Charging Cymbet EnerChip Batteries - AN-1002
AN-1002
Application Note
Guidelines for Charging Cymbet™ EnerChip™ Batteries
Introduction
Charging Guidelines
Cymbet™ EnerChip™ thin film, solid state batteries feature
all solid state construction, are packaged in standard integrated
circuit packages, and can be reflow soldered for high volume
PCB assembly. They are ideal as rechargeable backup power
sources for clocks, memories, microcontrollers and other lowpower circuits where data or timing information must be
retained in the absence of primary power.
The charging time of EnerChip batteries is short compared to
that of conventional rechargeable batteries. Figure 1 shows
the typical percentage of full charge vs. time during constant
voltage charging.
As with other rechargeable batteries, discharge capacity and
cycle life are a function of charge voltage, discharge cutoff
voltage, depth-of-discharge, temperature, and other factors.
The system designer must understand the effect of these
factors when designing the charge control circuit.
•
Never apply more than 4.3V across the battery terminals.
There is no need to externally limit the charging current of
small surface-mount batteries. The intrinsic cell resistance
is sufficient to limit the current to an acceptable level as
long as the applied voltage does not exceed 4.3V.
•
The charging voltage and charge time determine the amount
of charge delivered to, and accessible from, the battery. A
higher charging voltage will deliver more charge, but will also
result in greater long-term capacity fade as a function of
charge/discharge cycling. Figure 2 shows tradeoffs between
charging voltage, charge capacity and cycle fade.
•
The batteries may be charged at a constant current (CC)
followed by a constant voltage (CV). During the CC phase,
the current may be set to any value that results in an acceptable charging time and does not cause the battery voltage to
exceed 4.3V.
•
CV charging will normally result in faster charging times than
the combined CC-CV approach. The latter may become
necessary with future, larger batteries with lower intrinsic cell
resistance. Please refer to the data sheets of these batteries.
Current in µA / Capacity in µAh
4.0
3.0
2.0
1.0
0.0
0
10
20
30
40
50
60
Time (minutes)
Figure 1. Typical battery charging profile; Vc = 4.1V.
60
Achieved Capacity (µAh)
50
40
4.3V
4.2V
4.15V
4.1V
4.0V
30
20
CBC050-Q8C,
25 µA load, 90%
depth of discharge
10
0
0
10
20
30
40
50
60
70
80
90
100
Cycles
Figure 2. Effect of charging voltage on battery charge and cycle fade.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111002 RevB
Page 1
AN-1002: Guidelines for Charging Cymbet Batteries
Charging Circuits
EnerChip thin film rechargeable batteries are conducive to a
variety of charge control circuits. The recommended charging
voltage is a constant 4.1V. The range from 4.1V to 4.3V is
acceptable, but the number of life charge cycles will be
reduced toward the top of the range. The range from 4.1V to
4.0V is also acceptable, but the full charge will be reduced
toward the bottom of the range. The range of acceptable
charging voltages is illustrated in Figure 3.
Circuits consisting of one or more diodes and a fixed power
supply may be used; however, fluctuations in the power supply
voltage and part-to-part variability in the diode voltage drop will
affect the voltage across the battery terminals.
Figure 4 shows the simplest of charge circuits, where the voltage
applied to the battery terminals is the power supply voltage less
the forward voltage drop of a diode. The purpose of the diode is
to prevent the battery from discharging through the power supply
line when main power is lost. The reverse bias leakage of the
diode must be very low.
Figure 5 provides a 4.1V reference to accommodate a wider range
of power supply voltages. Figure 6 uses a linear regulator to
achieve the same end.
V+
+ Battery
4.1V
GND
Figure 4. Simple charging circuit.
Figure 3. Allowable charging voltages.
Linear Reg.
V+
IN
OUT
COM
4.1V
+
Battery
GND
Figure 6. Charging circuit with a linear regulator.
Discharge Cutoff
D1
BAS116
V+
Vout
R1
2.7k
D2
BAS116
D3
BAS116
U1
ZR4040
GND
D4
BAT54S
+ Battery
4.1V
In order to preserve the cycle life and other important
characteristics of the EnerChip, it is important to terminate the
battery discharge when the battery voltage reaches 3V. This is
particularly important when discharging at very low current – for
example, below a few microAmperes. Although > 90% of the
battery capacity will have been depleted when the battery voltage
reaches 3V at low drain current, the battery will nevertheless
continue to supply current below 3V. If discharged continuously
below that voltage, the battery will be damaged.
GND
Figure 5. Charging circuit with a voltage reference.
Simple circuits utilizing a discrete or MCU-embedded reference
voltage to control a series FET switch, for example, could be used
to disconnect the load when the battery voltage reaches 3V.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111002 RevB
Page 2
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