On board Equalizing Charge

There were no forums when Classic model (Gen1) was around. All we had in those early days was basically just a Yahoo & eGroups board. That non-threaded interface was clumsy. Topics would easily get lost.

To make matters worse, participation naturally dropped anyway. As people discovered Prius was actually more reliable than traditional vehicles, they left.

And now, the Classic owners are grossly outnumbered. How would they be heard over the massive chorus of new Prius owners?

Of course, becoming less and less common is the making for a collector's item...

 

Joe, I unfortunately don't have ready access to a Gen 1 otherwise I'd fish around in Techstream to see if this functionality existed.

That said, the battery ECU will keep block pairs balanced. If by equalization, you are talking about individual modules and cells (6 cells per module, two modules per block pair), let's take a look at that.

A Gen 1 battery can be thought of at 19 blocks of 12 nimh cells. The overall state of charge of each block can be managed by the computer by Coulomb counting, with a lookup table for current@voltage@temperature for conditions at higher or lower states of charge. The computer will shuffle charge from blocks that appear to be strong, to blocks that appear to be weak.

It is theoretically possible to have imbalances develop inside a block, however. One failure mode is that a cell will be different enough from the other 11 that, over a long period of time, it will have a different state of charge than the others. The usual analogy is having the bartender line up and fill 12 shot glasses, though his device that can only fill 12 shot glasses at a time. They all may look to have the same quantity from a distance, but they may be slightly different levels on closer inspection. If a patron comes by and takes a sip, then the balance has been lost, and the glass cannot be refilled with the bartender's device.. With the battery, then, this may lead to the cell reversing on discharge, or being overcharged during heavy regen. If a battery can be equalized, then it might be possible to avoid that sort of single cell failure.

Unfortunately, equalizing NiMH is not the same as equalizing lead acid. NiMH heats up significantly as it nears full SoC. If the charge rate is too high, then pressure will build inside the module and it will vent electrolyte, which cannot be replaced easily. The only way to equalize it without ruining the battery is to force charge it with the car (i.e. in Drive, one foot on brake other foot on gas), then hook up a high voltage trickle charger at a 325 mAh rate for several hours. Most NiMh can be safely float charged at C/20. This will allow all of those shot glasses to be filled very slowly to overflowing, and the overflow rate is small enough as to not make a mess.

I can't see this being possible with Techstream. It would take all day to do it right, and Toyota is extremely cautious about having technicians do anything other than replace batteries that have coded. At 99mpg.com, Mike makes a high voltage charger that can do this equalization. On older Hondas, you almost have to do this, because those cars manage the SoC based on one group 120 cells, rather than smaller groups. I don't know if Mike's charge can be extended to a Gen 1, however, due to the high operating voltage.