Running the numbers on battery specsJuly 18, 2007
I’ve been doing some research lately on the different battery technologies that are being formulated for EV use – all proven lithium ion chemistries and cell formulations. If you have a modicum of understanding about math, electricity and engineering, you might find what I found out rather interesting, because there’s stuff out there that isn’t well explained – or well publicized.
1) Lithium ion with cobalt oxide cathode- This is the most proven technology – energy densities up to 200Wh/kg! It’s the technology being used in the Tesla Roadster due to its high capacity and wide availability. But there are drawbacks – the cells are inherently unstable, and can explode or burn when overcharged or physically damaged. That’s why Tesla has extensive technology in place to protect the LiCoO2’s from both the environment and themselves. Cobalt is also rare, expensive, and found in China. It’s also used in the widening Chinese steel industry, so there’s competition for it on the horizon.
Here’s where the numbers show some interesting things:
The Tesla pack has a capacity of 56000 Watt-hours. With 6,831 laptop cells in place that translates to each cell containing about 8.2 Watt-hours. Divide that by the typical LiCoO2 voltage of 3.7V, and the individual cell current capacity comes out to 2.2 Amp-hours. In other words, these. A cell with an energy density of 175Wh/kg.
So that’s the culprit cell. Now, multiply each one of these sucker’s weight (46.5 grams) by 6,831 – and you’ve got 317.6kg, or 698 pounds.
But wait! The total pack weight is well known to be 450kg, or 990 pounds. In other words, the other parts of the system – the extensive monitoring and protection components – weigh in at 292 pounds! That’s 30% of the entire pack weight!
What this means is that even though Tesla’s choice was driven by finding the best high energy density, putting these cells together into a cohesive package is still a very heavy and complicated venture.
2) Lithium iron phosphate
This chemistry is newer, but has a compromise. The advantage is incredibly good power density, as well as plentiful, environmentally-friendly materials. Also, it’s got great safety characteristics. The cells don’t explode when damaged and are much more stable under high charging/discharging stress. The folks at Killacycle have never replaced a single cell. But the cells have lower capacity than traditional lithium ion cells.
Again, the numbers present an interesting case once you get past the surface. Detractors to this chemistry – ahem, Tesla – point to the low energy density as a deterrent. However, citing energy densities as being “less than half” those of cobalt cells isn’t accurate – in two ways, at that.
But first, the caveat. Large format cells, like Valence’s U-Charge, DO have energy densities of less than 90Wh/kg. Similar pitiful numbers are observed in C-cell LiFePo4’s (down to 60Wh/kg!)
However, a scientifically better comparison is to not just compare different chemistries, but different chemistries using the same cell format. An 18650 format lithium iron phosphate battery from Valence has an energy density of 117Wh/kg, versus a cobalt oxide’s of 175. That is nowhere near as bad a difference as critics make it out to be. Additionally, the chemistry’s theoretical maximum tops out at 140Wh/kg (80% of lithium ion’s most stable offering). Automotive format cells of this capacity from A123 are anticipated to be used in the Chevy Volt. So saying that one winds up with “twice the weight” as Martin Eberhard has stated, is entirely inaccurate.
What is more, the weight of the eventual battery pack also has to factor in the heft of the safety equipment. Cooling and padding still help to prolong any pack’s life, of course, but given LiFePo4’s superior stability, a less radical amount of technology is needed to control it versus cobalt oxide. Thus, a less energy dense pack may still wind up with a great capacity rating for the whole car – not to mention a greater safety rating.
3) Lithium nanotitanate spinel
AltairNano’s new anode technology, despite being marketed as revolutionary, is actually predicated on existing, 10-year old innovations in manganese-spinel cathode technology. Both these chemistries have lower energy density, but offer extremely fast charging and good safety characteristics.
All the numbers I’ve punched come out to an energy density of around 85Wh/kg (based on a 900-pound, 35,000Wh battery pack). This isn’t that great, but one has to consider the clear difference that cell format makes in changing this number. Valence’s large format cells are fairly comparable to Altair’s in terms of capacity, and their 18650’s are considerably higher. Additionally, the large format cells that Altair and Phoenix uses have NO active temperature or safety management, if any at all. Put that in a heavy, steel truck or SUV, and the fact that you get a range of “about 130 miles” is pretty impressive. Putting the same pack in a much lighter car undoubtedly would push the range higher – well into Tesla territory.