Inventus Power, Bren-Tronics, Navitas Systems, and Ultralife Corps

Battery management is increasingly common in video games like Deus Ex, Tears of the Kingdom, and Crysis. Gadgets allow you to turn invisible or power an airplane—but only until your batteries run dry.

But battery capacity isn’t just a game mechanic for sci-fi super-soldiers. Advanced sensors, communication systems, jammers, navigation systems, small drones, medical devices, directed energy weapons, and propulsion systems all increase electrical demands on soldiers in the field. And fighting in Ukraine has revealed that even small, frontline infantry squads and platoons in the trenches need their own drones and drone-jammers to have acceptable survival odds.

While the latest military vehicles incorporate auxiliary power units (APUs) to help keep their systems running even with engines off, troops on foot are stuck lugging lots of heavy batteries. That’s a problem, as soldiers already average at least 60 pounds of gear each—and sometimes dozens of pounds more.

Army research into more convenient portable batteries began in the early 2010s. Finally, in 2021, the Army handed out contracts to Inventus Power, Bren-Tronics (EnerSys ENS to Acquire Bren-Tronics), Navitas Systems, and Ultralife Corps (ULBI)to compete for a contract worth up to $1.25 billion. The objective: a 2.5-pound Conformal Wearable Battery (CWB) that could be easily integrated into the front, side, or back pockets of body armor with ballistic plates inserted, distributing battery weight with minimal discomfort for soldiers.

The challenge was clear—not only did this battery need to achieve extraordinary energy density, but it needed to be ‘bendy’ to conform with body armor and had to pose no safety hazards to soldiers using them in the least-safe places imaginable. The requirement documents specify testing the effects of penetration by 7.62-millimeter bullets, immersion in salt water, and exposure to salty fog and explosive decompression, amongst other situations.

The CWBs are particularly aimed at powering Nett Warrior, a combo of smart phone (most recently, the Samsung S20) and PRC-154/A secured radio used by dismounted infantry and cavalry leaders. The phone uses Android Tactical Assault Kit software to graphically display the position of friendly and adversary forces on a geo-referenced map, and communicates with other leaders by transmitting/receiving texts, audio, GPS coordinates, and video streams. Testing of the conformal batteries used for Nett Warrior in the mid-2010s found that they extended usability by 20 hours and required 4.5 hours to charge.

While 150 watt-hour CWBs are presently being fielded, considerably more capable solutions are in the works. Inventus’s offering uses a promising alternative battery cell type known as silicon anodes, which in principle offers ten times the energy density of carbon-based graphite anodes used in most lithium-ion batteries (LIBs).

According to Don DeRosa, CEO of battery materials research company Eonix, energy-dense silicon is basically required by the Army’s “ambitious” requirements for a 300 watt-hour CWB. “Silicon is the most near-term commercial solution that can be prototyped today,” he told Popular Mechanics. “It isn’t a question of whether [the CWB] uses a silicon anode, but how much silicon is needed for a silicon anode.”

The reason silicon anodes aren’t already widely used is that they’re infamous for expanding up to 300% on contact with lithium, which can lead to material cracking. That’s a big problem.

Amprius Technologies is one of the suppliers of silicon anode batteries that have overcome what seemed to be an insurmountable physical principle. According to Amprius’ CTO Dr. Ionel Stefan, the secret sauce is a proprietary silicon anode that’s designed to accommodate the expansion experienced during charge and discharge of the battery, preventing contact between silicon nanowires.

“That was the innovation: in addition, we developed an electrode that makes the cell much safer,” Stefan told Popular Mechanics. “Inside the cell, you have a number of anode and cathode layers. The anode isn’t a dense crystalline silicon. It’s more porous, amorphous and fluffy, like snow compared to ice. The second part is they don’t touch due to the distance between each layer—even if there’s some material expansion in each wire. [Other batteries using silicon] rely on spherical particles contacting each other. So when they expand, they push into each other and expand the cell causing damage. Ours don’t do that.”

“It’s very important to pair the silicon material, a high-energy density anode with a high energy density cathode to get the best at the cell level,” he continued. “High-energy density cathodes are now what we work for. The ratio between capacity of anode and cathode is now 10:1.”