4.20V+4%=4.368V and as recently I’ve measured 4.362V on plus and minus poles of the BPP-L503 when (or rather if charged) with some external 4.35V type of universal battery charger. Therefore as mentioned already, as I actually asked before about LiHv batteries (in particular), 4.35V in our usage scenario, allow for a higher cut-off charging voltage (please read below linked article as it explains, in several details, knowledgeable approach from Purism Team, very well thought out development approach, even unique):
Now and when above understood let us take important things (for example, and as accustomed to the usage of some/any Android based smartphone, how to extended battery life if it charged only up to 81% of its nominal capacity) seriously:
This very important post from @dos and in particular this “typical process” refers to the Librem 5 only and as such (with its build-in battery charging components, taking care of all necessary, all well thought out in advance, for customer benefit, as mentioned above) it provides following as well:
Now let me transfer/switch from “termination current” to “cut-off charging voltage” (as correlated and happening within very same time-frame), to the point (or millisecond) when actually red LED light that was on turns off.
Now we might even distinguish Librem 5 (ID 316D) and its belonging BPP-L5 (01,02 or 03) as two different end products where only Librem 5 regulates correctly (that I know of today) its final cut-off voltage (recently I’ve measured 4.137V, not sure yet if that was right after it was fully charged) up to the max. of 4.20V:
@guru, thanks a lot for your thread! Thanks actually for your clear awareness, to include @fsflover contribution, about correct voltage output on BPP-L503 + and − poles (just right after red color light turns off, the red one from its RGB LED)!
This correct (maximal value) cut-off voltage number of POWER_SUPPLY_CHARGE_FULL_DESIGN=4199160 aka 4.20V (and below) is firmely written down within Librem 5 Linux phone main driving software, called PureOS, as shown here (from: $ tail -n+0 /sys/class/power_supply/*/uevent):
If I’m allowed to write (perhaps 
): not quite, as 95% or even 100% (right after red LED light off) are just about very fine numbers as well (while my above thoughts related to the Librem 5 only). And finally, @Gavaudan, content of this post (message that I wanted to bring over to you) is important, as not that I’ve even tried to clearly rewrite what already noted within this Forum, therefore hope this post helps (somehow), including @amarok “alarm” script contribution here, helpful one (that I’ll test when more time for it), to understand that even when BPP-L503 battery charged up to 100% (no red LED light although original USB Type C 5.0V power supply plugged in) it is well taken care of Librem 5 battery health (cannot happen to be overcharged within Librem 5) and care taken about max. capacity that is actually used from it as well (few % below 4500mAh, when original battery full up to the max. allowed of 4.20V, as officially and already presented).
P.S. I’ve mentioned some external (intelligent) battery charger above, but still none of them is singing one, not based on here contributed Linux 
script from @amarok. Thanks therefore!
P.P.S. @Gavaudan, I do not drive around some EV, not yet, but if and once this happens I’ll recall about your 81% post for sure. Thanks therefore!