The multi-cell thermal battery (MCTB) is a device that can recover a large fraction of the thermal energy from heated regolith and subsequently apply this energy to heat up cool regolith. The individual cells of the MCTB contain a thermal storage media that is specifically designed for optimal performance at a given temperature range. Each of these cells is charged with thermal energy from hot regolith that has been used in a lunar ISRU application. Once the MCTB is charged, the heat is transferred from the battery to newly harvested regolith. In this manner over 85% of the heat can be transferred from the expended to the new regolith. This is a large improvement especially considering that this reduces the heating requirement to produce 1000 kg of O2 from lunar regolith from an average of 1 kW to only 0.15 kW (assuming 3% O2 recovery by weight). The other irreducible power consumption of lunar ISRU O2 production is electrolysis which consumes at least 0.3 kW. Hence, using the MCTB decreases the irreducible power consumption of lunar ISRU by 65 %.