I have read that it is strictly not recommended to charge Li-ion cell packs in series without using a balance charger. My question is, can a balance charger be avoided by keeping the total charging voltage slightly lower than the actual full charge level? For example, if for a 12.6 v Li-Ion Battery pack if we use a supply level of 12.5 V, can we hope nothing untoward may happen?
Balancing can be avoided if you can meet certain conditions:-
the cells must be well matched for capacity and internal resistance, so that their voltages track as the battery is charged and discharged.
On the the first charge you must monitor the cell voltages, and balance the battery if they are not all equal (within 0.01V) when fully charged.
The battery must be properly cared for so that no cell becomes damaged by over-discharge, over-current, high operating temperature etc.
If the cells are well balanced and have equal capacity then they should stay that way for a long time. However they may age at slightly different rates, or a cell might go leaky and lose charge. Therefore you should regularly measure the cell voltages to check the balance, preferably on every charge.
Using a battery protection circuit module (PCM) is recommended. This monitors cell voltages and disconnects the charger if any cell goes above (typically) 4.28V, and also protects the battery against over-current and over-discharge (though they usually allow the voltage to go lower than is good for cycle life, so this feature should not be used as an end of discharge cutoff).
Charging to a slightly lower voltage will allow more voltage variation between cells without risking any cell going above 4.23V. It also improves shelf life if the battery is left fully charged for long periods, especially at high temperature. The down side is reduced capacity. If a PCM is used it will allow the battery to go more out of balance than when charged to a higher voltage. Therefore you should still measure cell voltages regularly to check battery health.
Without balancing you are relying on the cells continuing to stay matched. Good cells will do this for many years if treated well. In my experience a battery that needs regular balancing usually has a faulty cell, and will get worse. Once a battery gets to this condition it is best to discard it, even if using a balance charger.