FEATURED ARTICLE
Marc Kohler, Valence Technology
Marc Kohler mans Valence Technologies booth at EVS 20, November 2003. On display are number of different lithium-ion batteries designed for EV applications.

Building a Safer Lithium-ion Battery - Part 2

Conclusion of interview with Valence Technologies Marc Kohler

By Josh Landess

Good Charging Practices... 10,000 partial cycles???

EVWorld: Let's say I called you and said I want a battery. What would you really emphasize to me that you would not do with your battery if I wanted to run the number of cycles it's supposed to run?

MK: We'd ask you which charger you were working with. That's one beauty about our batteries is they can use existing lead acid chargers. You don't have to have special five step algorithms or anything. So if you have a charger that can be programmed for a certain voltage, depending on your pack, we'll tell you which voltage to go to. Now if your charger runs away, has a failure or whatever, you can kill the cells, which you may be upset about, but there's not anything that I can tell you to stop.

EVWorld: Well for example, Tom was saying for some reason he found that not topping them off helped extend battery life.

MK: Well what they were saying there is, in a battery cycler application for maximum energy, you will charge them up to the highest voltage they can take safely in a Cobalt cell, and discharge it from there and that's the way you get your maximum range. But if you want maximum life, the higher the voltage you go, the quicker it breaks down, the electrolyte or so forth. So in that case he says, "Yeah, I'll only charge to 4.1.... I may only get 90 percent out but I…"

EVWorld: "I still have so much to spare...."

MK: Exactly.

EVWorld: Well what about fast charging? Is that going to damage battery cycle-life?

MK: We are doing some tests with Cobalt cells now and we do see it's degrading a lot faster than ours, so we're excited that ours don't seem to be as affected by fast charging or discharging as others, because you've got to remember the original cells were made for ... very slow charging rates.

EVWorld: The big excitement with the tzero in part was that it could help me meet the specifications out there that were laid down by CARB and others that were kind of assumed to be impossible for battery electric vehicles to meet. Now with fast charging, if it could be done such that you didn't destroy your expensive battery pack, you could do a ten minute charge for example and go 100 miles or something?

MK: It depends on the size of the pack. It depends on your charger but you can charge very quickly without damaging.

One good thing about Lithium Ion, especially our batteries; you don't have to top them off all the time like lead acid. Lead acid likes to be charged all the time. If you want to live in the 50, 60 percent state of charge range, that's fine with Saphion You're not gonna hurt the battery. In fact it'll probably last even longer.

EVWorld: What is the number of cycle-lives of your pack?

MK: Down to 80 percent depth of discharge, greater than 2,000. Where the 80 percent came from is lead acid. If you take 80 percent out of lead acid, at the very end ...the voltage drops. [The last 20 percent] ...there's not very good useable energy. So, left over from the lead acid days people stopped at 80 percent discharge, but with Lithium Ion you can go all the way to 100 percent without damaging the battery.

What's good is that with the Lithium Ion battery, ours, when you do partial depth of discharge, [such as] 50 percent, you may get over 10,000 cycles. So whereas a lead acid may go 300 to 500 cycles, you can see we're at 20 times more cycles. In an EV application where people are driving around, very few times do they go 100 percent every single time. They go to the store and they come back: It's all random.

EVWorld: More often than not you wouldn't discharge fully.

MK: You wouldn't. You would do some partial [dis]charge, right? So you're not going as far down.

And the other thing is – since you have more energy on board ...you're unlikely to use it all anyway so if you had a 40 mile range before and you needed that 40 miles, now you put in a Lithium Ion pack, you may have 80 to 120 mile range and you're still using only that 40 miles on the commute but you have some reserve. So that partial discharge really helps out the cycle life cost analysis. So the battery should last as long as the car.

EVWorld: Do you have a demo car?

MK: A company called Electric Vehicle International has delivery trucks that they make. Their primary market is Mexico City. They make electric trucks and they purchased a pack and put it in for evaluation. We're also working with the telecommunication industry and utility substations. They're looking at decreasing the size and maintenance of their backup systems, and that's where the K-charge came in.

EVWorld: Have you had a feedback from Electric Vehicle International about what kind of range they're getting and what kind of recharge time and so forth?

MK: Two and a half times the range of their original lead acid packs. They did require a certain range increase for the application but that wasn't as important as the load carrying capability so instead of carrying around 2,000 pounds of batteries they were able to carry around 1,200 pounds of payload so that was what was interesting to them. Recharge time rate didn't change very much because the chargers they were using still put out the same amount of power. You're limited to your charger and your plug and your infrastructure, so if you can only charge at 20 amp rate...

EVWorld: ...you're not implementing any of the fast charging that we've seen here.

MK: Right.

EVWorld: I want to know more about rapid charging just because if you can fit the specs then you could compete against any of these fuel cell vehicle claims and gasoline car claims but can you give an idea of some numbers? If a guy owned a Saphion powered EV and he had to stop for electricity, how much time it would take him to charge up to go X number of miles, or is that too specific?

MK: Well it depends on lots of factors. For example, if you had a big enough charger… Say you could recharge our batteries back to 90 percent within an hour.

EVWorld: Oh wow.

MK: But, if your car has 30 kilowatt hours on board you better have a 30 kilowatt charger, and that's very rare. 30 kilowatt hours, if your car is small, may give you a 200 mile range.

EVWorld: It's rare, but it's being talked about more. I don't know if that's dramatically less energy efficient to do fast charging. Maybe you can tell me?

MK: Not that I'm aware of. In fact our battery's Coulombic is almost 99 percent efficient return. What you put in you get it back out.

CONCLUDED IN PART 3

Times Article Viewed: 9767
Published: 10-Jan-2004

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