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Re: Is my battery dying ?

Posted: Thu Apr 18, 2019 2:45 pm
by Gary12345
Oh and i've got a dual 10A RC charger and a separate 10A discharge capacity tester, so in ~5 hours I can do 1 full charge, 1 full discharge and a storage charge (50% charge once they have been tested).

Re: Is my battery dying ?

Posted: Wed May 15, 2019 5:27 am
by DBMandrake
Gary12345 wrote:I will charge to 4.1v and then discharge to 3v. The Yuasa spec sheets show that they test between 2.75-4.1v but there is very little below 3v. That should give me a good idea if the cells are usable in a car.

Curious to know whether you have received your block of 8 cells yet and if so whether you have had time to test their Ah capacity ? I'm still toying with the idea of replacing four of my cells. Two are quite bad - both high internal resistance and significantly lower capacity than most of the others, a 3rd is nearly as bad capacity wise but resistance seems OK, and a 4th is not far behind the three worst ones. There's then a significant gap to the next worst cell with most of the remaining 84 cells being relatively closely grouped so probably not worth replacing any of the others.

I removed, fixed and replaced my battery pack on the drive without a hoist - its fairly involved but its definitely doable - I had the car up on 3 concrete blocks (the 4" thick ones) under the wheels so the whole car was 12" up, removed the big orange fuse then the thick orange cables, then bolted trolley wheels on a wooden pallet, put this under the car, used 2 trolley jacks with 2x4" on top jacked up underneath the front and back of the pack, then undid the pack bolts and electrical connectors and carefully lowered the jacks with the wood and battery pack on top down onto the wooden pallet, then slid the battery pack out.

I've been following along and helping with the diagnosis in this thread where a CMU is being replaced:

https://www.speakev.com/threads/c-zero- ... ir.137750/

He has tried two different approaches - both which seem doable to me. The first was to raise the car up high on ramps and stands and then use a hand operated hydraulic pallet mover to lower the pack down and withdraw it from the car. So as long as you can hire one for a weekend this seems like one way to do it, although lining everything up when raising it up again could be fiddly.

Later in the thread he came up with a second method where four of the main mounting bolts are replaced with very long threaded rods with a spacer tube and nut such that once fitted you remove all the other bolts and simply unwind the nuts to lower it down onto a small trolley.

Slower than the hyraulic lift, but very minimal resources required, and in theory it should make getting the battery back up into exact alignment easier as it will be getting lifted up on the threaded rods into perfect alignment. I could definitely see myself able to use this approach so I'm now considering again whether I should do a cell swap to prolong the useful life of the car.

My Ah capacity is down to 34.0Ah now and still falling relatively rapidly, DC rapid charging speeds are much slower than two years ago (due to the two high resistance cells reaching peak voltage too soon) and I believe that were it not for the 4 bad cells the capacity would still be up around 38Ah now.

Tempted...

Re: Is my battery dying ?

Posted: Fri May 17, 2019 4:17 am
by Gary12345
Sorry my cells are still not here, i'm relying on a favour to get them shipped 400 miles for free, so basically have to wait until the guy is close to me to get them ! Hoping for soon.

Thanks.

Re: Is my battery dying ?

Posted: Tue May 21, 2019 7:34 am
by DBMandrake
In the last few days I reset the BMS in my car, as described in the following thread:

viewtopic.php?f=23&t=3943&start=60#p38994

In hindsight not a clever idea because it resets the Ah capacity back to the factory default of 45.8Ah, and the car is very slow/reluctant to re-learn the true capacity when it differs so much from the factory new capacity! :( (On a positive note, we now know that the factory new capacity of these packs is 45.8Ah! Thus we can confidently use this figure to calculate % SoH)

Due to this, the BMS grossly over estimates the range of the car and will try to drive the battery right down to the 3.0 volt cutoff point at which point you are very suddenly restricted to 7mph. Not good! So to solve this in a timely fashion I had to do the battery calibration procedure as detailed in that thread. For anyone wanting to force the BMS to re-measure the Ah capacity of the battery in a safe way - just do the battery calibration procedure under maintenance, not the full BMS reset! (battery replacement procedure)

I decided to analyse some of the new data I've found from the latest calibration and recent Canion voltage graphs regarding the state of my cells. Earlier in this thread I noted that I have two cells that are particularly weak and a third that is well on the way to being weak, and that my suspicion for a while has been that these two or three cells are responsible for the majority of the seemingly accelerated capacity loss I've had over the last year, and I think I have the data to prove that now.

