PTC NTC Fuses
The ProTek devices in the circuit protection components manually is specifically listed for military and Aerospace application.
This article specifically addresses circuit protection for satellite power systems. From section 3.2 under resettable fuses, the non-linear PTC effect is discussed.
A test conducted shows the clearing time of fault current is relatively long and the voltage sag is limited.
Further circuit tests in this article (eg. Figure 12) shows that PTC is a valid circuit protection device and may be valid to be used in space.
Page 415 Figure 11 is a reference bus voltage with voltage figure of a unregulated bus during a reference orbit.
Interesting article about fuses in Aerospace (no content on NTC/PTC fuses) https://s3-eu-central-1.amazonaws.com/centaur-wp/theengineer/prod/content/uploads/2012/10/09100700/Fuse_for_Aerospace_Industry.pdf
Considerations for LEO
- Clyde Space uses e-fuses and specifically says NO PTC - CalPoly CubeSat Interface Review/Specification asks about presence of PTCs inside cells but doesn't state which case is desirable (also asks about CID) - NR lists them as an "effective interrupt device" alongside other devices
Do our batteries have these already? Some sources say all Li+ have a PTC and CID, others disagree
Difference from Thermal Cutoff
Thermal cutoff ie bimetallic strip is a temperature-driven temperature cutoff, so regardless of environment it trips at a certain temperature as desired. Polyfuse is a temperature-driven current cutoff, so the temperature-current relationship can change with environment and change the trip current (I think). I guess the question is, is this a problem? The reason we limit current is to limit heat buildup. If the thermal environment is such that the PTC will heat faster or slower relative to current, the other chips will likely be similarly affected and therefore the current SHOULD cut off sooner or later. However, the exactness of the solution is not preserved.