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 Post subject: Improve battery with cell replacement?
PostPosted: Thu Jul 23, 2020 7:00 am 
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Joined: Mon Jun 29, 2020 3:56 am
Posts: 2
I haven't found a similar topic on this forum, apologies if it's there and I have missed it...

I have a MY13 with 223 000 km on clock, SoH 25.4 Ah. According to PhevWatchdog statistics, this significantly below the expected SoH at this milage. So I am wondering if there is a reasonable way to improve capacity with replacing the weakest cells in the package. The BMU was reset 400 days ago by the previous owner.

According to PhevWatchdog the maximum cell voltage difference ever measured is 88mV, but from these overall statistics it is still not clear to me which cells might worth to be replaced, if any.

I have already ordered a miniVCI, and plan to use it with MUT 3, so I was wondering if I should perform a DBCAM, and check the cell voltage at the low end of the DBCAM process, and also check which cells stop charging first during the charging phase. But how could I log the cell voltages during the DBCAM? As far as I know PhevWatchdog only logs the min and max voltages, but not all the cell voltages separately.

-what might be the best way to identify cells to be replaced?
-does this plan make sense at all? (replace the weakest cells -> BMU reset -> smoothing -> DBCAM)

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 Post subject: Re: Improve battery with cell replacement?
PostPosted: Sun Aug 02, 2020 2:28 pm 
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Joined: Fri Nov 13, 2015 3:05 am
Posts: 83
Not familiar with your software, but there's two fairly standard ways to "eyeball" a weak cell.

1. Testing remaining capacity. A cell that has reduced capacity will be first to fill as well as first to empty. Most reliable way is to get the battery perfectly top balanced, and watch it as it discharges. The ones that drop in voltage fastest are the candidates for being most degraded. However, allow it to rest before making judgement, as internal resistance can drag some cell voltages lower than its capacity might suggest. A few seconds of no load will fix that.

2. Speaking of internal resistance, get the battery to an even state, and put a high load on it. The ones that drop in voltage more are likely to be more worn.

Both of these are "eyeball" methodologies, and without cycle testing, it's still just a best guess.

I dropped off this forum for a while, but came back as I am thinking of adding in a secondary battery. Mine is a MY14.5, and this (Australian) winter, is the first one I have noticed a significant loss in range. So I will probably be doing the same diagnoses as I am recommending you as soon as I either find or get a new OBD-II connector. Also trying to find a cheap MUT3 box too, as I understand you can use it to force a balance.


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 Post subject: Re: Improve battery with cell replacement?
PostPosted: Sun Aug 02, 2020 11:25 pm 
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Joined: Mon Jun 29, 2020 3:56 am
Posts: 2
thank you very much for your reply Sunder!

Currently I am using PhevWatchdog, which is an absolutely amazing tool, but during discharge it can only register the voltage and number of the highest and lowest voltage cell, not the voltage of all the cells. I have ordered a miniVCI, with that I should be able to use MUT 3, but I don't know whether MUT3 is capable of registering all cell voltages during discharging.

What tool would you use to register individual cell voltages during discharge?

For test #1, do I understand correctly that first the car should be charged fully(until EVSE shows zero Amp load, so balancing has also completed), then discharge it for some time, then stop the load, leave it rest for a couple of minutes, then compare the cell voltages?

For test #2, does "getting the battery to an even state" means the same as with #1, full charge, until balancing completes, right? Then high load can be some intense acceleration, DURING which the voltages should be registered, right?


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