Variations of lithium-ion technology are the cutting edge for battery-electric vehicles, but there will continue to be combustion-powered and hybrid-electric vehicles for many years to come, and lead-acid battery technology is continuing to advance for these vehicles too.
To hear about some of the developments in this area, Design News spoke with Alistair Davidson, Director of the Consortium for Battery Innovation to get his thoughts on the direction of familiar lead-acid battery technology.
As Director of CBI, Davidson manages all the consortium’s work programs. He attended the University of Oxford and obtained a Ph.D. at the University of Edinburgh and has lectured at Washington State University and the University of Chongqing, China.
Design News: What are the primary opportunities for lead-acid batteries in the automotive market?
Alistair Davidson: We see the biggest opportunity for lead batteries in micro and mild-hybrid vehicles. Lead batteries continue to be the main power source for cars with petrol or diesel engines for engine starting, powering electrical systems in key-off mode, and for security and emergency functions. They are likely to remain in that position for the foreseeable future and in addition, there is a huge aftermarket for replacement batteries which is much larger than the OEM market.
Virtually all cars with a conventional powertrain now use start-stop, idle start-stop, or micro-hybrid systems to reduce tailpipe emissions of carbon dioxide. This has massively increased the duty cycle for the battery. Instead of one cold start per journey with the electrical systems being powered by the alternator, each time the vehicle comes to a standstill the engine is stopped and then restarted when it moves off.
During the engine stop, the electrical systems are powered by the battery and then the engine is restarted by the battery. The restart is a warm start so the current required is not as high as the cold start but nonetheless, the duty cycle is very intensive compared to a standard vehicle.
The lead battery industry has developed new types of battery to improve performance in start-stop applications.
These are either Enhanced Flooded Batteries (EFB) or Absorptive Glass Mat (AGM) batteries. They have successfully responded to two technical challenges; one, to withstand thousands of small charge and discharge cycles and, two, to be able to recover braking energy by accepting charge from the alternator very rapidly.
DN: What improvements are still possible for the conventional 12-volt automotive main battery for combustion vehicles?
Alistair Davidson: CBI has identified improving dynamic charge acceptance (DCA) as one of the highest priority goals for our research program. The challenge for the industry is to improve the shallow cycle life and in particular the ability of the battery to accept charge - DCA.
Simple start-stop systems are being superseded by micro-hybrid systems where more energy is recovered and this all puts a greater demand on the battery. This is being achieved by improved battery designs and changes to the electrochemistry. Special carbon additives to the plates are playing a major role. We’re working with companies, research and testing institutes, and universities across the world to explore ways to achieve this.
DN: What technological improvements can make lead-acid batteries competitive for use in hybrid vehicles?
Alistair Davidson: Lead batteries have a natural limit in a micro-hybrid system and continue to be developed as described previously. This remains the most cost-effective way of reaching the emissions standards as measured by dollars per gram of carbon dioxide reductions. CBI recognizes that this market is crucial for lead batteries, and our current research program has a number of research projects specifically looking into improving performance for micro-hybrid applications.
DN: How do the differences between power batteries and energy batteries make lead-acid a good technology for hybrid vehicles?
Alistair Davidson: Lead batteries are better suited to lower levels of hybridization. The power density of lead batteries is one of their natural strengths which is why they are excellent at cold engine starting.
It should be noted that all-electric vehicles (HEVs, PHEVs, and pure EVs) utilize a 12-volt lead auxiliary battery to support 12-volt systems on the car and provide power for emergency, security, and communications systems if the main battery is disabled. So, lead batteries remain a vital part of the electric vehicle market.
DN: Are there changes to battery chemistry that are being made or could be made to make them better suited to long periods of disuse, as seems to be happening during the pandemic?
Alistair Davidson: A standard automotive battery should have a shelf life from being fully charged for 6 months. The best practice would be to recharge quarterly and modern vehicles have intelligent systems that reduce the quiescent load in a key-off mode. Changes to the grid alloys can extend the shelf life and are used for specialty and for batteries used for defense applications.
The best solution if a vehicle is not used very often is to take it for a short drive. Alternators are capable of very high outputs and a 10-15 minute drive will provide a useful charge. A simple test is to drive the car until the start-stop system is enabled. The vehicle electronics measure the battery condition and tell the stem to start working when the battery reaches a suitable state-of-charge.
DN: What are your expectations for the automotive market in terms of shifting from purely combustion-only to at least mild hybrid as the baseline?
Alistair Davidson: The high-level challenge for the automotive industry is to meet the overall requirements for emissions at least cost. This means that there will be a mix of vehicles. Pure EVs and PHEVs provide the largest contribution to emissions reductions per vehicle. HEVs are in the middle but with a suitable mix, start-stop and micro-hybrid vehicles will stay important for a long time.
DN: Do you think 48-volt mild hybrids will gain popularity or is that a neither-fish-nor-fowl middle ground that may never take off?
Alistair Davidson: 48-volt is a half-way house and provides an emissions gain above a micro-hybrid with a 12-volt EFB or AGM battery. It will have a level of uptake but the car market is in transition and there will be a large number of different solutions offered in the next several years.
DN: If it does take off, what is the suitability for lead-acid batteries for those cars?
Alistair Davidson: There are some interesting developments for lead batteries that are applicable to 48-volt batteries, in particular bipolar batteries where the battery is built as a stack rather than from individual cells. This increases power and energy density and this may be a target market for the companies with this technology. CBI’s research program has a dedicated project investigating and benchmarking bipolar battery performance for the automotive market.