DBMandrake
Posts: 147
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: 147
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: 147
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: 147
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: 930
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

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