The many cells that make up a lithium battery pack are not all equal; some will degrade and die faster than others. New research out of Stanford has found that the whole battery can live much longer if each cell gets an individual charging treatment.
There are many reasons why individual cells in a pack might become weaker than others. Maybe there's variances in manufacturing or materials. Maybe some are more exposed to heat sources than others, or located in spots that are harder to cool. Either way, the average single battery cell lasts longer than the average battery pack, and it's these weak cells that take the whole ship down with them.
“If not properly tackled, cell-to-cell heterogeneities can compromise the longevity, health, and safety of a battery pack and induce an early battery pack malfunction,” says Simona Onori, an assistant professor of energy science engineering at the Stanford Doerr School of Sustainability, and author on a new study aiming to keep lithium battery packs useful for longer.
Fast charging and discharging events are stressful for battery cells, and while they're designed to take that stress, these are the moments in which weaker cells suffer and deteriorate the fastest. So the Stanford team wondered whether the standard technique of charging all a battery's cells at the same rate might be accelerating battery death.
The researchers painstakingly designed a computer model to test their theory over an accelerated time frame, resulting in what they believe is an unprecedented level of simulation detail. They attempted to accurately represent the physical and chemical state of a battery, as well as the changes that occur in relation to a range of stresses across its whole lifespan, including both changes that happen in seconds, all the way up to others that might take months or years.
“To the best of our knowledge, no previous study has used the kind of high-fidelity, multi-timescale battery model we created,” says Onori.
Using this model, they ran a number of simulations comparing a standard, set-rate charging approach against other approaches, in which each individual cell's capacity served as an indicator of how much charge it could take. The theory here was that only the strongest cells should be subjected to the highest stresses; cells that had already begun to degrade early – for whatever reason – should be treated much more gently, in the hope of staving off their eventual decline.
The team found that by individually setting the charge rate of each cell, they could minimize temperature increase and cell degradation, to the point where these packs could handle at least 20% more charge/discharge cycles than a battery that charges uniformly – even using frequent fast-charging.
The drawbacks here are fairly obvious; if you're fast-charging your EV or phone battery, you want it to charge as fast as possible, so you can get back to whatever you're doing, and under a model like this, a certain number of cells in your battery simply wouldn't be charging as fast as they would normally. If you see your batteries as more or less disposable items, and your cars as something that gets replaced every few years, you can see how plenty of consumers wouldn't care if they're accelerating the death of their battery packs. It's somebody else's problem.
On the other hand, it's not like you're generally fast-charging all the way to 100% when you're in a hurry, and the majority of cells in the majority of batteries are just fine, and capable of taking a fast charge. So the difference in the state of charge at the end of half an hour on a supercharger might not be hugely different under this charging model, and if batteries can be coaxed into a longer useful life, that's better for everybody – particularly given the lithium squeeze that's projected to put pressure on decarbonization efforts in the coming decades.
The researchers say their charging model can be easily rolled out through existing electric vehicle designs, or used to guide the development of next-gen battery management systems. They also suggest the same model could be applied to the discharge cycle, asking less from weaker cells and more from stronger ones, for further benefits to the lifespan of any battery pack that gets subjected to high stress loads. Indeed, one of the study authors is now working as a battery researcher at eVTOL developer Archer Aviation.
“Lithium-ion batteries have already changed the world in so many ways,” says Onori. “It’s important that we get as much as we possibly can out of this transformative technology and its successors to come.”
The study is published in the journal IEEE Transactions on Control Systems Technology.
Source: Stanford University
So, I've got an Eneloop NiMH battery set in older and newer varieties. The newer allowed individual batteries to be charged while the older one charged them in pairs. I'm wondering if this is simply a matter of making battery more modular - where individual cells can be simply popped in/out without soldering. Would individual charging require a BMS to have a direct connectíon to each cell rather than them being connected in series? Does this massively increase the production cost?
How close are we to building safe, large battery cells that can store more energy so packs can be made of fewer cells?
I just charge the whole lot in one go - Which is fine as usually crash or lose the drone before the batteries need replacing!
If they want battery packs to last longer, they could add an extra cell or two--although marketers would hate that because it lowers the bragging numbers--and swap which cells are connected at a given time. This would lower stress on the weakest ones, and squeeze a bit more life out of the whole pack. That would be for situations where battery longevity is the top priority (hard to replace locations, etc).
A better technique for improving longevity might be to allow a controller setting that sets full charge at less than 100% and a 'charge now' level at higher than approaching 0%, and a charging rate at less than absolute maximum. Last time I checked, Li-ion cell longevity depended on depth of discharge, maximum charge, and rate of charge (and temperature). So, push the button for 'maximum charge as fast as possible' or the one for 'least expense in the long run'.
I'm guessing this "study" might be related to an invention or patent or manufacturer who's already planned the integration chip that every *cell* will require to suit this charge and discharge approach, and the study is just a sneaky PR campaign to make pack-builders look for and use their new chips.