This application note describes a low cost, two-cell Li-Ion/Li-Pol battery charger. An effective cell-balancing algorithm during
both charge and discharge phases is presented. This charger can be used either as a standalone application to charge a
battery pack with two serial connected Li-Ion/Li-Pol batteries or embedded in residential, office, and industrial applications.

Introduction

A modern portable system requires more operating voltage
than a single-cell Lithium-ion (Li-Ion) or Lithium-polymer (Li-
Pol) battery can provide. A serial connection results in a
pack voltage equal to the sum of the cell voltages. To
increase the battery pack capacity, the cells are connected
in parallel. For many applications, two cells in series are
sufficient, with one or more cells in parallel. This
combination gives nominal voltage and the necessary power
for

laptop

computers

and

medical

and

industrial

applications. Problems can occur when the cells have
different capacities or charge levels. During charging or
discharging, the cells in the battery pack do not have
matched voltage every cell. Therefore, the battery pack is
not balanced. The unbalanced charge between cells causes
the following problems:



Reduced overall battery pack capacity to the value of
the cell with the least capacity. During the charge
process, this cell reaches the maximum charge level
before the other cells, and during the discharge process
this cell is depleted before the other cells in the pack.



Reduced overall battery pack life. The charge or
discharge of cells at different values increases pack
imbalance.



Cell damage, which occurs if the charger monitors only
the summary voltage. For example, if the lower cell has
a capacity deficiency of at least 10 percent, its cell
voltage begins to rise into the dangerous area above
4.3 volts. This can result in additional degradation of the
cell or a safety system response that greatly reduces
pack capacity.

This application note describes a two-cell Li-Ion/Li-Pol
battery charger. An effective cell-balancing algorithm is
designed. It avoids the issues that appear in battery packs
with two cells in series. Through modification of the
configuration parameters, the cell-balancing algorithm can
easily be adapted for various applications and selected
batteries. The unique architecture of the PSoC

device

provides an integrated hardware solution for a two-cell
battery charger and a flexible

μC-based, cell-balancing

algorithm with minimal external components at a very
affordable price. The CY8C24x23A PSoC device family
used in this implementation reduces the total device cost
even further.

When you want to use algorithms for the latest charging or
cell-balancing technologies, only the firmware needs to be
modified. PSoC Designer’s in-circuit and self-programming
capabilities make these operations simple.

Specifications for a two-cell Li-Ion/Li-Pol battery charger with
cell-balancing support are listed in

Table 1

on page 2.

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This manual is related to the following products:

CY8C24794 AN2309 CY8C27x43