2012 Microchip Technology Inc. DS01463A-page 1
AN1463
INTRODUCTION
In spite of the considerable media attention to the
concept of recyclable power sources, usage of
rechargeable Nickel-Metal Hydride (NiMH) cells has
been lim it ed. The combi ned ch arger an d cell cost have
been a mitigating factor in favour of primary cells. The
high internal impedances of primary cells are not
suitable for the surge currents needed for digital
cameras, DC motors and walkie-talkies. NiMH cells
exhibit low internal impedances and, as such, are
particularly viable in these applications. The digital
camera user may be more discerning about using the
NiMH batteries, but the toy market is still dominated by
the primary cells solely due to cost considerations. In
the long run, rechargeable batteries are more
cost-effective. Commercial NiMH chargers can be
classified into two areas:
a) Fast chargers that take 15 minutes to three
hours to charge two or four cells.
b) Trickle chargers that take about 15 hours to
charge two or four cells.
PROFILES OF NiMH BATTERIES
UNDER VARIOUS CHARGE
CONDITIONS
Figure 1 shows the charge efficiency at different
charge rates. This is a variable quantity dependant on
the state of charge and the charging current. The
maximum charge efficiency is seen at charge rates
between 0.5C and 1C. The details of this data are
beyond the scope of this application note, and
additional information is available in the listed
references.
FIGURE 1: CHARGE EFFICIENCY AT
DIFFERENT CHARGE
RATES
During the char ging and overcharg ing proc ess, oxyge n
is generated at the nickel electrode. This oxygen
recombines with the metal hydride electrode, thereby
preventing the build-up of cell pressure. However, at
high charge currents, the rate of oxygen generation
may exc eed the recom bination proc ess. The acco mpa-
nying the rma l build up and accum ulati on of oxyg en will
lead to cell pressure build-up leading to cell damage
and th e elect rolyte w ill leak out of the case. De ta ils of
this electrochemical reaction are available in the
Section “References”. Figure 2 shows the cell
pressure profiles at various charging rates.
Note 1: A cell is a single physical unit with an
anode and cathode to produce the rated
voltage. A battery can be a single cell, or
a group of multiple interconnected cells to
produce the required voltage. The term
cells an d batte rie s are us ed int erc han ge-
ably in this application note.
2: A battery’s capacity is measured in
Ampere-hours and the C rate is the cur-
rent which a battery delivers if it is com-
pletely discharged in one hour. For
inst ance, a 2 000 mAH battery, char ged at
200 mA, is said to be charged at 0.1C
rate.
Authors: Siva Periasamy
Tony O’Byrne
Microchip Technology Inc.
Reprod uced with perm ission of th e copyright ow ner
Elsevier B.V.
NiMH Trickle Charger with Status Indication
AN1463
DS01463A-page 2 2012 Microchip Technology Inc.
FIGURE 2: CELL PRESSURE PROFILES
AT VARIOUS CHARGING
RATES
FAST CHARGERS
Fast chargers employ a constant-current charge
manage ment al gorithm with charge curren t s from 0.5C
to 6C, and are of ten based on s witch-m ode c ontrol lers.
The design would involve consideration to temperature
management, EMI and safety. In view of the large
charge currents and build up of cell pressure, as seen
in Figure 2, dete ction of the charge st atus an d cut-of f is
crucial to avoid cell and charger damage. EMI, safety
compliance testing, materials procurement,
manufacturing complexity and test complexity add
additional costs. Typically, the charge management
algorithm would detect charge completion and change
the charge profile to a lower charge current, while
monitori ng for fau lty ba tteri es . Th e f oll owin g c on diti on s
are detected and displayed visually:
• Charge completion
• Insertion o f primary cell
• Insertion of a fully charged battery
• Insertion of a shor ted cell
The algorithm can charge a partly discharged battery
without overcharging it. Some chargers may have
additional features to detect special NiMH cells which
can handle high charge currents.
A visual display in the form of LEDs or LCDs with an
audible alarm may be implemented. The reader can
refer to the references at the end for schematics and
det ai ls of these ch argers.
Their higher cost limits their sales to the more
discerning customer.
TRICKLE CHARGERS
Commercial trickle chargers do not have any charge
manage ment al gorit hm and the typica l ch arge cur rent s
are limited to 0.1C or lower. The charge current is
limited by a res is tor i n c onj unc ti on w i th th e tra nsf orm er
impedance, as seen in Figure 3, which also explains
the basic conc ept of a trickle charger.
FIGURE 3: TRICKLE CHARGER
Reproduced with permission of the copyright owner
Elsevier B.V.
R1
1R6
D1
1N4001
CELL 1
CELL 2
R2
1R6
D2
1N4001
CELL 3
CELL 4
AC MAINS
TR1
TRAN-2P3S