Makita max charge voltage?

[hman88]

Greetings fellow community members!

 

I just acquired a Makita impact driver (BTD142) and drill (BDF452) with (2) batteries (BL1815) for only $30. One of the battery packs is faulty and won't take a charge which was disclosed on the seller's listing. I disassembled it and found one of the (5) cells was dead and won't take a charge when I connect an extra 18650 cell in parallel.  

 

I am curious to know why when I charge the working battery does it only charge to 19.4v and not to 21v given there are (5) cells with a max charge state of 4.2v? I would assume this is by design in order to prevent premature degradation but just wanted to see if anyone could explain why the charger does not charge the battery packs to 21v. 

 

I am acquiring (2) more open-box 5ah packs today for $100.

 

Thanks in advance for your responses!