A little over a year ago, a BMW iX electric SUV fitted experimentally with a dual-chemistry Gemini battery pack from Our Next Energy (ONE) drove 978 km (608 miles) on a single charge – an impressive achievement for such a large, heavy car, writes Peter Donaldson.

The Gemini battery features high power-density LFP cells and high energy-density, ‘anode-free’ cells. The robust LFP cells handle day-to-day driving up to 150 km, after which the more sensitive cells are switched in, adding 450 km of range, with the DC/DC conversion system transferring energy between them.

In another example, Chimera Energy is developing a battery that features both lead acid and lithium-ion cells to produce an affordable battery with a better balance of performance and longevity than either chemistry can for cost-sensitive applications, such as electric quads and other off-highway EVs, static storage and even aerospace uses.

Such dual- and even multi-chemistry batteries are a very attractive idea as, like any other hybrid, they promise to compensate for the weaknesses of each technology with the strengths of another. There is one obvious objection to the idea in that it seems a lot of effort to expend when the same result might be achieved by fitting separate batteries with complementary chemistries.

However, using separate batteries brings its own challenges, as each one typically requires a BMS of its own, adding to the complexity of integration and control – possibly necessitating a third, supervisory BMS. It also tends to bring duplication of other components such as thermal management pipework and pumps, wiring and connectors.

A further drawback of independent systems is their reduced flexibility in operation, as dynamic balancing of performance across chemistries is much more difficult, limiting overall optimisation. Inevitably, before integrated, multi-chemistry batteries can reach their hoped-for potential there are multiple technical hurdles of their own to be overcome.

One of these is complexity in power management, particularly when it comes to balancing chemistries and designing appropriate switching topologies. Different chemistries have unique charge/discharge characteristics, energy densities and thermal behaviours, so creating a control system to transfer loads and charging currents among them dynamically and efficiently is challenging.

Also, developing switching mechanisms that are reliable and not overly complex is critical to ensure seamless operation across chemistries and to minimise energy losses.

Further, cells with different chemistries tend to work best at different temperatures, so devising a thermal management system that can avoid overheating and inefficiencies in the combined system is tough.

Multi-chemistry batteries pose a similar conundrum for the battery management system (BMS) when it comes to state-of-health (SoH) and state-of-charge (SoC) monitoring.

In the longer term, it is crucial to harmonise the lifecycles across chemistries with different degradation rates to ensure consistent performance of the whole battery, avoiding premature failure of any component.

Safety concerns loom large because integrating chemistries with different safety profiles increases the potential for thermal runaway or other catastrophic failures if not carefully managed. Furthermore, isolating faults so that problems in one part of the system do not cascade to others is critical, particularly in modular and scalable battery designs.

Achieving economic viability is another major hurdle as multi-chemistry battery designs are likely to be more expensive to produce initially, largely due to the need for multiple supply chains for raw and processed materials, along with more complex architectures and control electronics. These factors are also likely to be a severe test of the industry’s ability to scale up such batteries for mass production that is economically viable.

Lastly, the bigger the variety of materials incorporated into a product, the more difficult to separate and recover them for recycling, so innovations in reusability and second-life applications are essential.

However, hybrid technologies offering the best of both worlds are perennially attractive, so long as the risk of suffering the worst of each is avoided.