Monday, September 29, 2014

14. WHY IS CHARGING WITH CCCV BAD?                                  

CCC...what? Yes, it is a mouthful and it stands for "constant current constant voltage." It is presently the charging approach used for lithium ion batteries. As I indicated earlier, it was invented in the 19th century for charging lead-acid batteries, and somehow it became the default charging methodology for present-day lithium-ion batteries. This is what your smartphone, tablet and your electric vehicle do to charge your lithium-ion battery.

As the name implies, the electronics in the battery management system (see the earlier post on BMS) charge the battery initially with a constant amount of charging current. The higher the charging current, the faster the charging time; and consequently, the higher the power; and therefore, the bigger the AC adapter (or wall charger) to accommodate the higher power rating. That's why a tablet adapter, typically rated around 12 Watts, is bigger than a typical smartphone adapter which is rated at or near 5 Watts. And that's why an electric vehicle requires a far bigger charger, rated above 6,000 Watts.

As the battery is charged, its terminal voltage rises. A single lithium-ion battery starts near 3 Volts, and as it charges, its terminal voltage will rise above 4 Volts. When the voltage reaches a predefined limit, often 4.35 Volts, the charging electronics will switch from a constant current to a constant voltage -- this is to ensure that the voltage across the terminals of the battery never exceed 4.35 Volts. Higher terminal voltages risk the trigger of unsafe failures. Incidentally, never charge a lithium-ion battery with a charger that was not designed specifically for lithium-ion batteries.

Now a lithium-ion battery's internal chemistry is quite complex. When the battery is being charged, lots are happening inside the battery. As I explained in prior posts, lithium ions are traveling from one electrode to the other and inserting themselves within the electrode. This is all happening within the battery and is transparent to you, the end user. Alongside this charging process, as you might imagine, there are other bad and undesirable things that are happening too. For example, it is easy to imagine that all the lithium ions will travel together and happily make the journey from one electrode to another. In reality, these lithium ions will "collide" and they will bond together to form dangerous  deposits of lithium metal. These lithium ions are now out of the picture and can no longer participate in storing electrical energy. It is analogous to traffic jams on highways because cars do get into accidents; the notion that cars on a highway will travel merrily at 65 mph and stay in their own lanes is somewhat naive and left only to a utopian universe.

Now, it turns out that CCCV charging is greatly responsible in how lithium ions travel inside the battery. There is sufficient proof now that CCCV is one of the key factors that accelerate the damage inside the battery exhibited by the loss of lithium ions. Think of it as the ill-timed traffic lights or poorly marked signs on the road that can cause unnecessary accidents.

So you might ask, how come this issue was not observed in the past? Well, lithium-ion batteries of yesteryear are akin to a rural highway with very few cars on it. So even if the highway signage was defective, there were relatively few cars on the road. Batteries from past years have low energy densities, and consequently have fewer lithium ions to go around, so they were more forgiving. But modern batteries are now packing higher energy densities, in other words, they contain a lot more lithium ions than their older sisters ever did, and thus are extremely sensitive to how they are charged. This combination of high energy density batteries with CCCV charging is a recipe for excessive damage, less capacity, short cycle life, and consequently, a very poor consumer experience. 

Think about it next time you wonder why your battery seems to be losing its freshness.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp