I'm designing a device that has 6 18650 elements (all parallel) in holders, made so that the end user will be able to pop out the batteries when they're discharged and plug in a set of new batteries. It's possible to put in 4 charged batteries when 2 discharged remain inside (the device is like 2-layer box with lids at top and bottom, having 4 cells in the top compartment and 2 cells + electronics in the bottom one, so the user has to open both to change the batteries), therefore allowing the charged batteries to charge the discharged ones, which AFAIK is a disaster because of high currents.
To protect the device (and the user) against this, can I just put a resettable PTC fuse between the 4-cell compartment negative terminal wire and 2-cell compartment negative wire? I'm not thinking about the scenario where the user mixes charged and non-charged batteries in the same compartment. Also, if the fuse approach is conceptually right, will there be any drawback to using the resettable fuse, t.i. would a regular non-resettable fuse perform substantially better in terms of safety?