.6Ah USABLE, 44.353Ah FULL

As mentioned earlier I suspect the BMS will report closer to the first figure for each cell, but in any case that is still good as the very best cells in my car are only 40.5Ah and the worst ones are around 33Ah.

The hold up now is getting a small trolley built to go under the battery pack to wheel it out (fairly easy, some timber, caster wheels etc, as the pack is only 164Kg) and figuring out a way to get the car up good and high yet still stable. (Not so easy)

Rather than trying to jack it up approximately 50cm on the axles (as I don't really have anything to put under the axles that would be stable at that height) I think I'd rather raise it up on the wheels, for example lower the wheels down onto some wooden sleepers (which I have lying around the garden) however I need to get it at least 30cm higher off the ground to have enough clearance which would be three sleepers high, so I'm not sure how easy or stable that would be.

So I'm still trying to figure out an easy and practical way to get the car up a lot higher than usual on a driveway - normal stands/ramps aren't high enough (only about 20cm) unless I put them on top of a layer of sleepers. I suppose I could get a second set of ramps so I have ramps for all wheels and jack the car up and onto the ramps on top of sleepers.. :?

If anyone has any good ideas of an easy way to get the car raised up about 30-40cm higher than normal I'm listening...!

The last couple of months I've been strapped for cash so after buying the cells I had to put the project on hold for a bit but the winter weather will be here soon so I need to get a rattle on and get it done now.

BTW if you're swapping individual cells, I presume you've diagnosed some individual low capacity cells using Canion as I have ? Do you have a MUT-III or have access to one ?

After a successful cell swap you'll need to initiate the "battery calibration" routine so that it will correctly re-measure the capacity of the pack as a whole and update the working Ah figure. If you make a big improvement to the usable capacity of the pack it will not learn this properly without the intervention of the battery calibration routine, which means you'll get little or no increase in usable range as the BMS won't allow you to use the extra capacity.

As I have the PSA version of the car I use Diagbox/Lexia 3 to do this.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

WReed82
Posts: 49
Joined: Wed Nov 28, 2018 10:05 am

Re: Is my battery dying ?

Fri Aug 30, 2019 12:46 pm

Hi Simon,

Here in the USA 12" wide x 16" long x 8" high concrete blocks cost about $3 each.
A stack of two of these with a piece of 3/4" ply on top gets you up to around 16" , or about 40cm.

Including the plywood cost would be under $50.
I would take some fiberglass-reinforced tape and tape together the blocks and plywood.

If you can scrounge/borrow some blocks to stack, and some plywood to cut, cost could be lower.


Cross-stacking the sleepers you have could work well.

Are you anywhere near Aberdeen?

Thanks and good health, Weogo

CZeroOwner
Gold Member
Posts: 34
Joined: Mon Jan 15, 2018 1:47 am
Location: Denmark

Re: Is my battery dying ?

Wed Sep 25, 2019 4:11 am

A couple of weeks ago our CZero was in for service. Part of the service was a measurement of the battery capacity. I’ve written the results up in a post on the battery capacity testing discussion. The shop also told me that their test instrument showed that the expected capacity of a new battery is 48Ah. While this is less than the nominal capacity of 50Ah, it is more than the 45.8 reported by the Diagbox. I still believe that a new cell right off the Yuasa assembly line at an operating temperature of 30oC has a capacity of 50Ah. However in the few months it might take to get the cell to the car and the car to the dealer that capacity may very well decrease to 48Ah due to storage degradation. And if the operating temperature is less than 30oC when tested the battery might show as little as 45.8 Ah. In other words, the measuring instruments are programmed so that we will not be disappointed when we measurer the capacity of a new battery.

DBMandrake
Posts: 223
Joined: Mon Mar 20, 2017 4:57 am
Location: Scotland

Re: Is my battery dying ?

Wed Sep 25, 2019 4:26 am

CZeroOwner wrote:A couple of weeks ago our CZero was in for service. Part of the service was a measurement of the battery capacity. I’ve written the results up in a post on the battery capacity testing discussion. The shop also told me that their test instrument showed that the expected capacity of a new battery is 48Ah. While this is less than the nominal capacity of 50Ah, it is more than the 45.8 reported by the Diagbox. I still believe that a new cell right off the Yuasa assembly line at an operating temperature of 30oC has a capacity of 50Ah. However in the few months it might take to get the cell to the car and the car to the dealer that capacity may very well decrease to 48Ah due to storage degradation. And if the operating temperature is less than 30oC when tested the battery might show as little as 45.8 Ah. In other words, the measuring instruments are programmed so that we will not be disappointed when we measurer the capacity of a new battery.

I think you're overthinking this and you'll find the difference between 48Ah and 45.8Ah is simply down to choosing the minimum discharge voltage when performing the test. In other words you'll get a different result depending on how you define the test...

The Yuasa datasheet states the cells are 50Ah if discharged all the way down to 2.75 volts. The car does not do this! ;) The car only discharges the cells down to about 3.63 volts, which is well above the minimum voltage that the cells can be taken down to. There are multiple reasons for this - one is that this is open circuit voltage, and the cell has to maintain enough charge so that the cell voltage doesn't go dangerously low under full load from the motor. Therefore you have to be more conservative.

When I measured my 2nd hand replacement cells (still waiting to go in the car) If I measured down to 3.63 volts I got around 40Ah, if I measured down to 3.0 volts I got around 44Ah. So there is at least 10% of the cells potential capacity below the 3.63 volts that the car is willing to go down to.

The BMS Ah figure reported by Diagbox will almost certainly be based on the usable capacity between 4.1v and 3.63v, not 2.75v. However if the cells are tested on an external discharge tester that will take them to a lower voltage you'll get a higher Ah figure that more closely matches the manufacturers claims for the cell.
- Simon

EV: 2011 Peugeot Ion
ICE: 1997 Citroen Xantia V6

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