Why Dynamic Charge Acceptance is Important
Lead Acid Batteries (LABs) are one of the longest lived industrial products; they are the “starter battery” for virtually every mass market vehicle today. They are a proven, low cost product.
After many years of relatively stable requirements, a new requirement for automotive batteries began to emerge around six years ago: the need for high Dynamic Charge Acceptance (DCA). Simply put, DCA is the rate at which the battery can be charged.
The ability to accept charge at high rates is essential for fuel saving technologies such as Idle Elimination and Regenerative Braking in the next generation of mass market vehicles: micro- and mild-hybrid cars. The better the DCA of the battery, the better the fuel economy of the car.
Better DCA improves fuel economy through:
Regenerative Braking. Recovering braking energy is a “low hanging fruit” for improving fuel economy, with a possible fuel saving of 10-15% available. Regenerative braking involves capturing a car's braking energy which would otherwise be lost as heat and re-using that energy to recharge the car's battery. The problem is that the regenerative braking system is only as good as its weakest link, and currently, the weak link is the battery. Today's affordable batteries cannot accept the large charging currents that the braking systems can deliver. By improving the charge acceptance, you improve the overall energy capture, and hence reduce fuel consumption and CO2 emissions.
Maximising Start/Stop opportunities. Another key fuel saving technology is “idle elimination” or “start/stop” - when the car is at rest, the engine is switched off. Sounds simple, but this feature is incredibly challenging for the battery:
- it introduces many more starts - the battery may have 10-100x as many starts per day than a traditional car; and
- the battery must support so called "hotel loads" (lighting, air conditioning, radio etc) while the car is at rest and the engine is off.
With start/stop it is therefore relatively easy for a car battery's state of charge to become so low that, if not managed, it could result in the car not starting after a start/stop event. Automotive companies naturally protect against this possibility and ensure that the condition of the battery is continually monitored. If the battery is approaching a low state of charge, the start/stop functionality is disabled until the battery recharges. While allowing the car to re-start, this clearly disables the fuel saving potential of the technology.
To maximise the fuel saving potential of start/stop functionality, automotive companies want batteries that can charge quickly (i.e. good DCA) to help keep the battery at a high enough state of charge at all times.
The Problem with Traditional LABs
The ability to charge a LAB typically degrades rapidly with use and within a few months it reaches a stable level. Unfortunately this level is only at a fraction of what automakers require in order to maximise the fuel saving potential of micro- and mild-hybrid vehicles.
Carbon materials used in electrodes generally have better functionality if heated and the higher the treatment temperature, the better the performance. The lowest useful treatment is for “carbonisation” and is typically at 1,200°C and “graphitisation” is typically performed at 2,500°C.
We treat carbon material at extremely high temperatures of around 3,500°C - you simply cannot get much hotter without vaporising the carbon. Critically, we can do this in a continuous, low cost manner and while it’s taken us nearly ten years to optimise this, it’s been worth the effort.
AACarbon's DCA Performance
The following graph presents our typical DCA performance compared to state-of-the-art LAB technology (i.e. Absorbative Glass Mat (AGM)).
Traditional Battery Tests
While car makers want batteries with much better DCA, they cannot accept any compromise on the achievement of the traditional battery tests. This provides a very real challenge for developing new battery technologies as some of the best ways to improve DCA can often lead to serious problems in other performance areas (such as high water loss, low "cold cranking amps" (CCA), and high self-discharge rates).
There are a number of alternative energy storage technologies that have good DCA performance, such as lithium-ion batteries and supercapacitors. However, the disadvantage of these technologies is their relatively high on-cost. It is reasonable to expect that the low cost technology that meets the automakers' requirements will become the market leading technology.
ArcActive's electrode has been designed to be a direct substitution for existing negative electrodes in flooded LABs. As flooded LABs are the lowest cost form of starter battery, even with the modest additional cost contributed by the AACarbon electrode, the resultant battery will still be a fundamentally low cost product. We expect that batteries using ArcActive's electrodes will be no more expensive than the AGM batteries that are the current start/stop battery of choice.
Being a flooded LAB design has an additional benefit: the volume of manufacturing capacity for flooded LABs is vast and ArcActive's solution will avoid the large capital expenditures currently facing the LAB industry as it builds AGM capacity to meet projected start/stop battery demand.