80-45% charge only? Why!?

Hi Sam. I've seen my battery at 100% charge (all green bars on display), and I've seen it down as low as 2 bars.

You may be thinking about the "all electric" mode. They have this option in Japan, but not in North America .... unless you buy a hack.

Toyota did state that in order to make the batteries last the life of the car (apparently important in NA, but not in Japan?), they could not use this option in NA. This way, they can still promote their great 8 year / 100,000 mile (I think it's 10 years/150,000 in California) warranty on the battery.

I hope that helped.

... Brad

 

FYI, 2 bars = ~45% and full bars = ~80% SOC.

Toyota Prius Battery Specs - EAA-PHEV

Charging and discharging NimH is a pretty tricky affair. Particularly over a wide range of temperatures. You avoid a lot of those problems and greatly improve the battery life by avoiding the very top and very bottom of the range.

On top of this, increasing the effective capacity of the battery has almost no effect on mileage. Its been tried. People have doubled it, and still get the same mileage. The DOE also found that a GEN1 in their test fleet was down to almost 50% capacity at 160k miles, which also had almost no effect on mileage. You are fundamentally limited by the efficiency of the system charging and discharging the battery. This is part of the reason that the true hyper-milers seem to always try and use as little of the battery as possible. EV mode is a fun gadget, but is detrimental to MPGs in almost all situations.

For PHEVs its just the opposite. Since you are offsetting gas with electricity, the size of your battery directly effects how much gas you can offset. In that case, you ideally always want to end your trip with a battery that just went empty, so as to offset as much as possible.

Rob

 

The EV Switch on non-American cars has nothing to do with the ability of the battery to charge beyond the 80% limit or discharge limit. You can easily purchase an aftermaket switch but honestly it is not all that helpful.

Running in electric only mode is actually wasteful in a lot of cases. The best milage will be had by trying to stay off the battery but not on the ICE either. Coasting and gliding are your best friends. Driving an extra 1/2-1 mile at the risk of premature battery failure does not seem very economical to me.

 

If you haven't already, check out calcars.com. They have an open source lead acid based PHEV conversion that runs ~$3-4k if you DIY for ~10-12 miles EV range. They are working on NimH as well, but its more $$. If you have the bucks, Hymotion just introduced their Li-ion pack at $10k installed.

There are two main methods of PHEV conversion currently. Most of the DIY'ers are using the Hybrid method where you parallel a 240V pack with the OEM pack to pull its charge up by exploiting a recalibration routine in the controller that kicks in above 242V. Many of the commercial conversions completely replace the OEM pack, but that means building a custom battery management system and controller.

Rob

 

ThanX Rob ... good stuff.

... Brad

 

Sam,

I believe you are correct about the Prius battery management system. When the display in the vehicle shows the battery as "full" it is actually charged to 80% of its true capacity. This is about as charged as the battery management system in the vehicle will allow it to get. If it gets any higher the vehicle uses a variety of techniques to reduce the charge in the battery (including wasting electricity using the electric motors to spin the Internal Combustion Engine "ICE"). Additionally, when the display in the vehicle shows "empty" it is actually charged to 40% of its true capacity. This is about as discharged as the battery management system in the vehicle will allow it to get. At this point the vehicle uses the ICE to spin the generator and charge the battery back up.

I'm not aware of anyone changing this, although I believe a few of the Plug-in conversion systems fool the battery management system into believing that the battery is at a particular level to get better use of the supplemental batteries they install.

Seeing as the existing range is typically less than 2 miles and frequently less than a half mile, doubling it wouldn't accomplish much. Furthermore, since the Prius can't be plugged in (without significant modification), running the battery down so far would simply reduce the fuel economy since the vehicle would eventually need to burn gasoline to generate the electricity needed to charge the battery back up.

The Prius is not designed to be an electric vehicle. It makes very good use of its hybrid nature, and as such the battery serves its purpose as an energy buffer very well. It is sized just right for the purpose of a buffer.

 

"Why limit the use of the battery pack?"

Consider the battery is a string of 168 NiMH cells (NHW20 - current model Prius). The Prius charges and discharges this string from the ends of the string (201V nominal). It can monitor the string in steps of 6 cells (28 modules of 6 cells each).

Consider what happens when you charge the string. If you try to go beyond 80% charge, some cells may reach full charge earlier than the majority. They will then be overcharging, turning all the charge current into heat, with the possibility of loss of electrolyte, and because the few cells doing this constitute such a small percentage of the total pack (mass and voltage), the temperature rise may not be detected soon enough to keep them from overheating, so they may vent electrolyte, loosing capacity and making the subsequent charge cycles worse. This is a classic start of a "death spiral".

Also, NiCad and NiMH batteries become inefficient at charging once they reach 80% charge. They start to convert perhaps 30% more of the charge current to heat. In such a large battery this can be difficult to deal with as the heat must be removed so the cells don't overheat and loose electrolyte. This 80% heat up is used in many "rapid chargers" to cause the charger to switch to "trickle" mode (handheld radios, electric shavers, power tools, cell phones, etc). In this case, if you remove the device from the charger when the light changes (80% charge reached) you will be operating those batteries at the 80% charge limit as well.

Now, consider what can happen when you discharge the string. What if one (or more) of the cells has a capacity 45% lower than the others, either from "wear", from overcharging (loss of electrolyte), or from not charging as efficiently as the others. If you discharge the string lower than 40% of "full", those cells will fully discharge, and if you continue to discharge the pack they will reverse charge. This usually results in cell destruction (often an internal short, permanently removing the cell from the pack).

