Safety

22Jan 2016

I described in the earlier post how adaptive systems turned smartphones into great cameras. Let’s now talk about how adaptivity and adaptive charging can make a battery perform far better.

Let’s start briefly with the basic operation of a lithium ion battery. The early posts of this blog describe the operation of the lithium-ion battery in more detail.  I will briefly recap here the basic operation and explain where its performance is limited. For the reader who wants to learn more, select “The Basics” category tag and feel free to review these earlier posts.

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16Dec 2015

What do Chappaqua, NY and Brentwood, CA have in common? Burning hoverboards….this is a new category of battery-powered levitating boards, sort of electrically-powered, self-balanced skateboards. In both cases, the fires and ensuing explosion were attributed to the lithium-ion battery. Amazon has discontinued the sale of such devices. So what is happening?

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21Aug 2015

One of the serious consequences of fast charging lithium-ion batteries is the formation of lithium metal on the surface of the anode (the negative electrode when the battery is being charged). While the battery industry has invested significant effort to ensure the mechanical integrity of the battery and avoid unintended fires in case of mechanical damage, the formation of lithium plating during fast charging is a new challenge to battery vendors. Some battery manufacturers take it very seriously, whereas others tend to be more lax if not somewhat cavalier about its risks.

Let’s be clear about…Lithium metal plating inside the battery creates extremely hazardous conditions that may lead to fires or even exploding batteries.  Lithium plating leads to the gradual formation of lithium metal deposits on the surface of the graphite (carbon) anode. These islands tend to grow over time, both across the surface area and in thickness forming dendritic-like structures. If they pierce (and they can) the separator — the porous plastic layer between the two electrodes — then an electrical short-circuit occurs leading to excessive heating and potential fires (in battery parlance, it is politely known as thermal runaway).

For the vast part of the history of the lithium-ion battery, lithium plating was not a major concern. Well designed batteries ensured that they stayed away from the precursor conditions to lithium plating. Some battery manufacturers implemented additional safety measures — such as special surface coatings — that are intended to reduce the risk of a dendritic short-circuit. But with the advent of high energy density cells and the rapid deployment of fast charging, batteries are often operating near dangerous conditions. And some battery manufacturers seem to intentionally skirt the problem as it is not visible during daily operation — that is until a fire occurs and the damage is done.

The next photographs show the anode surface of a dissected polymer lithium-ion cell — in fact, two identical cells, one charged at a slow charge rate (left side), and the other charged at a higher rate (right side). The cells were cycled 100 times before cut open and inspected.

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On the left side, the surface of the graphite anode is pristine. On the right side, bright stripes of lithium metal are apparent on the edges. That’s where lithium metal tends to start forming — the current density on the edges tend to be higher (concentrated electric field lines) thus presenting favorable conditions for the formation of lithium metal. Additionally, manufacturing defects are more likely to be present on the edges, also presenting “seeds” for plating.  As the cell is further cycled (and aged), the lithium plating propagates and covers more of the anode surface, creating increased risks of a catastrophic failure.

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08Jul 2015

One of the most recognizable electric vehicles on the road, the Nissan Leaf, has a battery capacity of 24 kWh. It has a rated driving range of nearly 80 miles, about ⅓ that of the Tesla Model S. It is therefore not surprising that its battery capacity is also nearly ⅓ that of the Tesla’s battery which I covered earlier in this post. Today’s post, with the help of some publicly available information, sheds some light on what powers the Nissan Leaf.

The battery in the Nissan Leaf is manufactured and assembled by the Automotive Energy Supply Corporation (AESC), a joint venture corporation between Nissan and NEC located just outside of Yokohama, Japan. Until Tesla’s Gigafactory comes on line, AESC’s factory remains the largest automotive battery manufacturer shipping nearly 90,000 batteries annually, mostly into the Nissan and Renault EVs and hybrids.

AESC discloses on its website some pertinent details about the battery and its characteristics. A teardown of the Leaf battery pack by Ben Nelson at 300mpg.org supplements this post with a nice step-by-step mechanical disassembly of this pack. The weight of the Nissan Leaf pack checks in at 648-lb, about ½ that of the Tesla’s pack, yet only ⅓ its capacity. I will revisit this point below.

