Maximize Cycle Life of Rechargeable Battery Packs
with Multicell Monitor IC
Jon Munson Rechargeable battery packs prematurely deteriorate in performance if any cells are
allowed to overdischarge. As a pack becomes fully discharged, the ILOAD RINTERNAL
voltage drop of the weakest cell(s) can overtake the internal VCELL chemical potential
and the cell terminal voltage becomes negative with respect to the normal voltage. In
such a condition, irreversible chemical processes begin altering the internal material
characteristics that originally provided the charge storage capability of the cell,
so subsequent charge cycles of the cell do not retain the original energy content.
Furthermore, once a cell is impaired, it is more likely to suffer reversals in subsequent
usage, exacerbating the problem and rapidly shortening the useful cycle life of the pack.
With nickel-based chemistries, an overdischarge of a set of series-connected
cells does not necessarily lead to a safety
hazard, but it is not uncommon for one or
more cells to suffer a reversal well before
the user is aware of any significant degradation in performance. By then, it is too late to rehabilitate the pack. In the case
of the more energetic lithium-based cell
chemistries, reversals must be prevented
as a safety measure against overheating or
fire. Monitoring the individual cell voltages is therefore essential to ensure a long
pack life (and safety with lithium cells). Figure 1. Simple load-disconnect circuit to prevent excessive discharge of a nickel-cell pack.
FDS6675 1k
1.25V 100nF 1k …