So, to avoid these problems, the Toyota engineers, in consultation with the Panasonic engineers, decided to limit the pack to about 40 to 80% charge operation. You can actually get a bit lower and perhaps a bit higher in extreme situations, such as running out of fuel. Which is why many panic if an owner reports he/she ran the tank dry.

Note that when new the pack has been "matched", that is, the cells are chosen as a set of closely matched units in capacity and temperature response. So it's not so critical in a new car. But the cells will "drift" with age, so an older pack is in more danger than a new one.

 

Wow David Beale,

That was an excellent discussion of exactly what happens during charging and discharging. Thanks.

 

Some of the cal-cars folks are looking at using these Nilar NimH which seem very promising:
http://www.nilar.com/be/__media/pageID_14/langID_1/Nilar MB 24V 9Ah.pdf
I believe 20 of those should give you a ~15 mile EV pack. A cal-cars conversion with these should run ~$6-7k if you DIY. The custom battery management system is greatly simplified, as the Nilar's contain a pressure sensor. This provides an early and easy to read signal that tells you when the first battery is reaching full. A BMS system for general NimH or any type of Li-ion is generally much more complicated, and currently not available DIY. There are commercial systems out there, but they are $$. If you want to go that route you are probably better off just buying the hymotion pack for $10k. Thats about what most of us can buy 5kW of raw Li-ion cells for anyway, and thats w/o any warranty then you still need a charger, BMS etc.

As far as getting more range out of the built in pack, I really wouldn't recommend it. A few people had been taking cells out of wrecked prius packs to use in full BEVs and found that they only lasted a few hundred cycles w/o all the protection that Toyota had built into its controller. The people who had expanded the built in capacity did it by adding a second toyota pack complete with its management controller. They were able to play some tricks and get both controllers to be up and running at the same time without causing problems.

Another good site to check out is the Electric Auto Associations Plugin Hybrid group:
Prius PHEV - EAA-PHEV
This is kind of the repository for a lot of the publicly available and/or open source info on PHEV conversions.

I know its not real sexy, but a lead acid based cal-cars conversion is really hard to beat for up front conversion cost. Its basically a battery, a charger, a big switch, and a controller to figure out when to open and close the switch. There are a few other details necessary to keep the Prius from freaking out but thats pretty much it. The one thing you could consider doing without is the controller, but you will get much worse performance without it. The Prius will only increase the amount of electric drive under very carefully controlled conditions, and their is really just no manual way to duplicate that. If you can't get the car to use the electric motor more often, it doesn't really matter that you have a bigger battery in the trunk. In the long run the cal-cars NimH will probably be the cheapest overall, as the lead pack will have to be replaced every year or two. The Nilar's should last a lot longer, but I'm not sure any info is available on that yet. Other conversion methods add some good features like a dc:dc converter instead of a switch (contactor), or a better battery and custom BMS, but at the cost of complexity and up front money.

Rob

 

You would spend at least $10,000, void the warranty, and get maybe 10 miles of range. Wait a little. You can probably buy a plug-in Prius in 2010 with perhaps 40 miles of range. Or a Volt, if GM does what it says it will do.

If you really want to be able to drive all day on electric alone, assuming that you mean something like 400 miles, it cannot be done. You can't pack enough of any known practical battery type into a car to get that much range. Even the $100,000 two-seater Tesla gets maybe 200 miles on a charge.

 

Tesla claims 250 mi on a charge, recharge time in the 3-4 hrs range. Which makes Klick and Clack's comment "Hey, in this car we could be in Mexico in a few hours" when they were test driving just a little misleading. I think Mexico was more than 250 mi away.

 

Thank you for your kind comments.

Now, I've been working in the two-way radio business for 30 years and no manufacturer ever thought of limiting the charge/discharge. The "word" was fully discharge and fully charge to avoid "memory". It's the Toyota and Panasonic engineers who had the breakthrough.

When I read my Sanyo battery manual, it clearly says NiMH batteries still have the "memory problem". Something I was told by others was gone with that chemistry. I have no idea why the Prius pack doesn't suffer from it. Perhaps it's controlling the temperature that does it.

At any rate, it's not my knowledge I'm posting about. I -will- take credit for making it understandable (if I did).

Perhaps I did learn a little while trying to recover those $250 Motorola hand held batteries, perhaps not.

 

In that case you want a Zap Zebra. See http://www.zapworld.com/cars/xebra.asp

One of the members here has one.

 

Very iffy even with the most expensive available aftermarket upgrade. Unless roughly doubling the cost of the Prius is of no consequence to you, you really should wait a year and see whether the 2010 factory model provides that capability.

 

As David mentioned, battery packs are a collection of cells, and therefore have what amounts to multiple personalities when it comes to charging. One of the trickiest parts of Lithium batteries (which is what we are likely to see in later generations of hybrid and electric vehicles) is that each cell has to be monitored individually such that efforts to charge the pack do not overcharge the cells in the pack with the highest charge relative to the other cells in the pack. The higher the nominal rate of charge, the easier it is for an imbalance to develop. While this is very critical with Lithium cells (because the tend to ignite when overcharged), it is also important with Nickel based cells, as the excess heat that overcharging generates is destructive to the chemistry.

In the end, it is not overly important that the Prius battery has constrained charging endpoints, because the battery is not used in the manner that batteries typically are (being fully charged, then fully used and repeat!), but as a buffer to capture excess kinetic energy, and to release it as needed).