Cell Balancing 101

Here are two articles from EE Times Design that covers the generic topic of cell balancing.

12/8/11 - "Balance hybrid/EV battery packs to extend operational lifetime: Pt. 1—Hardware and strategy"

http://eetimes.com/design/automotive-design/4231232/Balance-hybrid-EV-battery-packs-to-extend-operational-lifetime--Pt--1-Hardware-and-strategy?Ecosystem=automotive-design" onclick="window.open(this.href);return false;

12/15/11 - "Balance hybrid/EV battery packs to extend operational lifetime: Pt. 2—Algorithm and lab tests"

http://www.eetimes.com/design/automotive-design/4233295/Balance-hybrid-EV-battery-packs-to-extend-operational-lifetime--Pt--2-Algorithm-and-lab-tests" onclick="window.open(this.href);return false;

Interesting reading. How "sophisticated" is Nissan's battery management for the LEAF? :ugeek:

The LEAF uses a sophisticated top-balance system with an array of MOSFETs that can bypass current around cells that are at a higher SOC than their neighbors, thus allowing those with lower SOC to "catch up" so that all cells have parity. This is all carefully controlled and monitored by the battery ECU located in the battery box.

-Phil

Ingineer said:

The LEAF uses a sophisticated top-balance system with an array of MOSFETs that can bypass current around cells that are at a higher SOC than their neighbors, thus allowing those with lower SOC to "catch up" so that all cells have parity. This is all carefully controlled and monitored by the battery ECU located in the battery box.

-Phil

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Is this just for charging balance or is the sophistication also for discharge? Thanks for your enlightenment...

I don't 100% know the inner workings, but I believe the top balancing is the only time this will occur. It would not be needed any other time.

-Phil

Is this top balancing done while reaching an 80% charge ? Or only when charging to 100% ?

Is it done at the end of EVERY charge, or triggered only under certain amounts of "imbalance" ?

Possibly.
Probably.

Probably not.
Yes, probably needs "enough" imbalance.

LEAFer said:

Is this top balancing done while reaching an 80% charge ?

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I vote no, since my battery definitely gets out of whack with partial (including 80%) charging. It takes several subsequent balancing efforts, typically longer period up to four hours.

Or only when charging to 100% ?

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I vote yes. My balance last night was only an hour, after many subsequent 100% charges.

Is it done at the end of EVERY charge

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No, only when meeting whatever pre-programmed criteria. Most times do not balance when doing 100% charges *and* leaving enough time after the charge for the balance.

or triggered only under certain amounts of "imbalance" ?

Click to expand...

Yep

How do you know if it is balancing or not?

TonyWilliams said:

LEAFer said:

Is this top balancing done while reaching an 80% charge ?

Click to expand...

I vote no, since my battery definitely gets out of whack with partial (including 80%) charging. It takes several subsequent balancing efforts, typically longer period up to four hours.

Or only when charging to 100% ?

Click to expand...

I vote yes. My balance last night was only an hour, after many subsequent 100% charges.

Is it done at the end of EVERY charge

Click to expand...

No, only when meeting whatever pre-programmed criteria. Most times do not balance when doing 100% charges *and* leaving enough time after the charge for the balance.

or triggered only under certain amounts of "imbalance" ?

Click to expand...

Yep

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How can I tell when the cells need balancing or how often should I balance the cells?
what is the proper way to balance the cells?

bernie82 said:

How can I tell when the cells need balancing or how often should I balance the cells?
what is the proper way to balance the cells?

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Cell balancing is automated. The only thing you have to do is charge the car to 100% and let it sit for up to about 4 hours. Leave the car plugged in during this time and you'll get a "top off" charge after it has discharged the highests cells during the automated process.

We don't yet have a tool to know exactly when, but it's coming (LEAFscan).

When the cells aren't balanced, there will be a significant reduction in available capacity. Unless you are using the entire useable capacity, you won't know or care.

I recommend once every week to once a month to allow the car to charge to 100% and give it that 4-ish hours to do the balance.

Is it true that if you do a timer override charge the battery doesn't balance? I read this somewhere on MNL.
If I do a timer charge to 100%, does the timer have to extend 4 hours past the end of charging?

No, it's not true.
Yes, the "balancing" happens after the LEAF's charge is complete.

grommet said:

No, it's not true.
Yes, the "balancing" happens after the LEAF's charge is complete.

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In my experience the battery will balance after a timer override charge, but you won't get the top-off charge at the end. At least in my Leaf, pushing timer override a second time after >4 hours has passed, the charging will start again and the gids will go a little up. Often from 279 to 282 or so, but not every time.

If range/capacity is not an issue for me, as I drive 10-40 miles a day, should I balance or just skip it? (I never charge to 100%). Thanks.

DesertDenizen said:

If range/capacity is not an issue for me, as I drive 10-40 miles a day, should I balance or just skip it? (I never charge to 100%). Thanks.

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I wouldn't worry about it. Maybe once a year?

jkirkebo said:

grommet said:

No, it's not true.
Yes, the "balancing" happens after the LEAF's charge is complete.

Click to expand...

In my experience the battery will balance after a timer override charge, but you won't get the top-off charge at the end. At least in my Leaf, pushing timer override a second time after >4 hours has passed, the charging will start again and the gids will go a little up. Often from 279 to 282 or so, but not every time.

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If memory serves, this is specifically advised against in the manual (charging from an already high state of charge), and is stored by the system to be reported as part of the annual battery check. Given the durability questions I wouldn't be trying to shoehorn in a few more watt-hours past the 100% charge termination. Best-case you're just heating the pack and causing undesirable secondary reactions. Worst-case the charger has a hard time recognizing the termination point and overcharges.

Nubo said:

Worst-case the charger has a hard time recognizing the termination point and overcharges.

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Any evidence of that? I find that doubtful, as voltage detection and termination is not all that difficult.

Is 4 hours prolonged? I don't want to "leave" my car sitting at 100% for any length of time.

ztanos said:

Is 4 hours prolonged? I don't want to "leave" my car sitting at 100% for any length of time.

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I am not a battery expert by any means, but I have read a lot of that written by the expers. While long term storage at at full SOC is detremental to the health of Li cells, I don't consider four hours every couple of weeks or so a big problem. Of course, the higher the temperature, the shorter the period at full SOC.

Further, the accepted maximum voltage for Li cells is usually considered to be 4.2 volts. The LEAF BMS charges to 4.1 volts per cell, so there is already a built in cushion.

I am OK with an occasional four hours at full charge - others are not. The battery will still balance.