App note: Switching battery chargers

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App note from Richtek about several aspects of switching chargers for single cell Li-Ion batteries. Link here (PDF)

Longer battery life and shorter charging times are some of the challenges in battery management in modern hand-held applications like Smart-Phones, Tablet PCs, POS and other portable equipment.

Devices with powerful processors are more power hungry and require larger capacity batteries to guarantee battery life. To quickly charge large capacity batteries, powerful high current chargers are needed. Linear chargers have too limited charge current capability for this purpose, so switching charger topology has to be adopted.

Li-Ion Tech Staring Into the Abyss with Note 7 Failure

Unless you’ve been living under a high voltage transformer, you’ve heard about the potential for Samsung’s latest phone, the Note7, to turn into a little pocket grenade without warning. With over 2.5 million devices in existence, it’s creating quite a headache for the company and its consumers.

They quickly tied the problem to faulty Li-ion batteries and started replacing them, while issuing a firmware update to stop charging at 60 percent capacity. But after 5 of the replacement phones caught fire, Samsung killed the Note7 completely. There is now a Total Recall on all Note7 phones and they are no longer for sale.  If you have one, you are to turn it off immediately. And don’t even think about strapping it into a VR headset — Oculus no longer supports it. If needed, Samsung will even send you a fireproof box and safety gloves to return it.

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Every airline has been broadcasting warnings not to power on or charge a Note 7 on a plane. Image Source: CNET

It should be noted that the problem only affects 0.01% of the phones out there, so they’re not exactly going to set the world on fire. However, it has generated yet another discussion about the safety of Li-ion battery technology.

It was just a few months ago we all heard about those hoverboards that would catch fire. Those questionably-engineered (and poorly-named) toys used Li-ion batteries as well, and they were the source of the fire problem. In the wake of this you would think all companies manufacturing products with Li-ion batteries in them would be extra careful. And Samsung is no upstart in the electronics industry — this should be a solved problem for them.

Why has this happened? What is the deal with Li-ion batteries? Join me after the break to answer these questions.

Lithium

No, we’re not talking about the song from Nirvana. We’re talking about something much cooler —  the third element in the periodic table! Lithium is part of the alkali metals group. All elements in the group have a single valence electron, which it loses easily. This makes elements in this group highly reactive. When a lithium atom loses an electron, the atom becomes a positively charged ion.

Check out what happens when you drop pure lithium into water. The electrons get stripped from the lithium atoms and causes the water to undergo electrolysis. It breaks in to H and OH groups, with the OH groups having a negative charge. This causes an ionic bond with the positive lithium ions to form lithium hydroxide, and the leftover H’s bonds together, eventually making hydrogen gas. The lithium hydroxide is soluble in water, and immediately breaks back into ions.

Long story short — lithium is highly reactive because it loses electrons so easily and forms positive ions. And this comes in handy when we want to make electrical current!

The Lithium Ion Battery

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BGR reported on this crispy Note 7 back in the first week of September

Using lithium for a battery is a no-brainer. You can find lithium ion batteries almost everywhere these days. From phones to laptops to tablets… even electric cars. The batteries work similar to a basic lead-acid battery, but have a much higher energy density.

They’re constructed by making many layers of cathode/anode pairs, with the cathode being a lithium metal oxide and the anode being graphite. The electrolyte is a lithium salt dissolved in an organic solvent. Each cathode/anode layer is separated by what is known as a separator.

The electrolyte carries the lithium ions through the separator between the anode and cathode. The separator is a permeable membrane that allows the tiny ions to pass, while keeping the anode and cathode physically separated. During charging, the lithium ions move from the lithium metal cathode, pass through the separator and are stored in the graphite anode.  During discharge, the ions move back to the cathode.

The Problem

Now that we have a basic idea of how lithium ion batteries work, we can begin to understand how they can fail.

The biggest fail point is obviously the separator. If a problem occurs with the separator, allowing the anode and cathode to touch, bad things will happen. Add to this the fact that the electrolyte is an organic solvent (most organic solvents are flammable) and you’ve got trouble.

Watch what happens when [JerryRigEverything] starts poking around the insides of a Note 7 battery. To be fair, he would get the same result with any Li-ion battery. However, you see as he pokes holes through the membranes, massive current flow takes place and ignites the organic solvent. And there is little one can do to stop it. It is inherent to Li-ion batteries.

In regards to the Note 7, it would appear that they pushed Li-ion technology too far by trying to cram as much energy as they could into a small space. Samsung reported a manufacturing error that “placed pressure on plates contained within battery cells, which brought negative and positive poles into contact.”

The Solution

One glaring oversight is that the battery in the Note 7 is not consumer replaceable. Imagine how easy the fix would be just to send everyone new batteries and set up a collection process for the old ones. Instead, Samsung now needs to recycle all components in the entire phone… 2.5 million times.

More industry-changing solutions include using safer forms of lithium derivatives, such as lithium iron phosphate. LFP batteries have a 14% lower energy density than typical Li-ion batteries, but are much safer. The oxygen atoms in the cathode are much harder to remove, giving it better thermal and chemical stability. Another interesting note, not safety related, is that they discharge at a constant 3.2 volts, reducing the need for energy hungry voltage regulators.

But the consumer phone market is a blood-sport, and battery life is usually the number one complaint of Android phone users. To reduce your energy density for a safer battery is a very hard sell for an engineering team to make. Well, it used to be a hard sell for the engineers — this incident should turn that around. But again, we thought the same about the hoverboards.

The ultimate solution begs for a major breakthrough. This can come from one of two sides: revolutionary battery technology that breaks through the power density ceiling while reducing the risk of a chemical catastrophe in your pocket, or orders-of-magnitude power draw reductions that would eliminate the need to carry around so darn much Lithium in a such a small package.

Sources

The main image is of one of the earliest reports, a posting on Baidu from August 24th, 2016.


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