Batteries that are 100% risk-free have yet to be designed, but the CSIRO is working on it.
How can a battery lose control? It’s a question the owners of the 300MW Tesla Big Battery in Moorabool, Victoria, want answered as they look over two containerised Megapack units damaged in a blaze in late July, only days after testing began.
When something goes wrong in a battery, clues may be found in its brain – the software that tells it what to do. Battery management systems control the charge and discharge of cells, modules of cells and packs of modules, so that these large storage assets can be controlled at different levels.
A battery management system (BMS) is like a doctor. It monitors what’s going on in real-time, collecting data on temperature, pressure, voltage, current and various other variables. Some of this information can be converted by the battery’s brain into coulombs, a unit of electric charge, for example, to calculate a cell’s or group of cells’ state of charge or impedance (a leading indicator of ageing).
A battery management system is also like a policeman, where boundaries are defined for currents and voltages so that the cells are never pushed above or below a predetermined range.
“Battery management systems are very simple things,” says CSIRO principal research scientist manufacturing Adam Best. “We like to break the systems down into bite-size chunks, so they can be controlled.”
If overcharging occurs, it may be due to a fault in the charger at BMS level or there may be a fault or short circuit within the BMS. A common cause, however, is tiny debris that has sneaked in during manufacturing.
“It’s a massive problem,” Best says. “As a cell charges and discharges you have to imagine it is like breathing – as lithium ions move into it, it is expanding; as lithium ions are moving out of the other electrode, it’s shrinking.”
This “pumping motion” inside the cell, of expansion and contraction from one side of the electrode to the other, can see foreign object debris move within a cell and cause a short circuit.
What’s the prognosis?
Things happen very quickly inside a battery and a battery management system that is crunching numbers can only respond to what’s already started. If something goes wrong, it can’t turn back the clock and undo it. “A BMS is a lagging indicator,” Best tells EcoGeneration. “If you’re measuring temperature, for instance, you’re probably measuring that at the cell terminal or the cell wall – you don’t know what’s happening inside the cell.”
It would be a great leap forward for the industry if battery management systems could see ahead, if only for a moment. Prognostics abilities that telegraph the rising probability of a wayward event are the hope of developers in the sector, Best says. “That’s something researchers are very keen on developing, such that you can see trouble before it comes.”
The more levels of connection required to peer into cells, such as optical fibre cables, mean that the evolution of prognostics abilities in battery software would be expensive. “More levels of connection requires greater cost and higher levels of complexity.”
It’s hard to imagine today’s battery technology being more complex, where electrodes can be separated by as little as 15 microns, or 0.015mm. “That’s thinner than a human hair,” Best says. Some companies are attempting to make separators as thin as 5 microns.
Any reduction in polymers and the electrolytes required to fill them would see a direct increase in energy density, as designers produce a smaller battery with the same clout.
Product designers are also relentless in their pursuit of thinner wall casings, in trialling the use of different metals in the tabs and thinner current collectors – the copper and aluminium used to support the active electrode materials. It’s all in the name of saving weight.
“The only thing that does the work in a cell is lithium,” Best says, citing an Argonne National Laboratories paper that estimated the lithium in a standard 18650 cell accounted for about 2% of its weight. “The rest of it is all the stuff around the edges.”
The challenge with lithium
When a lithium-ion battery is charging, lithium is being inserted into the interstitial planes between the graphite layers in the negative electrode (anode). “When you lithiate graphite you’ve got to do that in a very controlled instance,” Best says. “Lithium can only diffuse so fast into those layers.”
If a lithium-ion battery is instructed to charge too quickly, lithium can be “plated” onto the surface of the graphite anode rather than be inserted into the interstitial layers. This deposit of lithium metal will assemble itself in a needle-like form, Best says, which can puncture a separator and contact the positive electrode, or cathode.
Bad news … a short circuit.
“Charging in a battery has to be done really carefully,” Best says, and a BMS is designed to control voltage and current in a cell “to exactly stop this from happening”.
The CSIRO is working on hardware and customised software to incorporate in Australian battery-maker Energy Renaissance’s superStorage range of products, which extends up to 1MWh modular units. Best and his team are developing safety features they hope can detect issues where a failure event might occur and then command a battery to “switch the system out” before that failure becomes catastrophic.
“Ultimately you’d like a cell to go to zero volts, where it won’t work but there is no fire or flame or any of those issues,” he says. “But that’s a challenge of lithium, unfortunately.”