I've recently seen 8.4V stacked super-capacitors which advertise themselves as "not requiring any balancing". Namely, the 196HVC series from Vishay.
How can this be true? Even if the cells are extremely well matched, wouldn't wear and tear eventually cause a sufficiently large mismatch to require balancing after all?
I actually bought some of these to try them out, and it turns out the cells slowly discharge from 8.4V (maximum voltage) down to around 7.8V over the course of 12hrs or so. Afterwards, the voltage stays at 7.8V for a very long time (i.e. months). This most certainly points to an internally balancing mechanism, using SAB MOSFETs, clamping diodes, or something else.
The verdict is still out as to how these particular super-capacitors implement the balancing.