Plug-in Charging

Yes. There are no plug-in 2010s available to the public. Only a few fleets will get them. The standard 2010 available to private owners are NOT plug-in cars.

Except in Alberta Canada, where we have a plug-in block heater.

 

For some reason, the battey used in the earlier Prius does not like to be charged fully, or charged from an electrical outlet? The new battery, designed for plug-in use, is stiull being tested, and my understanding is Toyota doesn't want to risk their reputation on it until it is proven reliable?

A discussion of battery chemistry/technology would be welcome here...

 

The battery is a series string of a bunch of cells. The GII battery is about 200 V and each cell is about 1.25V, you do the math. It's difficult to control or even closely monitor the charge level in -each cell-. The cells are "matched" re capacity and internal resistance at the factory, but do drift with age.

Now, the death of a NiMH cell is -usually- caused by one two things barring catastrophic failure:
1. The cell is overcharged and looses some electrolyte each time, eventually becoming useless as it is so different from all the other cells in the battery. Because the first overcharge can cause the loss of some capacity the next one is worse, etc. Note that the cell with the lowest capacity or highest charge level when charging begins is the one at risk.

2. On discharge if you go too far one cell will fully discharge before all the others. At this point if the battery continues to be discharged -that cell- will be reverse charged. That is death to NiCd and NiMH cells. It usually results in them "whiskering" (a short of nickle develops across the cell, shorting it out of the battery). I've seen them and they do look like whiskers.

To prevent both of these from happening, the Toyota engineers, with assistance from the Panasonic engineers, designed the car so the battery never charges past about 80% and never discharges below about 30%. This ensures the problem won't appear for quite a few years. Eventually some cells loose capacity and the problem occurs. So far it appears in the GII car this won't become a problem in the majority of cars before at least 6-10 years, though it's the -number- of charge/discharge cycles/cycling that causes it, not age or mileage stricktly speaking.

Note that on charge, NiCd and NiMH cells become very inefficient when they reach 80% charge. They convert a lot of the charge energy to heat. This is often used as a "charge control" system, both by monitoring the temp. and shutting down to trickle charge when the temp begins to spike, or monitoring the battery voltage and shutting down to trickle when the voltage dips (the cells have a negative temp. coeffient and this causes the voltage to dip at about 80% charge). This is called "negative delta charge control". In both cases the battery temperature DOES climb a lot so the battery life is shortened because of this. Toyota use a charge measurement to determine when the charging should stop, so they stop -BEFORE- the battery temperature spikes.

There is no reason a plug-in Prius couldn't still use charge measurement to ensure long battery life, but you won't gain a lot with the current battery capacity. The car usually runs with battery capacity around 60%. Charging a 6.5 A-Hr battery from 60% to 80% isn't going to take you very far. My understanding is the plug-in cars are using a "double pack" (13 A-Hr), but even that isn't going to take you very far. Perhaps 10-15 km. And the cost is rather high, I would guess around $4000 extra retail. Tough to recover that in fuel savings.

My read is that Toyota are putting the plug-ins out there to gain experience in their operation in the "real world", in preparation for the eventual arrival of much higher capacity less expensive batteries. They would make cost recovery much easier.
Note that there are still a lot of people who don't care about the cost, just about reducing the buring of gasoline. This isn't wrong, just not fully practicle in a prodution environment just yet. (my opinion).

 

Very nice technical explanation, David.

 

Does this mean that the single cell undergoes a reversal in its polarity when fully discharged?

By inefficient I take it you mean more resistant to additional charge uptake?

 

Yes, but from the discharge current flowing through it from all the other cells - which results in reverse charging. Draw it out and you'll see what I mean. Note that you can't draw any power from it as it can't store much when reverse charged.

Well, not resistant really, it just converts a lot of the charge energy to heat instead of chemical storage of energy. So I suppose you could call that "resistance to additional charge uptake", but we just look at it as heating and inefficient charging. You can still take it to 100% if you try hard enough, but not in the Prius!

Note that most of my experience with this was with hand held radio batteries. They have a lot fewer cells in series, yet they still have problems. The more cells in series, the harder it is to sense what's happening, as the voltage change vs temperature can be larger than the voltage across a cell when you have a lot of cells in series, so if you monitor the voltage you're never sure if the change is caused by temperature or a bad cell. Toyota monitors the cells in groups of 10, as I recall. Even so, it is difficult to detect these problems (but not impossible, as the dealer can tell you if you have a "bad module" (group of 5 cells).

 

Has anyone had a chance to do some battery testing yet? I'm curious about the self-discharge rate for the new packs compared to the old ones. I wonder if the change in pack design is having any real effect. If they were able to lower the self-discharge rate, it should help improve overall batter life by reducing the total number of charge/discharge cycles and charge delta per cycle. They added the EV mode to the US version so I'm assuming they found some efficiencies somewhere.

 

The repackaging of the new battery was to improve the removal of heat. It shouldn't have any effect on self-discharge. I read the cells are unchanged. I doubt self-discharge would have much effect on battery life, as the discharge rate is so low. Any effects from that would be swamped by actual -use- by the HSD.