Hello.
There is three sets of five series cells in these batteries.

That supply 20 V at triple amperage.hours or 60V by connecting the three 20V groups in series or in parallel.

Measuring; they reach 18 Volts (paralleled) after the charger completes its cycle. Anyone knows if that is normal or the battery or the charger are at fault ? That is 3.6V/cell; not 4.2V
Its charge indicator shows two of three lights on. I have a couple of them behaving the same. The charger works with the internal groups paralleled. Only when inserted to a 60V tool; the groups are switched into series.

 

Have you measured each bank of 5 individually at the cells? Initially 18v would seem like you may have a duff cell - not uncommon in used kit.
Some of my genuine Milwaukee batteries seem to develop duff cells - interestingly only in the 6Ah versions though.

I’d ignore the on board charge indicators, all my Dewalt and some of my Milwaukee batteries have failed indicator circuits.

 

Thank you.
A couple of days ago the charged cells measured between 3.55 and 3.65V.
Yes, did measure each one of the 15 cells inside individually this morning and all are between 3.65V and 3.69V ; surprisingly uniform readings just minutes after removing from charger when finished overnight. No corrosion or anything wrong inside.
Today, the difference is the on-board charge indicators show full scale (3 lights)
Perhaps will measure again next after-charge to see if they keep 'improving'

 

3 sets of 5 cells in series...3.6 volts per cell...5 * 3.6 = ???

Am I missing something?

 

...3.6 volts per cell...Am I missing something?

Maybe. Nominal is 3.6V for old cells(carbon cathode).
Newer cells (NMC with graphite cathode) are allegedly 3.7V nominal, but up to 4.2V while charging.

A fully charged string of 5, fresh off a fast charger, could be over 20V for a few minutes!

 

Consider first that we see no claim as to any specific battery technology, Lithium, , NiCad, or NiMh! So no clue there. Then also consider that the brand, while OK, is not quite premium.
As pointed out already, 5 x 3.6 volts =18 volts at some state of charge and some level of load current, for close to new batteries.

 

3 sets of 5 cells in series...3.6 volts per cell...5 * 3.6 = ???
Am I missing something?

For 20V use and for charging they have 3 paralleled groups of 5 lithium cells in series; supposed to be ~4.2V each cell yielding 20 to 21 V = 5 x 4.2 and they are showing 18V which means get charged to 3.6V instead of 4.2V

 

Maybe it would help to leave them on the charger longer so the cells can balance? I'd expect them to get close to 4.2V/cell when full; 3.6V is 50% charge for lithium-ion.