AutoblogGreen Q&A: Denise Gray talks batteries, state of charge and more
Prior to the opening of the LA Auto Show this week, AutoblogGreen sat down with Denise Gray of General Motors to talk about batteries. Denise is the director of Hybrid Vehicle Energy Storage Systems and oversees all the battery development going at GM for vehicles ranging from the new Two-Mode hybrid SUVs to the Chevy Volt.
AutoblogGreen: Why don't we get started with the current status of battery development for the E-flex program. Bob Lutz recently mentioned in an interview you have received the first pack from CPI. Where do things stand right now?
Denise Gray: Well, just to step back a little bit in the May-June timeframe, we got our contracts together, we worked with our two chosen suppliers CPI and Continental and we have been working those programs feverishly. We have come up with designs that are buildable, if you will, for our first mule build, or our first bench build that is probably the more proper name, what that design should be composed of and CPI delivered that on October 31. I was over in the lab, in fact, when they called and said, "Hey, they are here. They are at the grounds. Come on. But our security guys routed them to a different gate, so they will be here in ten more minutes."
So it was kind of like waiting for the birth of a baby. They brought it in, they had their big truck they brought it in. We had our forklift. The guys went and got it and I said make sure you handle it very well, so it was brought in.
Check out the rest our conversation after the jump to learn about how GM determines the state of charge of a battery and the current status of the PHEV Saturn Vue.
DG: I was actually at CPI that Monday prior to the Wednesday that they delivered to also see it there and to understand how they had progressed as well, and judge its readiness to come over to our lab.
So it is in the lab, the teams, the GM and CPI team had been working collectively in the lab since that time doing our characterization, making sure that we can monitor all of the voltage and currents from each cell, making sure that as we go through our safety handling of the battery that all of the necessary things are working, because again, there's lots of energy, lots of power. I don't want to have anybody at risk with this first bench pack, so we were walking through with our checklist of things that we want to make sure things works with these sensors, that we have got these thermal couples. They are all working. The redundancy that we have to have in our lab to test these parts are in place and the team has actually got it on the cycler now.
The contactors are closed and we are beginning methodically and charging and checking to make sure everything is correctable. We will work through that in the next couple of weeks to a point where we feel comfortable before we will go and do deep discharges and charging up and going through different kind of systems.
ABG: Now, this first pack, is the packaging of the pack the same as what you are going to put in to the first mules, or is it more of just an open set up for bench testing?
DG: No, I think it will be capable of putting into a mule, but not completely capable. There are still going to be things that you don't want to do if you will because it is made for the bench. There are structural things that are not completely production intent, so I don't expect for this thing to go through a battery of Belgian blocks and heavy vibrations and that kind of thing, but I think it will be decently able to be used to go into a vehicle eventually for mule integration type test.
ABG: And when do you think that you will get the first units that you will actually put in the vehicles. When do you expect that to happen?
DG: We have got a roll out over the next couple of months with the packs coming in from CPI, some additional packs coming in from Continental. Over the next six months, there is a roll out of a pack this week and another pack that week. We are going to use this pack for this and that kind of thing, so it is really ramping up now with the first one on October 31 and over the next month or so there will be packs coming in from the different suppliers to facilitate different needs.
ABG: The first pack that you have, is that essentially complete? Does it have a cooling system and everything?
DG: The cooling system is in. We don't have a coolant flowing through it and there are still some things that we want to check out and make sure that all of the points are very well tapped, so there are no leaks and that kind of thing so the cooling system is intact. But we don't have coolant flowing through it yet because that is another part of the checkout and making sure those pieces are in place. We can actually operate it without the cooling system actually active at this point in time.
ABG: If it's sitting on a bench, you could air cool it and as long as you are not cycling it too aggressively, it probably would not be a problem.
DG: So there lots of learnings, right now, even without the cooling system working.
ABG: When do you expect to get the first packs from Continental?
DG: They are scheduled sometime around the end of the year, and that date is still being defined and we are still working through the readiness of that pack for delivery.
I think the first pack was the most important and getting the hardware and so we could see what it is looking like and we can actually do our own testing to make sure that our assumptions were correct. I think this exercise that we have been undergoing over the last couple of weeks have taught us what we want the next packs to do.
It might modify the timing quite frankly based on what we are learning here. I don't have to quickly get this in, I can let them do a little bit more finessing of the design, so my timing is going to be really a function of what else I need to learn to determine if I want them to keep it longer or to release it earlier to me, so that is kind of variable right now. It is kind of how I have got this going.
ABG: Getting a little bit more technical, one of the issues dealing with battery packs for hybrids and electric vehicles is monitoring the state of charge to maximize the life span and safety of the pack. You want to cycle it between a certain charge and discharge level, can you talk a little bit about how you estimate the state of charge of a battery pack?
DG: Actually that is done in-house. We are not asking the suppliers to provide that level of design from them. We are actually developing that in-house.
ABG: The control software?
DG: The control software as well as - all I am asking the suppliers to do is to provide me information, current, voltage, temperature. Then my scientists and we have been doing this for the past year, it is just not from the E-flex we have been having lots of learnings on nickel metal hydride where my scientists characterize the behavior of the different chemistry. We characterize and model that. We then develop an algorithm that models the battery that we can then put into our software. I am asking for information from the battery pack, current, voltage, temperature and those kind of rudimentary parameters and then my organization determines what the state of health is. What is the state of charge for that battery.
