As an example, the Ingenuity helicopter on Mars charges its batteries each sunny day (which is every day except when it isn't) and discharges them substantially each cold night (which is all of them) to keep themselves from freezing damage and to keep other critical goodies from getting too cold. Solar powered Mars rovers and landers on Mars do the same, which is all of them except for MMRTG-equipped Curiosity and Perseverance that use heated fluid circulation.
A thermal battery is:
...in general something that can store thermal energy and while an electrical battery can serve this purpose, a pot of water, wax, or any other material that undergoes some kind phase change within a useful temperature range with a sufficient enthalpy of change can potentially be used as a thermal battery.
From Wikipedia's Thermal battery:
A thermal energy battery is a physical structure used for the purpose of storing and releasing thermal energy—see also thermal energy storage. Such a thermal battery (a.k.a. TBat) allows energy available at one time to be temporarily stored and then released at another time. The basic principles involved in a thermal battery occur at the atomic level of matter, with energy being added to or taken from either a solid mass or a liquid volume which causes the substance's temperature to change. Some thermal batteries also involve causing a substance to transition thermally through a phase transition which causes even more energy to be stored and released due to the delta enthalpy of fusion or delta enthalpy of vaporization.
But in that case it was called a "thermal capacitor" rather than "thermal battery" for some reason.
The nice things about a thermal battery include
- You "charge it" with heat and there's sort-of five times as much heat in sunlight as there is electricity to be had from it; solar photovoltaics don't get much better than 20% when all real-world effects are taken into consideration (optical transmission & dust, de-rating for non-optimal temperature, power conversion and charging efficiency of the battery, etc.)
- Phase changes like melting/freezing don't "wear out" a simple bulk material the way repeated charge/discharge cycles wear out an energy density optimized electrical battery will. You're not going to get "tired water".
Question: What would a solar thermal battery system look like for an aircraft on Mars? Roughly how much mass would it add? How much would it replace?
Remember that Ingenuity's battery's mass and energy capacity are driven by the need to both have a usefully long and satisfying flight and still have enough energy to stay warm overnight along with a good margin for safety.
Adding a thermal battery may substantially reduce the required energy capacity and therefore mass of the electrical battery, so adding the thermal battery is not all "doom and gloom."