Before using them in seires, you want to have protection, and you prefer they be very similar in Capacity,
one way to do that, is to at least get them close to the same charged voltage.
even if you charge them as singles , in parellel, or in series, you somehow want to start with Similar things.
so they end more similar.
for charging in series - - - -
you will eventually need to balance out the 2 of them.
Quickly:
individual protection, or pack protection in seires can keep the battery from overcharge and overdischarge. battery still usable and good.
but it takes "balancing" to get them both to be charged properly IN SERIES, so they are at about the same voltage when the charge is complete, putting them closer to similar capacity.
Then when they discharge they will discharge more similar GOOD
. But they can still end at different times :-( just because "balancing" only gets the charge similar, it cant Match the cells capacity perfectally, so one can end before the other still.
quick examples:
Unprotected
say you pump in a 8.4v max voltage charge or cutoff point for the 2
in perfect world you get 4.2 + 4.2 = 8.4v and everything is perfect
once you leave utopia , you could have 4.0 + 4.4 and ruining a cell.
now Protected but unbalanced
the cells wont go much over the 4.2, they are kept from way to low discharge.
when leaving utopia you can STILL have final charge state of 3.9v + 4.2v Not = 8.4V because with protection the cell that reached the protection end, cuts out the power.
because the series is a "chain", when any one protection cuts out , power to both is terminated. Battery still protected, charge voltages arent kept similar.
When you have Very well matched, good age , cells , they match eachother really well, and it SEEMS like Balancing isnt nessisary, then after much use , or just lesser cell, they start getting further off. Balancing can put them back at similar charge voltages. Balancing always can keep them together much better , during charge, which helps during discharge.
Protected and Balanced
the cell wont discharge to far and get ruined, the cell wont charge to far and get ruined
PLUS on charge the final charge voltage Say 4.2+4.2 = 4.2v ?? because each cell is dealt with more individually during balance, even if it is charged mostly as the whole 8.4v (depending on the charger) they are finished up by getting each of them to 4.2v or very similar.
same thing applies if you were say charging only to 4.1V , they would all be 4.1 - 4.1 - 4.1 when finished, or say 4.18 4.17 4.18 , which again is very close.
