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mwalsh
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How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 2:30 pm

The other day I was having some real range issues. I got to work with ever so slightly less than I would normally see, but going home my range was falling WAY more rapidly than it should have. When I eventually got home (I had to take it easy the last 10 miles), I unplugged the Gidometer and plugged my ELM in instead, only to see that the voltage range between the lowest and highest cell was a whopping 68mV! It is HIGHLY unusual for my pack to be over 20mV, and it's generally between 15mV and 17mV. So pretty obvious why I ran low on charge much earlier than I would have expected.

But the question is...how can I mitigate this in the future?

Well, clearly it would help if I was using the ELM all the time, instead of the Gidometer. If I'd seen my pack was either out of balance already before leaving home, or noticed that it was edging towards becoming unbalanced at the mid-point, I might have been able to do something about it. But I do have my issues with the Android device - the first of which is I find it difficult to read the screen while wearing polarized glasses. Perhaps an OBDII splitter might be the answer to that (if there is such a thing?), so I can keep both devices going at once.

But the bigger question is...what exactly can I do if I found my pack appeared to be badly out of balance? Would any charging event attempt to re-balance it, assuming the BMS sees the imbalance (and hopefully it would!)? Would it need to be a charging event from a relatively low SOC? Would I have to just tough it out until the next time the BMS decides a rebalance is needed or could I force the BMS to rebalance?

Edit: Oh, one nugget I can give you is that my pack "seems" to be more out of balance on Mondays, especially after it's sat at a relatively low SOC all weekend because I haven't needed to use it.
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Stoaty
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 2:55 pm

mwalsh wrote:But I do have my issues with the Android device - the first of which is I find it difficult to read the screen while wearing polarized glasses. Perhaps an OBDII splitter might be the answer to that (if there is such a thing?), so I can keep both devices going at once.
I tried to run two devices at once with a splitter. Didn't work and was told I shouldn't do that. My suggestion: Get the LeafDD. You can easily read Gids and pack temperature on screen 1 (even with polaroids on), can switch to screen 3 to see max, min and average cell voltages.
But the bigger question is...what exactly can I do if I found my pack appeared to be badly out of balance? Would any charging event attempt to re-balance it, assuming the BMS sees the imbalance (and hopefully it would!)?
Apparently it is balancing all the time, but the best balance if you need the full range is to charge to 100% and keep the charging going for several hours afterward, during which time it should balance. It may also be that the pack is out of balance because you have a weak cell which is exposed at low SOC. What is the balance like at 80% or 100% charge?
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mwalsh
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 3:01 pm

Stoaty wrote:What is the balance like at 80% or 100% charge?
At 100% it's spot on. Or at least it always has been when I've sporadically checked it. 80%...no idea. 80% charge doesn't work for me, so I never go there.
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KJD
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 3:06 pm

How often do you see LBW ?

How often do you see VLBW ?

Do you normally charge with L1 or with L2 ?

The more often you see a low SOC, the more often the pack will be out of balance. Every car is different, but my car will balance better if I run it down to LBW then recharge to 100% using the 120 EVSE. It also seems to help to let the pack cool off for several hours after driving it. End only timer works well for this.
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mwalsh
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 3:26 pm

KJD wrote:How often do you see LBW ?
Every day.
KJD wrote:How often do you see VLBW ?
Rarely (for now).
KJD wrote:Do you normally charge with L1 or with L2?
L2. Could probably do L1, given that I have 15 hours from needing the car one day to needing it the next. Except that I'm too lazy to drag the L1 charger out every day, and the pack battery wouldn't get any rest/cooling off time.
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TomT
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 4:26 pm

I run two devices with no problems whatsoever using a splitter, the Elm and the Lincomatic V2.
Stoaty wrote:I tried to run two devices at once with a splitter. Didn't work and was told I shouldn't do that.
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KillaWhat
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 4:36 pm

TomT wrote:I run two devices with no problems whatsoever using a splitter, the Elm and the Lincomatic V2.
Stoaty wrote:I tried to run two devices at once with a splitter. Didn't work and was told I shouldn't do that.
I also run a splitter, with a Leaf DD and a WattsLeft meter.
No Problems at all.
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Nubo
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 6:36 pm

I only had this happen once (range falling more rapidly than expected near the low end), so I attributed this to an imbalance condition. A couple of full charges (including a 4-hour "balancing" opportunity), seemed to do the trick. When balancing takes place you'll get a 2nd "charging complete" message if you have notifications set in Carwings. I got that notification on 2 subsequent attempts, but not the third, so it seemed I needed 2 sessions to get back to normal.

I have made it a point since then, to give the car a "balancing opportunity" once a month, which includes the 100% charge and at least 4 hours of plug time afterwards. Even if I don't specifically need the full charge. Sometimes I get the 2nd message; more often not. The car seems to know when a rebalance is needed, and just needs the opportunity to exercise it.

I don't have any kind of Gid-meter, fwiw.
I noticed you're still working with polymers.

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Boomer23
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Re: How to mitigate a badly out of balance pack?

Thu Nov 14, 2013 7:56 pm

Speaking to the root of the problem, Mike, I think that you only saw the large variance in cell voltage because you were at very low charge. In my experience, I've only seen a large variance at very low SOC. So my guess is that you wouldn't have seen that large gap earlier in your trip, even if you had been running the ELM app.

A couple of other comments. I run the app on an old Droid X2 and a newer Samsung SCH-I405, and neither one has a problem with viewing the screen with polarized shades. The screens on both of these phones seem to have the interference set up at a 45 degree angle, rather than on portrait or landscape orientations. You might keep an eye open for an opportunity to swap your Android device with someone else who doesn't need to view the display with shades on (slim chance, but I thought I'd throw it out there).

And lastly, I'd concur that the best balancing opportunity is to have the car charge to 100% with no end timer set and allow the car to remain attached to the J plug for a few hours after the charge is complete. Doing this for a couple of charge sessions has done the trick for me.
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DaveEV
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Re: How to mitigate a badly out of balance pack?

Fri Nov 15, 2013 1:26 am

Boomer23 wrote:Speaking to the root of the problem, Mike, I think that you only saw the large variance in cell voltage because you were at very low charge. In my experience, I've only seen a large variance at very low SOC. So my guess is that you wouldn't have seen that large gap earlier in your trip, even if you had been running the ELM app.
Yep, 68 mV isn't necessarily out of line, depending on exactly how low the SOC was. So what was it? Screenshots are a great way to get all the data across.
Boomer23 wrote:And lastly, I'd concur that the best balancing opportunity is to have the car charge to 100% with no end timer set and allow the car to remain attached to the J plug for a few hours after the charge is complete. Doing this for a couple of charge sessions has done the trick for me.
Do you have any before/after data to show the effect?

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