Two Quick Questions

No, NiMH batteries are not the same as NiCd. They both have nickel. Many things contain nickel.

Yes, the Gen II and Gen III have many technical differences.

Tom

 

NiCad is inexpensive and durable vs. NiMH. This is why your power tool does not have anything fancy to manage its battery, while Prius has a separate ECU dedicated to this purpose.

The Nickel-based battery, its dominance and the future

 

To add to the above, the Prius traction battery is 100% recyclable, and Toyota pays a "bounty" on used batteries to reimburse you for the bother/expense of sending it back to be recycled.

 

Nickel Cadmium batteries are cheap and they provide a very quick discharge rate. They can be discharged completely with no issue. They have a nasty tendency to develop a memory, discharging it completely clears any memory. (This is the reason why so many iPod users had battery issues, since many iPod sales people tell customers to drain before charging... oops...)

Because of this characteristic, Ni Cads are still one of the better batteries for power tools and R/C cars.

One other issue is they tend to discharge relatively quickly just sitting there.

Nickel Metal Hydride batteries contain caustic base (opposite pH to Acids). So they do need to be recycled properly. But since recycling these batteries is cheaper than making new ones, rest assured the economics and environmentalism are in sync.

Nickel Metal Hydride batteries can not be discharged as quickly. They can develop a small amount of memory, that can easily be minimized. They die if fully discharged. They don't like being over charged. They cost more but have a much higher power to weight and power to volume ratio.

But compared to Lithium Ion and Lithium Polymer batteries they are much more robust and cheaper. So they end up in EV batteries.

Although in recent years cheap electronic charge circuits have resulted power tools having NiMH and Li-ION batteries. But that charge circuit still make these batteries expensive.

Computers and cell phones use Li-ION and Li-Pol because they are light and compact and these devices don't drain very highly.

Digital cameras are the evil ones that Li-ION and Li-Pol don't work well with. Companies that use this tech, will usually add a big capacitor so the flash doesn't kill the batteries.

 

The only thing in a NIMH cell is nickle in two forms and KOH (potassium hydroxide, with a little sodium hydroxide). They are quite safe.

Actually there is very little difference between NICAD and NIMH batteries. The cells have the same voltage. Specific sized cells can hold about the same charge (though manufacturers are only developing NIMH now as cadmium is carcinogenic so NIMH cells now can be obtained in higher capacities in a specific size cell).

Both types of cell behave the same in charge and discharge though there -are- subtle differences. For example:
1. When charging both types start to become inefficient at charging when they reach 80% charge, but the NIMH is a little more inefficient than a NICAD.

2. NIMH seem to self discharge a little faster than NICAD, but it still takes many months. Self discharge in either cell is not "life threatening" to the cells.

3. Both types exhibit a negative temperature coefficient - that is the voltage of the cell drops as the temperature rises. NICAD cells vary three times as much as NIMH. This characteristic is used in some chargers to detect when they reach 80% charge (the inefficiency mentioned above heats up the cells). An inexpensive NICAD charger won't detect the 80% point in a NIMH battery so you can't use those chargers for NIMH cells. There are similar chargers for NIMH packs, and the modern ones for AAA, AA, C, and D cells will safely charge both types. This is called "negative delta charge termination". Other charges use a thermistor contained in the battery pack to determine the heat rise and therefore the 80% charge condition. Either type of cell will work in these chargers without modification. Both types of chargers switch the charge rate to "trickle" when 80% charge is reached, which is usually a 16 hour charge rate (takes 16 hours to charge a fully discharged battery). This rate will not harm the battery if left on indefinatly.

4. -ANY- type of cell, including NICAD, NIMH, and even lead acid and lithium cells is susceptable to a serious problem when discharged too far. -ONE CELL- in a pack will always have a slightly lower capacity than all the others. As the pack is discharged -that cell- will eventually run out of energy. When that happens AND the pack is continuing to be discharged -that cell- will be "reverse charging" (draw it out and you'll see why - the other cells will be driving current through it in reverse). This is death for NICAD and NIMH cells. It causes them to "whisker" (nickle "whiskers" form across the cell shorting it out).

So "fully discharging" a pack is not a good idea. Single cells, fine. Either type of cell, NIMH or NICAD can be fully discharged without harm. They can be stored fully charged or fully discharged. NEVER allow a rechargeable LITHIUM battery to fully discharge. They will not recharge if they fully discharge. In fact most lithium rechargable cells have an electronic circuit built in that disconnects them from discharge when they reach a critical low charge level. If not charged within a few months, internal self discharge will fully discharge them and they will become poor paperweights. If not used, charge them every 6 months at least. You can leave these batteries on charge indefinatly without harm with most charging systems. RTFM though.

With all this known, the Toyota engineers, with help from the Panasonic engineers (the Prius batteries are made by Panasonic), do not use "negative delta" charging, or thermistor limited charging. They watch the current into and out of the pack and calculate the pack charge state. They use other factors to calibrate this system from time to time automatically.

They match the cells before assembly so they are equal in capacity and other factors.

They also don't charge the pack more than 80% or discharge below about 20% (30% most of the time).

They also fan cool the battery.

This ensures the pack doesn't over heat and doesn't reverse charge a cell, for a long time. Eventually the cells "drift" in capacity and finally cells begin to be reverse charged. Your car panics with (!) among other things, and you will need a new traction battery.

But DON'T PANIC!! This is unlikely to happen for "the life of the car", usually considered 5-10 years. So far GII cars are still going strong after 5 years. GI cars had poorer battery management systems and poorer batteries, and seem to fail a little sooner.

-SOME- batteries will fail sooner, some later. YMMV

 

Thanks for that very informative essay, David. Too bad it wasn't on thread with a little more informative title so everyone interested in this sort of thing (like me) would see it. I found it by accident.

 

As an interesting aside on cadmium, a lot of the formerly used defense sites (ICBM, radar, etc) have cadmium contamination due to the battery servicing