iMiEV Battery Cell Balancing

[JoeS]

I opened up this thread so we can find it in the future. Too much good information on this Forum is buried in unrelated thread topics.

Whenever the iMiEV battery is fully recharged, the last hour or so is spent by the battery management system individually bringing each cell (we have 88) up to its proper voltage and stabilized current. This is called "top balancing" This manifests itself to us as a decreasing ac input current into the car's charger.

To properly do this, Mitsubishi has recommended (in a February 2012 letter to US owners) that the battery be taken down to two bars and then left alone to fully recharge, and that this be done once in the first year and then once every other year thereafter.

Now, here's a slightly different perspective:

Citroen's manual says that at least once in two weeks you should fully recharge your battery.

Adding to the confusion is a statement someone had made that when the car is down below two bars and in turtle mode that "bottom balancing" is taking place, something I have not seen substantiated anywhere.

So, Zenelec, what is the year of your Citroen C-Zero and is there a date on your manual and exactly what does it say? This entire area of care and feeding of lithium batteries is evolving (witness the 787 debacle), and maybe there is some updated information out there we should all know about?

 

[Zelenec]

Well, I think there are two possible reasons for differencies:
A) PSA (Citroen + Peugeot) just don't know what they are selling!
B) Battery in EU version isn't the same as the I-one.

I prefer answer A. When I took test vehicle in last august, nobody was able to answer some simple questions about regenerative braking, what is the best battery usage, and many more basic issues, salesman or at least tecnician really should know. I waited for every answer few days and some of them were pretty dumm.

 

[Don]

If you could post exactly what your owners manual says about it, we could possibly make an informed decision as to which of your options is the correct one. Don't bother with anything anyone at the car dealership told you . . . . we know that most of that is just garbage

Don

 

[Zelenec]

Joe, if you're asking for default battery maintenance, the manual and service officer say for the first year:

Obligatory full charging at least every 15 days! (My friend is really pro in Li-Ion batteries and found that very strange.)
After one year or 20.000 km (which comes first) cells balancing!?!
That's all regarding battery. Any other info I found on internet.

 

[Zelenec]

I found online Manual: see page 85

 

[Barbagris]

And same for Peugeot:

To optimise the service life of the main battery, recharge it completely every 15 days.
For a recharge to be complete, follow the procedure for a normal recharge (domestic power point) without interruption, until it switches off automatically (charging lamp goes off in the instrument panel).

(Pdf, page 16 and 83)

 

[Barbagris]

For me it is hard not to make a full load.

My daily commute is about 50-60 km (30-40 mi), in a mountainous area, which eat half or more of the battery (so I recharge daily)

Because electricity is much cheaper at night, I leave it plugged in to charge with a timer. The charger put in little more than 3 kw (220V 16A), and in three hours or so the battery has all its bars, and cells begin to balance.

And at that time I am very sleepy :lol: This means that in the morning the car always has battery at 100% and cells matched.

The seller said there was no problem, is designed for it... but I suspect sellers know less of their products than buyers...

 

[JoeS]

Zelenec, and Barbagris thank you for posting the link.

I find myself distrusting this information, especially as the instructions in the second paragraph go against Lithium-ion conventional wisdom which is that for long-term storage the battery SOC should be kept in the 25%-35% region. It is deleterious to the battery to keep it fully charged, especially in a hot climate.

Even though fully charging on a daily basis and then driving the car shortly thereafter is probably not an issue (especially since we know the full charge is safely below the maximum cell capacity), if you read some of the Leaf forum information on battery management you'll understand our concern. Some of us plan to keep our iMiEVs for a very long time...

Here is the Mitsubishi US letter on this topic, posted by Wee John:

http://myimiev.com/forum/viewtopic.php?p=1031#p1031

Perhaps further queries to Citroen/Peugeot by our European friends may be in order?

 

[Don]

The once every two weeks advice is also found in the owners manual for our USA version iMiEV's on page 1-19 - "Fully charge the vehicle by regular charging every 2 weeks"

It sounds like they want the BMU to be able to do it's complete cycle at least once every two weeks . . . . for proper cell balancing I suppose

It also advises "It is recommended that you perform regular charging from 2 bars or less on the energy level gauge to full at least every three months. This lets the energy level gauge adjust to decreases in battery capacity and correctly show the remaining energy in the main drive lithium-ion battery."

This should still be 'current' - It's not anything they told us to cross out in our owners manuals

Don

 

[Barbagris]

I think, as Zelenec, that Peugeot/Citroen simply don't know the car... is a Mitsubishi!

On the other hand, the recomendation

(in a February 2012 letter to US owners) that the battery be taken down to two bars and then left alone to fully recharge, and that this be done once in the first year and then once every other year thereafter

seems not so much about the health of the battery, but to recalibrate and "maximize accuracy of the SOC indicator"

 

[Vike]

It also advises "It is recommended that you perform regular charging from 2 bars or less on the energy level gauge to full at least every three months. This lets the energy level gauge adjust to decreases in battery capacity and correctly show the remaining energy in the main drive lithium-ion battery."

That's the current information. I got my car in September 2012 and the above statement appears in three different places, including the manual and two supplemental documents. I can only assume the statement about following this procedure once every year or two is obsolete.

As for the "allow to fully charge at least once every two weeks" I'm sure I manage that by forgetting to set the timer just about that often , but perhaps I should note on the calendar when that happens just in case.

 

[BillThompsonMIEV]

OK. Now I am confused. I routinely fully charge once per week. I do not normally let battery get below two bars. Two times I have let it go to two bars and did a full charge (per insert discussed above); I always begin my next journey when battery is at full charge within two hours of the full charge. Typical RR after full charge is 90 miles or higher. I have owned the car for seven months. Am I helping or hurting the health of the battery?

