ΓενικεSOME INFORMATION ABOUT REGULATORSς πληροφοριες  

The purpose of this article is to explain, in simple terms, what"imbalance" is in a "LiPo" battery pack,how it is created,and what balancers can do.

First some definitions in order to understand balancers better.

LiPo is a short name for Lithium Polymer,which recently is popular chemistry for batteries.
A LiPo cell is the building block for LiPo packs and has a nominal voltage of 3.7V. The maximum permissible voltage is in the region of 4.2 V (+/-20mV) and the minimum voltage is in the region of 2.9V.
A LiPo pack consists of cells connected either in series or in parallel, in order to obtain higher voltages and or higher capacities than a single cell.
A LiPo balancer is a device which acts as a current shunt at a preset voltage (4.18V-4.21V) thus protecting the LiPo cell across which it is connected during charge. The term "balancer" is not quite right and the term "current shunt" is more explanatory, but it is commonly used and that's the reason we use it here.

WHY BALANCERS ARE NEEDED.
The end-of-charge voltage of a single LiPo cell is approx. 4.2V (multiply by the number of cells for multi-cell packs). It is vital not to exceed this voltage limit, in order to avoid trouble (problems range from a reduced performance and cell life, to fire or even explosion).
So, at the end of charge each LiPo cell in a pack should be in the region of 4.2 V (+/- 20mV). Chargers suitable for LiPo packs look for this condition in order to terminate the charging process. For example,for a 2-cell pack they terminate at 8.4V, for a 3-cell pack at 12.6V, and so on.
Since the charger connects to the pack only by two wires (the pack's plus (red), and minus (black) terminals), the only voltage measurement available is the sum of the individual cell voltages.
So it can be seen that there is no way for an ordinary charger to know the voltage of each individual cell,and although they should be equal, they usually are not.
After discharging the pack in our application, because of cell differences inside the pack ,it is probable that not all the cells will end up with the same voltage.
For example,the physical location of the cell in the pack results in lower or higher cell temperature affecting it's internal resistance resulting in different discharge capability. Another possibility is having "tired" cells together with "healthy" cells which results in lower voltages for the "tired" and greater for the "healthy" ones.
Note that imbalance problems occur if cell voltages differ more than 0,005V.
So when we charge the pack,the cells that started with a lower voltage are left behind (below 4.2V) and the cells with a higher voltage are overcharged (above 4.2V) at the end of the charge cycle, since the charger will end its charge cycle relying on just the total voltage presented at the pack's terminals.
The above explains imbalance in a multi-cell pack. In order to avoid this condition, it is required to monitor each individual cell during charging, giving the lower-voltage cells time to reach the end-voltage; meanwhile enabling a shunt to absorb excessive charge from each of the higher-voltage cells which had reached 4.2V earlier.

The balancer does exactly that. It restores balance in a pack in order to have all cells ending charge at 4.2V. This of course can be done only while charging the battery pack.There is no guarantee that if you start charging with identical cell voltages that you will end it having all cells in balance if you do not continuously monitor and adjust them.So the pre charge "equalising" is not fault proof in most ocassions as the cell's absorption capability may vary.

We need as many balancers as the number of cells in a pack.
For example a 2-cell pack (2S 7.4V nominal) needs 2 balancers, a 3-cell pack (3S 11.1V nominal) needs 3 balancers, and so on.

For packs having series and parallel cells for higher capacity, we consider only the series connected cells and treat the parallel cells as one, since they have the same terminals and voltage.

WHY A HIGH-CURRENT BALANCER
Usually chargers nearing the end of the charge voltage region, continue to supply a small current to the pack (0.1A - 0.2A). So,in this case,there is no need to handle large bypass current.
This is not true in case the imbalance is high. What happens is that the cells with the higher voltage activate their balancers earlier, when the charge current is in the 1C region (C=battery capacity) and this could be considerably higher than 0.1A or 0.2A.
So,if the balancer cannot handle high current, problems occur,like burned balancers/overcharged cells. This is why with the LB1 balancer you have better protection of the pack even if high imbalance exists.

 

 

 

 
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