Batteries Go Underground
A Saft expert evaluates various Li-ion chemistries and strategies – battery swapping vs. fast charging – for electric mining vehicles.
Diesel-powered mining vehicles add risk and complexity underground. Operators need extensive ventilation infrastructure to manage the exhaust gases, and this becomes exponentially larger as mines go deeper. That can be significant in terms of cost and space for today’s deepest mines, which reach depths of 4 km (2.5 miles).
Around 80% of the energy demand for underground mining is associated with load, haul and dump (LHD) machines and other vehicles that transport people, equipment and materials. Diesel engines are a source of CO2 and particle emissions, so switching to battery power is an opportunity to decarbonize and improve air quality underground.
Despite these advantages, operators have held back from electrifying vehicles because of past limitations on battery technology. However, today’s lithium-ion (Li-ion) battery technology has matured to the point where it is economically viable and reliable.
Battery swapping or fast charging?
What operators want is total reliability from vehicles during five years of heavy use in the toughest environments. Batteries need to deliver high power over shifts of around four hours. They also need to support frequent charge and discharge cycling, 24-hour operation and high ambient temperatures.
Two schools of thought have emerged as solutions to deliver this: battery-swapping and fast-charging. Battery-swapping calls for two identical sets of batteries, with one installed and powering the vehicle and the other charging. At the end of a four-hour period, the vehicle will pull up to a swap-and-charge station, where the battery will be swapped out for the fresh battery. This normally takes 15 minutes or so while the driver has a break.
This strategy uses slow charging that doesn’t place a significant burden on the mine’s existing electrical infrastructure. However, the battery changeover adds an additional task for the underground team and requires lifting equipment for safe handling.
The other approach uses fast-charging batteries that are permanently fitted into the body of the vehicle. The latest Li-ion electrochemistry is amenable to fast charging within around 10 minutes during breaks and shift changeovers, as well as “opportunity charging” during brief pauses in normal operation when the vehicle is waiting.
This approach requires specialized charging stations with a high-power grid connection. Therefore, operators may need to upgrade the mine’s electrical infrastructure or install wayside energy storage, especially for large fleets that may need to charge simultaneously.
Choosing the right Li-ion blend
Li-ion batteries are familiar from consumer devices and electric cars, but Li-ion is an umbrella term that covers a wide range of electrochemistries. These can be used singly or blended together to precisely deliver the right properties across a range of five factors: energy density, cycle life, calendar life, fast charging and safety.
The most common Li-ion types have a positive electrode made from lithium nickel-manganese-cobalt oxide (NMC), lithium manganese oxide (LMO) and lithium iron phosphate (LFP). Their negative electrode is typically graphite or another form of carbon. Of these, NMC and LFP are most commonly used in underground mining vehicles, particularly the swappable types. Both provide long autonomous operation with charging taking less than one hour.
To compare the two at the level of individual battery cells, LFP has a lower voltage. When scaled up to a large EV battery system, this means that LFP needs around 30% more cells than NMC for the same system voltage and energy. However, the raw materials in LFP batteries are more abundantly available, giving them price stability for mine operators.
A new addition to the Li-ion family is lithium titanate oxide (LTO). It is similar to NMC but instead of graphite, the negative electrode is based on LTO. This gives it the ability to accept very high charging power, making charging times as short as 10 minutes. Another benefit of LTO is its ability to withstand three to five times more charge and discharge cycles than other Li-ion batteries. This makes it particularly well suited to underground mining, as does its inherent safety, even when subjected to electrical abuse such as short circuit, mechanical damage and even deep discharge to zero volts.
A drawback of LTO is that its energy density is lower than both LFP and NMC. However, the ultra-fast charging means the batteries remain in place inside the vehicle, so they don’t need to be mounted in an accessible location for swapping.
Another important factor for mine operators is the battery management system (BMS), which constantly monitors the voltage and temperature of cells and manages charge and discharge to keep temperature constant across the entire system. This maximizes the lifetime and ensures consistent performance. The BMS also monitors the state of charge (SOC) and state of health (SOH). Both are important measures for operation and maintenance.
Vehicle designers face practical challenges when considering battery power. Large underground mining vehicles typically need a voltage of 650 to 850 V. That’s likely to remain the case for the foreseeable future as it avoids the increased systems costs for higher voltages. At these voltages, designers will need systems with energy storage capacity of 150 to 250 kWh. Some manufacturers are looking for 300 kWh or higher, depending on the vehicle.
Vehicle manufacturers also typically want to use a modular approach. The logic is that they can design one electrical system as a basic framework. They can then apply it to multiple vehicles in their portfolio. This minimizes the amount of development time and type testing required for each vehicle, helping to keep costs and time to market under control. Recognizing this, Saft has created a battery system as a standardized 48-V building block that incorporates the Li-ion cells together with voltage and temperature monitoring systems. The solution will be available in NMC and LTO electrochemistries, providing a basis for vehicle designers to build bespoke systems.
Inside the modules, Saft is using cells that are prismatic in shape, rather than the traditional cylindrical shape. Because prismatic cells are rectangular in cross section, there are no gaps between cells, allowing maximum energy density and battery performance to be packed into the available space.
The battery system also can be complemented with components such as a heavy-duty enclosure, thermal management system or fire suppression system, depending on the OEMs needs. In addition, operators can monitor the performance of the battery systems across their entire fleet of vehicles.
Evaluating LHD scenarios
As a practical comparison, Saft evaluated two alternative scenarios for an underground LHD vehicle using swappable and fast-charging batteries. In both cases, a LHD machine was considered that weighs 45 tons unladen and 60 tons fully loaded with 6-8 m3 of material. The evaluation also envisaged vehicles with batteries of similar weight and volume of 3.5 tons and 4 m3 in an envelope measuring 2 m x 2 m x 1 m. This enables a like-for-like comparison of the two approaches.
For battery swapping, the battery system was based on NMC or LFP chemistry. It would support six-hour shifts for the LHD vehicle from a battery rated at 650 V and with 400-Ah capacity. When swapped off the vehicle, they would require three-hour charging. They would deliver 2,500 cycles over a calendar life of 3 to 5 years.
When applied to fast charging, a fixed LTO battery of the same size and weight would be rated at 800 V with 250-Ah capacity. It would deliver 3 hours of operation with a 15-minute break for charging. The LTO battery would last 20,000 cycles, giving it a calendar life of 5 to 7 years.
While these scenarios provide an interesting comparison, a real-world vehicle designer can apply the modular approach to develop a battery system that will precisely suit the preferences of their customers and the needs of individual vehicles. For example, they could extend the duration of the shift by increasing the size of the batteries.
Ultimately, mine operators will choose the vehicles that are best suited to the configuration of their mines. That will require a fine balance to consider the available space underground, as well as the accessibility of high-power electricity. The TCO of the vehicles and the infrastructure needed to support them also will influence decisions. That’s why it’s important for battery companies to provide vehicle designers with flexibility.
Xavier Iraçabal, mobility product manager for Saft, wrote this article for SAE Media as part of the annual Executive Viewpoints series.