Traction battery refreshing???

Yes your own Wikipedia reference.

John (Britprius)

 

Hi John. I know that fully discharging a cell (at a controlled rate) will not damage it, but the question remains - with so many cells in series what is the best way to discharge the whole pack without reversing any of the individual cells?

My thoughts are that you'd need to do it at a reasonably steady rate (not too fast) while monitoring the voltage of every (6 cell) module and halt when any module falls below some predetermined voltage. Not sure of the best voltage here, but I'm thinking about 6.0 to 6.2 volts. How does that sound?

 

No it's the same thing. When you discharge a large number of cells in series, if you let the overall voltage fall too low then it becomes likely that the most healthy cells will eventually reverse bias the weaker ones.

Say for example that you have 6 cells in series and you discharge the overall module to 5.0 volts. It's very probably that if one cell is significantly weaker than the other five that at the end of the discharge cycle you'll have 5 cells at about 1.2 volts and one at about -1.0 volts. Note that 5*1.2 + 1*(-1.0) = 5.0 volts, so the negative voltage is implied by Kirchhoff's voltage law.

 

Think about this. You discharge a module to 6 volts. In that module "before discharge" you could have a cell with a charge of 500mAh and all the others could be at 4500 mAh. This must in use give a fault code for that module (and it's partner), because as soon as you begin to use the pack the voltage from the low charge cell will fall rapidly. (the voltage curve of an Nimh cell is like falling off a cliff when it is at the end of it's charge.
When you discharge to 6 volts you can reasonably assume the other cells will have an average voltage of 1.2 volts each (6 volts) since there maximum charge in the Prius (with 8 green bars) would only be at 80% charged so there would be a risk of reverse charging that cell that is at zero volts slightly by discharging it with whatever charger/discharger you are using.
Now do the same calculation with a module that has a cell with a low capacity of 3000 mAh with all the others at 4500 mAh "our serviceable in use battery". The number of cells or modules in the chain does not come into the equation as long as you count the number of cells and discharge to 1 volt or even 1.2 volt per cell.
In fact the discussion should really be at what voltage do we end the discharge as per uart's post, 1.2 volts per cell would be the safest as it would be most unlikely at that voltage (7.2 volts for a module of 6 cells) we would ever get into the area of reverse charging. However would this exercise the cells enough.

John (Britprius)

 

Yes but this can reverse charge a cell within the module depending on the amount of difference in charge level within the module, but most unlikely in a working battery. Something we cannot measure because the connections between cells are not available at the surface of the module. If they were we could charge individual cells to bring them into line.
In the end this is the same if you discharge one module to 6 volts or 28 modules to 168 volts.

John (Britprius)