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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