 

[JoeS]

OK. Now I am confused. I routinely fully charge once per week. I do not normally let battery get below two bars. Two times I have let it go to two bars and did a full charge (per insert discussed above); I always begin my next journey when battery is at full charge within two hours of the full charge. Typical RR after full charge is 90 miles or higher. I have owned the car for seven months. Am I helping or hurting the health of the battery?

Bill you are treating your iMiEV fantastically well, as evidenced by your great RR and that you are driving her soon after fully charging. Generally speaking, what's not good is extensive pedal-to-the-metal driving, especially at low charge levels, and especially fully-charged battery exposure to high ambient temperatures. Our iMiEV battery and BMS design so far appears to be quite robust, with no known issues other than Remote compatibility with some third-party EVSEs.

 

[jray3]

FYI- a string from Seattle EVA that may shed light. In short, conflicting info from Nissan about Level 1 charging and battery life was apparently because Nissan feared that Level 1 users would deeply discharge their batteries more often and spend more time at low SOC. BUT, level 1 charging would otherwise be better for long-term battery life.
-snip-
I was aware of this information from Nissan
By no means am I perfect but I usually don't just assume to be correct so here is the story.
I looked into this almost two years ago do to conflicting information directly from Nissan.
My main concern at the time was Nissan not going to warranty their charger if it failed using 120. There has been chargers in the past who died using 120. Brusa which is a high end company had a mass failure in their earlier chargers when using 120 to charge over 240. The dozens of blown up Brusa chargers were out of warranty and up to the owner to take care of.
I bought two Leaf's and had planned on loaning them to drivers who would only have 120.
What I learned was Nissan was very concerned about drivers not being able to recover enough capacity to be in a higher state of charge during the majority of their driving. In other words driving in the low end of the pack. If you use spend a majority of your driving time in a low SOC it does not mater what size pack you have. It will shorten its service life.
Nissan as it turned out had really had no issue with using 120 to charge your Leaf 100% of the time. It is understandable Nissan put out this information because most of their sales are going to be to drivers who have little to no knowledge of batteries.
By the way Nissan confirmed I was correct that lower current charging was better for the pack.
It took me several weeks of contacting them every other day. Nissan provided a very limited amount of information and would put nothing in writing. These questions had to go up a chain of command and Nissan was very guarded in their verbal replies.
To me the really good news is they claim their charger is rock solid and can handle mass amounts of 120 charging. I was told they would not void the warranty on the charger but again nothing was in writing.



Don Blazer








>Your maximum cycle life will be with lower charge rates. In the Leaf it would be using the 120 Volt charging.



Please note that at least when the Leafs were introduced Nissan said that the 120 Volt trickle charging would not maximize battery life, and specifically recommended against that approach, stating that for maximum life one should:



Use 240V 3kw charging using the 80% setting.



Not continuously use “top off” charging after only driving a few miles.



Wait a few minutes after operating the vehicle to let the battery cool down before beginning charging.



I have no idea why Nissan was recommending against continual use of trickle charging, unless that somehow defeats their pack balancing circuitry, or something? In any case Nissan now seems much less paranoid about battery life issues – except in Arizona.

 

[Kryten]

They are all correct in various aspects!

I haven't looked at the battery balance mechanism (all Li-Ion battery systems really need one), but most work at the end of charge by detecting cell voltage and bypassing the cell through various mechanisms. Controlled shunt resistors are popular in EV conversions.

The cells used in the i-miev are LiMn2O4, which is slightly more energy dense than LiFePO4, common in home EV conversions and similarly safe and long-lived. The 3.7 volt terminal voltage is typical of this chemistry. The chemistry has been tweaked to address the relatively poor lifespan of typical LiMn2O4 cells.

The issues and principle are universal:
The cells have a very flat voltage vs charge state relation for most of the charge cycle, so the battery manager can't determine the charge state of individual cells if the battery is in a mid-charge state all the time.

There are two ways to balance the cells:
1) bottom balancing. The idea behind this is to balance the cells so that all become discharged at the same time, when the battery is flat. I consider this to be very dodgy because a) completely discharging the cells will kill them. b) the lowest charge cell will need energy put into it, at the time the battery is nearly flat. I will leave this idea to madmen and fools.

2) top balancing. Here, the battery is charged fully with those cells that reach a full state of charge bypassed so that the less fully charged cells can reach the full charge state without over-charging the first to arrive. This is simply achieved with switchable shunt resistors or other charge shuttling mechanisms. This is a well proven and reliable technique.

If the battery is always maintained in a mid charge state, (eg always charging only enough for the next trip) the top end balancing does not occur and the cells may become out of balance, resulting is significantly reduced battery capacity. The battery will have to shut down when the lowest SOC cell becomes discharged or it will destroy that cell.

It is necessary to perform this charge balancing every so-often. I would estimate that a few times per year is sufficient, so many estimates are over-kill. With a 120km (or so range), this is unlikely to be an issue for most i-miev users.

I suspect that there is no chemical reason that the battery must be fully discharged periodically, The 2-bar suggestion makes the remaining capacity about 15%, so does not approach minimum closely. This type of requirement was common for Ni-Cd dry cells, where an oxide layer is deposited in the cell at the low charge mark. repeated low discharges cause this to build up and reduce the cell performance as it discharges past this point. This is not an issue for Li-Ion chemistry.
A deep discharge will show up any faulty cells (reduced capacity), which is useful.

The battery university: http://batteryuniversity.com/learn/article/types_of_lithium_ion
has a lot of good information on Li-ion batteries, including LiMn2O4.


By the way, Cessnock Mitsubishi is a bit confused as to what they are selling:
http://www.cessnockmitsubishi.com.au/new-vehicles/i-miev.aspx
CVT and VVT, interesting - I must be missing something