My overview of what actually is happening when we charge was written and placed here:
http://99mpg.com/blog/whatactuallygoeswr/
I copied it here as well.
IMHO:
Hybrid battery packs develop several problems as they age, which are aggravated based on temperature history, and length of time spent inactive.
Inactivity allows the cells to self discharge, and since all cells are going to do this to one degree or another, and the rate of self discharge can be quite different from one cell to another, a typical pack that is setting codes like the 1449,1447,1433, will have cells that have fallen way behind others , and the IMA control system which works to keep the cells in the 20% to 80%, has no mechanism to rebalance them.
All Nickle based batteries develop this self discharge because they can spend a lot of time sitting at a middle SOC, where Nickle dendrites (little whiskers of nickle) will grow and gradually provide a leakage path between the + and - plates, eventually leading to a shorted cell.
Another thing that happens when a cell drops down in the SOC relative to the other cells is it develops an effectively reduced capacity that is recoverable called Memory effect.
Memory effect capacity loss is reversible by fully cycling the cell from 100% to less than 1V/cell.
So the code setting pack has this unmatched set of cells with widely different SOC, and wide spectrum of effective AH capacities, which reduces the effective capacity of the pack as a unit to a value that sets the code.
When the car charges the pack to full, (80% SOC) it stops when it thinks the pack is full, with the assumption that all the cells are the same, where in fact some cells are still only partial charged.
The discharge limits are also skewed so the pack stops allowing assist long before the highest capacity cells are empty because one of the low capacity cells has dropped out.
When that happens the rest of the pack discharges through the empty cell effectively reverse charging the cell.
The IMA safeties kick in causing the SOC guage to drop to the bottom, and aggressive charging begins to pull the reversed cell back from sure destruction.
The longer the pack is used like this, the worse the pack gets.
Another factor in this picture is heat.
Once a cell gets to 140F the internal pressure can get so high that it vents potassium hydroxide and the cell will permanently loose AH capacity. No amount of charging can bring back capacity lost due to venting.
The charger and discharger system will first fully charge all cells to 100% SOC, for the first time since the pack was installed. This can take a long time, as we have to limit the charge current so the cells that fill up first do not overheat.
When a cell is fully charged, all the charge energy becomes heat, which is why the cooling fan should always be used during charging.
Getting the whole pack to 100% will fix some packs, especially ones that have become unbalanced due to just sitting and self discharging, and have not developed any memory effect.
Hypermileing where you do not use the pack much can cause the pack to develop the same self discharge unbalance.
Most packs have both memory effects and real capacity loss, as well as differing degrees of self discharge, so the discharger aspect of the system becomes a great tool to extend capacity on the memory effected cells by cycling the pack between full and the detection of the dropout of the weakest cells in the pack, The idea being that the weakest capacity cells are the ones causing the problems, so since they will be the cells that get the deepest discharge, if the issue is memory effect those cells will recover capacity with each cycle, and the minimum voltage reached during the discharge should get lower and closer to the 1V/c level which would be 120V on a 120 cell Insight or first gen civic. A well balanced pack even if at low capacity will discharge uniformly so the discharge would stop at near 1V/cell.
In the real world that is seldom is seen, since all packs will have several of these issues happening at the same time, so there will be a point where the discharge will repeatidly stop at the same elevated voltage, like 138V. At this point the cell that is dropping out is likely one that has lost capacity due to venting or simply age, and to get that pack back in balance, we look at the other cells when that cel drops out to see if they are also ready to drop, or if the majority of them are still quite good.
At this point, if the car is still setting codes, we need to take to process from the car to the bench.
Using a procedure that we are still tweaking, we can use the charger/discharger, and datalogger to discharge the pack with the ends open so we have access to the ends of each stick, to make voltage measurements.
If the other cells are still well over 7V replacing the stick that dropped out with one of higher capacity, would then allow the cycles to continue to reduce memory effect and increase capacity of the rest of the pack.
Comments corrections additions?