So we use that knowing how we want to use it in the vehicle so that we can provide its state of charge, state of health. The level of, the depth of discharge that we want to use it based on how we understand that chemistry and how we also want to use the overall energy in the vehicle.
So that is all done in-house.
ABG: The reason I asked, in the old days before we had nickel hydride batteries for consumer electronics with alkaline cells and things like that, as they discharge, the voltage you got from the battery would drop gradually over time. With nickel hydride batteries, in particular, the voltage tends to stay relatively constant until they are done and then it just kind of drops off, so for batteries like that, what sorts of things would you look at to determine the state of charge of the battery. Obviously, the voltage by itself is not sufficient, how would you determine the state of charge?
DG: Well, there are tests that we run outside of the normal - we call them characterization test and every chemistry, be it lithium iron phosphate or be it manganese spinnel, we characterize that chemistry when it comes to current and voltage and overall performance under hot conditions, cold conditions and just its performance through the depth of discharge kind of test, so we run a battery test in order to understand how to characterize that chemistry, and then from there, we recognize where the fall off zone is, and everybody knows, for iron phosphate for example, the curve is somewhat flat in certain areas and with manganese spinnel, it is a little bit different. It is a little bit easier.
For nickel metal hydride, it has a curve where you can understand where it is at as well, so we do a battery of tests outside of actually real-time from a vehicle perspective, so we do that outside, we get lots of samples from the suppliers. We do that characterization, samples over a wide variety of lots to make sure that we understand the variation, and we go through and model that, and that helps us to understand where that knee of that curve actually is going to occur and we make sure we stay away from that if you will so that our useful life is somewhere other than that point.
ABG: So even though the curve tends to be more flat than what you would get with an older style battery, there is still enough of the change there over time that you can measure that and see where it is going.
DG: In fact, I got an email, I was walking out to the parking lot two days ago and I ran into my engineer who is actually doing that modeling, and I said, "How is it coming with the different chemistries?" and he said, "I think I got it." And so they have continuously trying to get more and more resolution in that curve so that we can clearly detect its state of charge, how it is going to operate in a different condition and that is being modeled and it is something that our organization has internally.
ABG: How different are the characteristics of, say, the iron phosphate batteries from A123 versus the types of cells that you are getting from CPI? What type of chemistry does LG chem use?
DG: Manganese spinnel.
ABG: So how different are those in terms of their characteristics?
DG: There are some very similar characteristics, and there are some, obviously like I mentioned that the iron phosphate is a lot more flat of a curve if you will. Those have been the only perceivable differences.
ABG: When you have got a flatter curve, like with the iron phosphate, would you do things like for example, factoring in the time? If you have charged it, factor in how long it has been running?
DG: Well, if I tell you I will have to kill you.
ABG: I guess that is a little bit too much to ask.
DG: There are many kinds of things, there are tricks that we have and there are different kinds of models to allow us to get better resolution of that to use different parameters in order to get a better feel for what that flat spot of that curve is.
ABG: Another question that you may or may not be able to give me an answer to, are the battery packs consistent enough from one to another that you can have one model and would not necessarily have to calibrate to a specific pack or would you have to calibrate each pack and get some characterization for each individual pack?
DG: No, our goal is to have a model that is chemistry dependent as opposed to pack dependent and we will go through a lot of development, a lot of sample, a lot of confirmation to try to get a good bandwidth, to catch the variation and the CPI chemistry versus the A123 chemistry. We will go through them. We will do that today. We will go through getting a big enough sample size of the chemistry and then validate if our models still applies and we will do that continuously throughout this entire development cycle and then there will be particular things that we have put in calibration that are going to be more vehicle specific if a vehicle-a configuration vehicle number one with these kind of tires versus vehicle configuration two with this different kind of tire, wants a different performance, then there will be some calibration factors that we will modify in order to get that application specific kind of characteristic, but our goal is to have the core algorithm applicable to a particular chemistry.
ABG: On a slightly parallel path, what about the Saturn Vue plug in, the two-mode plug in, how is that program coming along?
DG: It is going pretty good. It doesn't get the big fanfare, but we actually have packs in the lab now and we have got packs of it a while ago, actually late summer time in the August time frame, we began getting bench packs, bench packs are here in Warren (Michigan), in my lab as well as packs out in Milford (Michigan, the GM Proving Ground where vehicles are tested) because we have got not just what the battery team has to do to make sure that everything works fine, but the rest of the control system and the rest of the control system happens out at Milford, so those packs have been delivered from both suppliers. Those suppliers are Johnson Controls-Saft as well as Cobasys/A123, so we have got work happening in both of those areas. We have got the same kind of modeling of the behavior of those chemistries, working through those as well and we have got some good model work happening and we have got some very good characterization that is occurring, so that one is progressing right on schedule as well as there is still a lot to learn, but yet both are progressing. So we get a lot of activity happening.
So those are running. They are cycling in the lab. They are actually in our environmental chambers, so they are a couple of months ahead of the E-flex one, but that is how our program started. That was started in January and this one started in the June time frame.
ABG: Okay. Thanks so much for talking to us today.
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