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?
Electrical – Is it possible to charge Li-Ion Cells in series without a Balance Charger, by using slightly lower 4.1 V per cell charging rate
lithium ion
Related Solutions
It's likely that balancing will be reasonably OK over a moderate number of cycles. You can measure the two cells occasionally and if there is a significant imbalance, discharge the higher voltage cell "enough".
A simplistic method that will work well enough is to place a 4.2xV clamp regulator across each cell. I say 4.2X rather than 4.2V as ideally it will be just above what the regulator sees as Vmax/2.
The regulator needs to be reasonably precise. A cheap and easy version is a TL431 shunt regulator diode with a Series resistor load in the Cathode to V+ lead and two resistors setting the voltage (Vref to ground and Vref to V+).
Setting the trip voltage to 4.25V at 25 degrees C should be "good enough".
TL431 Datasheet
Figs 21 27 30 relate to this application.
A TL431 by itself has limited sink current.
Fig 30 shows how to easily and cheaply extend the current capability.
If doing this, use of the 0.5% part is recommended plus 1% (or better) voltage setting resistors.
http://www.baronerosso.it/modellismo_articoli/show/622/lipo-balancer.html
An implementation of the above concept.
Here is a simple method that should be effective.
From here
Note that this loads the battery at all times when powered on.
A slightly tidier implementation - but probably no more effective. Allows higher rebalancing current due to transistor buffer.
If you charge them as individual packs like the picture bellow, you should not run into particular issues.
But ! The safe discharging of the 4 batteries that you separately charge will rely on the quality/precision of your battery charger. Since the only way your complete battery is balanced, is by reaching the maximum voltage of each cell to that of the reference voltage of your charger. Thus if your charger charges to 4.2 V +/- 2%, you should be totally fine, if it charges within 10% (which might be the case) you could get up to 3.4V of difference between two of your 4 cells pack if one gets recharged 10%% to much and the other 10% to little.
I have used this strategy for a 12 cells battery that I recharged as two 6 cells packs and never had a problem. But I would encourage you to be careful, and take precautions and measurements to make sure all voltages are close enough to each other before attempting to discharge the battery.
One major disadvantage with this charging scheme is that you ALWAYS HAVE TO charge all batteries to their very maximum voltage and very maximum capacity (since most chargers are not smart enough to charge to a precise and balanced "in-between"), so that you do not end up exhausting weaker cells even quicker due to their reduced capacity and tendency to not fully charge at the same pace to that of good cells, in less ideal charging scenarios.
Best Answer
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.