Faster Discharge?

From everything I've read and know there are only two ways to balance cells in a pack.. Monitor voltage of individual cells during charge or discharge and stop each cell when it hits the min or max safe voltage for that particular chemistry - which is the best method, - but we can't do that unfortunately. The only other way is to overcharge to bring the weak cells up to to match the others. This requires a slow enough "trickle" charge phase to not damage the good cells and I've seen ranges of 0.05-0.1C for this.
In the rc heli community we use lipos, we can only use the individual cell cut off method. Battery packs are designed with a balance plug so a smart charger can access all cells. Trickle charging is a huge no-no because lipo will store all energy and you'll start a nice fire if a cell goes over 4.35v. I don't know much regarding nimh in other branches of the rc community. Do you also have balance plugs or do you have access to cells another way? I've seen some interesting home brew rigs to balance discharge (down to 0.6v) large airplane packs using diodes to protect from reversal a getting good results in increased capacity but shortening life, so no good for us on the deep discharge front.


iPhone ?

 

I agree. I just saw a post where someone charging a prius module at 2C heard a boiling sound inside (o. The method being used here is too fast on the balance portion of the charge cycle and are damaging these packs more than helping. People are able to get on the road with them due to replacing the worst modules that have completely failed below the ECU's threshold, but they're degrading the rest of the modules a little and not helping things. I think I'm going to start another thread to address this as my original topic isn't as relevant and this is an important issue. Those weak cells are hitting the top voltage (it's the weak cells that get there first) and baking, further degrading them. The good cells might be catching up with the 7250mah limit people are setting but it's still hurting at such a high end of cycle charge rate. It MUST be slow charged at .05C to do this right.


iPhone ?

 

Yes! C is an easy way for people to deal different battery capacities, and how fast (or slow) you should charge them. For example, if a battery’s capacity is 250 milliamp hours (mAh) and you want to charge it at 1C your current into it would be 250 milliamps (mA). In theory it would take exactly 1 hour to charge that battery from empty to full (empty being the minimum safe voltage of the cell). If you wanted to charge it at 2C, you would charge at a current of 500mA – it would then take only 30 minutes to charge it. Now say you have a large battery that holds 10,000mAh. To charge this at 1C your current would be 10,000 milliamps (which is 10 amps). Just like the small battery it will take exactly one hour to charge the battery. To charge at 2C the current would be 20,000 milliamps (20 amps) – charging the battery in 30 minutes. Why does this matter? Think of the large battery as a tall empty glass, and the small battery as a thimble. You can pour juice into the tall glass fast and get it close to the top no problem. But if you poured the juice at the same speed into the thimble it would immediately overflow and spill everywhere. So how do you fill the small thimble? Obviously fill it slowly. If you start filling them both at the same time and they both top off at the same time, you’ve filled them at the same C rating even though the flow (current) and capacity were different.When I'm referring to .05C that's a VERY slow charge, it would take 20 hours to charge any battery at .05C. I strongly feel this is how slow we need to charge the modules in the traction battery to fully balance them and do it safely, and I believe the advice given is risking damage to modules that are borderline failing. (Note, some people express .05C as C/20, its the same thing. 1/20 = .05 C/10 is the same as .1C (ten hours to charge). I also strongly believe an occasional balance of the modules would reduce failures. Every failure 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.

For the those still reading this thread (should I start a new one?):

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%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.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."

The increased pressure and venting is exactly what we see when we overcharge these modules at 2C. We have to slow it down.