Slow Down the Balance (maybe).

This is mostly a cut and past from a post in another thread about discharge rates. I think that what I'm now discussing is off topic and deserves a fresh start:

Ive seen some people mention charging modules as fast as 1C using various methods to determine when to stop charging, this is too fast to safely achieve a balanced module. I also strongly believe an occasional preventative balance of the modules would reduce failures. Most if not all traction battery failures (where its the actual battery failing and not the ecu or a sensor wire etc) I've seen on here is due to one or two modules that are ruined due to cell imbalance. You cant keep rechargeable batteries alive forever, but balance issues can be reduced, and stopping them early is critical. How many pictures of bulging modules during the rebalance process have we seen? That's the effect of charging them too fast, further degrading them. We need to balance using a charge rate of C/10 at most. This translates to a charge current of 650ma at most - ten hours to fully charge one module one time. Using a two stage charge to charge fast early then switch to a slow charge would be good of course, but we have to be careful to not charge too fast too long. Here are a few excerpts from some great papers. They're all fascinating reads. I wish we had access to individual cells in the modules, but we dont, so we have to do the best we can, which is a slow (slow!) charge to balance. What I dont know is what is a safe high rate charge rate if we do a two stage charge. I suppose using the maximum current the HV battery can sink can give us that number. Obviously if you're broken down and you need your car back you want the rebalancing process to go as quickly as possible, so we can cut a lot of time if we can get the safest high speed charge and discharge rates then switch to the 0.1C (C/10) rate to balance at the end of the charge cycles. I would love to know how to build a grid balancer and just condition my battery while it is still in the car. Has anyone made such an animal?

How to charge Nickel Metal Hydride Batteries. "The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10."

http://www.zenspider.com/~pwilk/aero/additional/duracell.pdf
"Charging at a constant current at the C/10 rate with time-limited charge termination is a convenient
method to fully charge nickel-metal hydride batteries. At this current level, the generation of gas will not exceed the oxygen recombination rate. The charge should be terminated after 120 percent charge input, or approximately 12 hours for a fully discharged battery."

They go on further stating C/300 is safe for a true "always on" trickle charge, something we would never need.

Technical Tips for NiCd and NiMH Battery Pack Users — CamLight Systems

"Critical to proper maintenance of your NiCd and NiMH battery packs is an occasional slow charge to help balance the voltages of the cells and regular discharging down to 0.9V/cell to allow a full charge to break up any large crystals forming in the cells that can affect performance. It also breaks up any high-resistance compounds that might have formed on the electrodes due to passivation. We recommend a monthly 2-stage charge and 2-stage discharge."

If your packs don't fit into a tray discharger, use a pack discharger to bring the pack down to approximately 0.9V/cell (not below 0.8V/cell and not above 1.0V/cell). Then rebalance the cells by doing a slow charge, 1/20C-1/10C, or a 2-stage charge."

Ours dont fit in a tray discharger (a device that accesses each cell in a module to monitor and stop discharge at a set voltage) so we must balance using the slow charge method)

http://focus.ti.com/download/trng/docs/seminar/Topic 2 - Battery Cell Balancing - What to Balance and How.pdf

"To make the matters worse, the affects of cell degradation caused by imbalance is auto-
accelerating, once a cell has a lower capacity, it is exposed to increasingly higher voltage during charge which makes it degrade faster so its capacity becomes even less, which closes the
runaway circle.
Note that not all battery chemistries are equally affected by cell-unbalance. While Li-ion chemistry is specially vulnerable because of its ability to store almost 100% of all energy delivered, Lead-acid, NiMH and NiCd-s are relatively tolerant to overcharge because they
can respond to increased voltage by internal shuttle reactions that are equivalent to a chemical short-circuit inside the cell. For example in NiMH battery oxygen and hydrogen generated after the end of charge recombine inside the cell building water. This causes extensive heating because all the energy of the charger is converted to heat rather than stored. Still, overcharge at high rates does cause increased pressure inside the cell and creates a chance for explosion or venting.