The first photograph shows the pack with its top protective metal case removed. The pack measures approximately 1570.5 x 1188 x 264.9 mm (61.8 x 46.8 x 10.4 in).

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Photograph of the battery pack for a Nissan Leaf electric vehicle. One can readily see some of the smaller modules that make up the pack. Courtesy: Benjamin Nelson.

The first observation we make is that the pack consists of smaller modules. In fact, AESC tells us that there are 48 of them, each measuring about 303 x 223 x 55 mm (11.93 x 8.78 x 1.38 in) and weighing about 3.8 kg (8.4 lb). These modules are arranged into three distinct sections, one near the back with 24 modules bolted to each other in a vertical position, and two other sections on each side of the pack, each with 12 modules in a horizontal position. Electrically, the modules are all connected in series. Bus bars (thick copper connectors) electrically connect together these three distinct sections.

Each module is made of four individual pouch (also known as laminate) cells, each cell like the one shown in the next photograph. The four cells are electrically configured as 2 in series and 2 in parallel. This prior post teaches more about series and parallel configurations.

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Photograph of the battery pouch cell used in the Nissan Leaf pack. Source: AESC

AESC shares some of the electrical characteristics of the cell. Each cell is rated at 32.5 Ah, or about 10X that of the 18650 cell used in the Tesla. It uses a different material for the cathode called lithium-manganese-oxide with nickel oxide (LiMn2O4 with LiNiO2) that is inherently safer than the lithium-cobalt-oxide cathode material used in mobile devices and the Tesla pack. The cell’s voltage chart shows a maximum cell voltage of 4.2V. Rated nominally at 3.75V, one pouch cell can store a maximum 122 Wh of energy, or about 10 times what an iPhone 6 Plus battery can store.

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So let’s do some math. Each module contains 4 cells, so that’s a total energy of 488 Wh. This is now a substantial amount and hence one should exert plenty of caution in handling or using such modules. The nominal voltage across one module is 2×3.75 = 7.5V, and the nominal voltage across the entire Leaf pack is 48×7.5 = 360V. The maximum voltage at the pack is 2×4.2×48 = 403V, though it is widely known that the Leaf only uses about 80% of the pack’s capacity (20 kWh out of the 24 kWh to preserve cycle life) making the maximum cell voltage closer to 4.0V, and the pack’s maximum voltage closer to 384V.

The voltage chart above shows that one cell can deliver at least 90A of current. That’s equivalent to a pack delivering more than 180A at 384V, or 70+ kW (95+ hp) of power to the drivetrain. This estimate is not far off from the Leaf’s vehicle rating of 90 kW (120 hp). In any case, one can see that both current and voltage values are high, warranting special design measures to ensure safety.

But for the added safety of the LMnO material, Nissan incurs some important penalties. First, the intrinsic energy density of the individual pouch is only about 320 Wh/L. Compare this to nearly 700 Wh/L for the Panasonic cells used by Tesla. Why does it matter? Energy density translates directly to range, and range, or rather lack of it, is right now the #1 challenge for electric vehicles. This is precisely why the Tesla pack weighs only twice more than the Leaf pack, yet delivers 3x more driving range. In other words, a Nissan Leaf using a hypothetical battery with cells at 700 Wh/L should be able to deliver a range of 120 – 140 miles, instead of the present 80 miles. I want one of those!

Second, the use of large pouches makes it necessary to have dual levels of packaging, one at the module level, then again at the pack level. This adds unnecessary weight and volume to the pack. Look at the energy density for the module and the pack. For the module, it is down to 131 Wh/L, and for the pack, it is a dismal 49 Wh/L.

Another way to look at this mechanical inefficiency: The total weight of the 192 cells is 151 kg (332 lbs) — that’s the part that really stores energy — to which the steel boxes, plates, wire harnesses and electronics add another 144 kg (316 lbs) for a total pack weight of 295 kg (648 lbs). In other words, that’s 316 lbs of added weight that contributes zero to energy storage. Each pound of weight in the Leaf battery pack stores 37 Wh of energy. By comparison, each pound of weight in the Tesla S pack stores 64 Wh of energy!….this Leaf pack design does not scale well for longer driving ranges.

This, folks, says that a Leaf is a good Gen 1 vehicle, but that Nissan needs to figure out major improvements to its battery if it is to become widely adopted beyond select green and affluent communities like our San Francisco Bay Area.

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