Using low-grade sand, the device is charged up with heat made from cheap electricity from solar or wind.
The sand stores the heat at around 500C, which can then warm homes in winter when energy is more expensive.
That said, its storing heat, not electricity? So not exactly a battery.
Low-cost electricity warms the sand up to 500C by resistive heating (the same process that makes electric fires work).
This generates hot air which is circulated in the sand by means of a heat exchanger.
Sand is a very effective medium for storing heat and loses little over time. The developers say that their device could keep sand at 500C for several months.
So when energy prices are higher, the battery discharges the hot air which warms water for the district heating system which is then pumped around homes, offices and even the local swimming pool.