So first some Canion voltage graphs. This first graph I've posted before and was taken at 10% SoC in April 2018 at about 41k miles, which was when I first started to notice abnormally fast drops in Ah:

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You can see there is a 50mV spread at 10% SoC - not terrible, but not good either. Fast forward to October 2018 at about 47k miles and it is now a 125mV spread for the same SoC. Ouch:

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About a week ago I recorded this voltage graph at a slightly higher 14% SoC, and at 52k miles. The spread is now 135mV at 14% SoC - as the spread increases at a lower SoC it's worse than it looks at about 10% SoC it's more like 150mV now. I've highlighted in red the cells that I have particular concerns about and propose replacing:

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Also of note is that some cells have high internal resistance. This can be tested by discharging the car to a relatively low SoC like 20% then Chademo rapid charging the car. Cells with high internal resistance will try to go over voltage very quickly. Here is a voltage graph taken only a few minutes into a Chademo charging session where the charge rate is already throttled back due to Cell 25 hitting the maximum allowed voltage. I've highlighted three cells that I have concerns about high internal resistance, not surprisingly 3 of the same 4 cells from above:

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The difference between 4.075v and 4.105v might not seem like much, but when repaid charging this can mean the difference between pushing the good cells up to 43kW (before reaching 4.105 volts) and having to throttle way back to 21kW to keep the high resistance cells from going over voltage.

In short, the weak cells are seriously limiting rapid charge speeds as well. In fact no matter what SoC I start at my charge rate only remains at 43kW for less than a minute, as shown in the following graph where I started charging from about 30% SoC - note that it doesn't even last 30 seconds at the full 43kW before throttling back, and it is painfully slow by even 55%:

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So I've known for a while the difference between best and worst cells is getting progressively worse but I couldn't put a quantitative figure on it as I can't directly relate the relative cell voltage near discharge to the capacity of the cells.

After running the battery calibration routine I've realised that it actually measures the individual cell capacities and reports the minimum and maximum cell capacities of the pack! And I also ran this calibration and kept the results about a year ago so now I have two data points. So a year ago it reported this:

Image

And now today:

Image

So in the last 12 months and 12k miles the best cell has degraded only 0.5Ah (better than I would have expected!) while the worst has degraded 3.3Ah. So the spread has increased from 2.9Ah to 5.7Ah in the last year. :(

I've added these new data points to my excel spreadsheet and graph and have come up with the following interesting display of the data:

Image
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The dotted blue line is actual Ah measurements taken by Canion or Diagbox, with the yellow line being a linear interpolation of the degradation rate. The red line shows the degradation rate of the weakest cells as reported by the battery calibration routine which pretty much exactly follows the yellow line.

The green line shows the degradation rate of the best cells in the pack - which is dramatically slower than the worst cells, and you can clearly see the gap between the green and red lines opening up...

The question is, if I replaced those 4 cells, what would the new minimum capacity be ? Since the weak cells seem to be significant outliers I think I could recover at least half the lost capacity if I replaced them with better cells - I'm estimating that it would go back up to about 38Ah, and more importantly and hopefully arrest the rapid degradation that I'm seeing at the moment. If I could get it back up to about 38Ah and reduce the degradation rate back to what it was before 40k miles it would add an extra several years of useful life to the car for me.

The way the weakest cells are degrading at the moment I will not be able to make my 35 mile daily commute next winter without a rapid charge on the way home every day - and at the reduced rapid charge rates I'm now seeing, painfully slowly...