ASalt and Battery: Long

Or, talking your battery wisdom with a grain of salt:

There is a LOT of information out there about the care and feeding of Lithium Battery packs. Much of it has some grain of truth contained within. How do you separate all of that out into a 'Best Practices' to maximize battery life and performance for your particular use pattern with your particular MiEV in your climate. It's a deep subject, so put on those hip waders.

There are at least four major types of batteries that are referred to a 'Lithium' and several chemistry/materials/build-process variations within each of those. So, if someone starts talking about what "All Lithium Batteries Do" you can start ignoring them now. Each type/sub-type is optimized for particular tasks, cost and service life.  Lithium Traction Batteries (for car use) are sufficiently similar that some generalizations can be made.  Almost any 'rule' applied absolutely will either damage the battery life or the usability of the product. This is a game of weighed tradeoffs.

First, remain calm. The Yuasa/Mitsubishi engineers seem to have done a pretty darn good job of constraining the operation of this LiMn based battery pack in useful, well thought out ways. For the most part you really can let the system operate itself, which, coming from the old DIY days where you had to watch everything all the time is an enormous relief.  There are of course exceptional circumstances that generate exceptional exceptions.

What do we know?  Some of it's really good stuff:
The MiEV system seems to do a good job of preventing overcharge (arguably one of the top killers of this battery type) both by having a per-cell tracking system and by limiting the maximum voltage per cell to a number well below (3.96V?) the Max charged spec for these cells (around 4.1v +/- temp.correction)
Granted that leaves roughly 4-6% of the potential usable energy storage untapped, but that's probably a VERY good tradeoff for overall battery life.
It also appears to do a good job of limiting the maximum discharge. I don't know the exact numbers, but it appears that the system hits 'Turtle Mode' where it begins to limit the available maximum current at somewhere around 15-20% charge and cuts it off entirely around 5% below that. Actually I suspect it limits current so that the lowest cell doesn't go below their selected 'Min' Voltage number as that's generally the best way to prevent cell damage.

All of the above requires that the pack stay in good balance. As a long serial string of batteries goes up and down the charge discharge cycle(s) together, each cell will act very slightly differently than the others. Cells that are warmer toward the middle of the pack or colder toward the edges, cells with slightly different variations within the manufacturing tolerances, maybe variations in the battery monitoring boards themselves, all can conspire to cause a particular cell to drift off from the average over time. Since the MaxV/cell the MinV/cell and the maximum current that can be drawn from the string are all limited by whichever cell has drifted off the most in a given measure, it's obviously very important that they behave as much alike as possible.
As the first cells reach full charge during the charging cycle, the battery board signals the charger to cut back on the charge current. The battery boards start to bypass current from the highest cells burning off the excess current with an onboard (per cell) resistor to keep the maximum voltage on that cell under control. This is called top balancing. It keeps doing this until all the cells have reached the correct voltage or a timeout occurs, which appears to be about an hour. Over time and a number of cycles this allows all the cells to reach the same charge state, and tends to keep them there.

Lets look at that for a moment. People who have seen and reported about the on-battery-board per-cell resistors make me doubt that they can dissipate any more than 5W and probably less. At somewhere around 4V/cell that translates into roughly one amp. We know the balance time is limited to around one hour. Therefore the maximum variation between cells that can be corrected by the balancing system is around 1Ah. Since the nominal capacity of each cell is around 50Ah the balancing system can handle variations of around 2% per charge cycle. I'm sure that's plenty enough to handle the day-to-day variation, but is probably woefully inadequate for people who are 'saving' their batteries by never (or only very rarely) reaching full charge. Granted that doing a full charge a couple times a day could reduce battery life by some amount, especially if it's allowed to sit (or run) at high temperatures on a regular basis.
How do we know for sure? We don't really. My personal guesses cause me to make sure I reach full charge/full balancing cycle about once every couple weeks. I might back off that a bit if I was living in the Arizona summer and the car was going to sit out in the sun for hours after reaching full charge. [ie: I only do the full balance thing when the car is relatively cool] Since cell-to-cell variations tend to increase as the cells age, generally speaking I'd be inclined to balance more often as the car gets older.
We also know the getting the pack down to two 'bars' on the full-O-meter occasionally allows the battery management system (BMS) to reset/revise some of it's capacity assumptions. So letting it get down there once a month or so (depending on usage patterns) is probably a good idea.

Q: Doesn't restricting the depth of cycling improve the cycle-life of the pack?
A: Sure it does. Say you keep it between 6 bars on the low end and 13 on the high end. Wow, you might get 5000(!) cycles out of it. Of course you're only getting about 30 miles/charge, but still, 30x5000? that is 150,000 miles.  Pretty good eh?
Imagine some other slob who charges all the way almost every time and runs it down to 2-3 bars... What a sap, maybe only gets *half* as many cycles out of his pack. Huh, that's 60-65 miles/charge and 2500 cycles. Darn, that sucker only got 150K miles out of it... ;-) 
Now granted maybe the more careful user would be getting even more cycles out of it or the other even less, at this point we just don't know. The numbers are semi-informed speculation. The slob definately had the easier time of it though and got to drive a bunch further per charge.

Accelleration kills batteries. Kills'em dead! ...or maybe not.
Again the Mitsu engineers have made some pretty good tradeoffs. It appears those cells can probably stand short term surges up to 10C (about 500 amps) without much problem. They seem to have crafted the acceleration profile so it doesn't surge much beyond the 150A (3C) maximum continuous limit. How do we know it limits to 150A? We don't exactly, but that Amp-meter on the dash appears reasonably linear (especially in the plus direction) and if you have been in an EV that has a motor current meter, you'd be pretty sure this is battery amps that's shown.
Granted keeping it pegged at 150A (Max on that 'power' instrument's scale) for long periods probably isn't good for it, especially if it's unusually hot or unusually cold (below freezing) or the charge state is low (below 4 bars) but accelerating briefly near the top of that meter probably isn't causing much harm. I try not to do that every time, or keep it above 100A for any extended period. Besides it DOES seem to kill the range!

So what do we know again?
Don't go pegging the power meter for extended periods, especially if v.hot, v.cold or v.low.
Don't let the batteries sit at full charge if it's real hot or for extended periods of time (I show 6-8 bars if I'm gonna leave it for a week, might go lower if I was going to leave for longer and I don't charge to full if it's over 100F)
Don't run it hard if it's extra cold out (wwwweeell below 30F)
Don't run it down below 2 bars on a regular basis.
Don't be topping it off all the way several times a day if you can avoid it.
Do let it go through a full cycle occasionally. Twice a month? Once a month? Something like that.

...it's not all that hard...