If the battery is "full" it will not accept energy from regenerative braking. It will also not charge via the ICE if it is "full". Also, "full" isn't always "full". The system will charge the battery to "almost full" via the ICE but it leaves a little room for regen to "top it off" since you will almost always be braking at some point it is pretty much a guarantee that there will be some regen down the line. When that happens it is better to have some "room" in the battery rather than force a run up of the ICE to spin it off. If you are rolling along with a full battery and press the brakes, the system will take that extra energy from regen and rev up the ICE to spin off the extra juice. You will see that described as "run up" here.

 

Some good explanations so far.

One way that NiMH is superior to LiIon is charge time.
With NiMH, the ones used in Hybrids, there is a 1:1 ratio.

Whatever you discharge in 2 minutes, can be recharged in 2 minutes.
So it will not take much of a downgrade to get your battery to the max.
But this is ideal for city driving, because you often start and stop within 2 minutes of each other.

LiION are great, since they have more energy per pound, but after 2 minutes of discharge, it will take 4-6 minutes to recharge to the same point. For plug-in hybrids, where you have hours to charge, this is great. No big deal.
But it's going to be really interesting to see how they do with a regular hybrid in city traffic. I think FE will not be as good as NiMH since the ICE based charger will have to run 2-3 times as often as NiMH models. This is why I think there has been no hurry to switch to LiIon in hybrids.

 

Technology is improving.
The old Li-ion poor image is not existing today.

Ken@Japan

 

I found information that connecting the cells in series as in Hybrid Vehicle packs requires special battery management.

The capacity of each cell must be the same (matched cells). This allows the cells to to charge and discharge at the same rate. The cell with the lowest capacity will discharge first. Then, the charge remaining on the other cell damages the discharged cell. The lowest capacity cell also reaches full charge first, so it is overcharged while the remaining cells are still charging.

This problem can be managed by monitoring the voltage of each cell where the battery management process stops discharging when the first cell reaches discharged and stops charging when the first cell reaches full charge. This method allows maximum battery capacity to be used, and can be used to identify/repalce weak cells. Cells do not need to be matched. It is very expensive!

It appears Ford measures total battery pack voltage, and sets the charge/discharge limits to satisfy the accumulated tolerances of each cell's capacity. A reasonable design, but a real blow to the battery capacity.

The Escape Hybrid battery capacity is 20% of the full battery capacity. That also means the power density is x5.

 

Fewer recharge cycles, more complicated battery pack design because when a cell goes bad it can explode during recharge if its not disabled. In laptop batteries this is usually just a fuse to disable the WHOLE pack, clearly not an option in a system with 250 cells that runs a car....

And... in terms of energy per unit volume, the advantages aren't nearly as high as you'd like. Energy per unit mass LiIon is a huge winner, but two packs the exact same size, not as great.

LiIon will get there someday for vehicle use, I'm not sure its really up to snuff for mass production at this point. Sure there are companies building replacement packs for PHEV vehicles.... but I think Ford and Toyota just didn't feel the technology was right when the vehicles came out. Next generation maybe.

 

The PHEVs use a battery pack in addition to the HV battery. This battery is connected to the HV battery through a diode. This means the PHEV battery can discharge to feed the motor/gen, but it cannot be charged by the motor/gen. The PHEV potential makes the HV battery management process think the HV battery is fully charged. When the PHEV is discharged, HV battery management works the HV battery in the normal way.

Cooling the PEHV battery is not needed since it is slow(er) charged by plug in.

The charging cycle can be more aggresive because the PHEV battery does not need to live through 10,000 cycles.