Posted on 01 September 2019

A Balanced Approach to Battery-Charging

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Correctly balancing delivers improved battery performance

Charge-balancing for series-connected battery cells means balancing the trade-offs across a number of techniques. Whilst battery cells connected in parallel tend to be self-balancing, cells connected in series will typically need to be balanced to prevent over-charging which can lead to over-heating, increased pressure and reduced performance.

BY Scott Dearborn, Microchip Technology Inc.


The individual cells within a battery may vary in performance, with certain cells limiting the charge and discharge performance. During the charge cycle, for example, a degraded cell will have a diminished capacity, and may become prone to overcharging. This can lead to increased temperature and pressure, which reduces the overall capacity, efficiency and reliability of the battery pack. Conversely, during the discharge cycle, it is possible for the voltage on the weaker cells to be reversed as they become fully discharged before the remaining cells. Fluctuating temperatures may also add to the electrical unbalance in the battery.

Lead-acid and nickel-based batteries, such as NiCd and NiMH, can withstand a certain amount of overcharge or overvoltage, without sustaining permanent damage. With these battery technologies cell balancing or charge equalisation can be performed by prolonging the charge time. This allows the fully-charged cells to release energy by gassing until the weaker cells reach their full charge. However, Li-Ion cells cannot tolerate overvoltage and therefore simply extending the duration of the charge cycle is not an effective option and cell-balancing may be considered.

Li-lon-LiPolymer Charge - Management Controller

There are a number of cell-balancing schemes. Active cell-balancing methods remove charge from one or more high cells and deliver that charge to one or more low cells. The balancing is performed sequentially and the down-side to this technique is that it can be very time consuming. Alternatively, passive balancing techniques are dissipative in nature and excess energy is removed from high cells through resistive elements, until their charge matches that of the low cells.

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Regardless of which cell-balancing technique is chosen, the state of charge for each individual cell will need to be determined. The simplest method uses the cell voltage as an indication of the state of charge, whilst more precise methods use Coulomb counting and factor-in the temperature, age and voltage of the cell.

Not all applications with series-connected battery cells will implement Although cell-balancing helps to improve battery reliability and performance, applications which require a low initial cost, or where battery replacement costs are not prohibitive, may decide not to use cell-balancing. These applications should, however, always include fail-safe cell-protection circuits.

Before deciding whether to use cell-balancing for series-connected battery cells, careful consideration should be given to the balancing the trade-offs between charging times, efficiency losses, cost and complexity. Correctly balancing these criteria, should deliver higher reliability and improved battery performance.



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