Devices and methods for protection of rechargeable elements

Details Devices and methods for protection of rechargeable elements Devices and methods for protection of rechargeable elements

1999-10-22

2001-12-18

Tso, Edward H. (Department: 2838)

Electricity: battery or capacitor charging or discharging

Battery or cell discharging

Regulated discharging

Reexamination Certificate

active

06331763

ABSTRACT:

FIELD OF INVENTION
The present inventions pertain generally to the field of overvoltage and overcurrent protection systems and more specifically to devices and methods for protecting rechargeable elements, such as rechargeable batteries, from overvoltage or overcurrent conditions.
BACKGROUND
Electrical circuits that protect rechargeable elements, such as rechargeable battery packs, are well known. However, such rechargeable elements, and in particular rechargeable lithium battery cells, can be dangerous if the operating voltage exceeds a safe limit.
For example,
FIG. 1
shows a typical charging curve, i.e., the voltage across the battery vs. time, for a common lithium battery pack (e.g., used for a wireless telephone handset) allowed to keep charging beyond its maximum safe level. As labeled in
FIG. 1
, this curve may be divided into three general areas.
The first area is represented by the region where the voltage, V, is less han 4.5 volts. In this area, the battery charges at a safe level, with the temperature of the battery remaining below 60° C. to 70° C., and the pressure inside the battery remaining below 3 bars.
The second area is represented by the region where the voltage is between 4.5 volts and 5.3 volts. When charging is in this area, the battery begins to operate in a dangerous mode, with the temperature rising above 70° C., and the pressure inside the battery rising to a range between 3 bars to 10 bars. Even at this slightly increased voltage level, the battery might even explode.
The third area is represented by the region where the voltage exceeds 5.3 volts. At this stage, it is too late to save the battery, which is subjected to internal degradation and may explode or combust. Notably, battery cells in a “fully-charged” state are more dangerous and susceptible to explosions than those in the discharged state.
In particular, in order to be sure that a lithium battery operates in its safe operating mode during a charging operation, at least one of the following three conditions must be met: 1) temperature <60° C., 2) pressure <3 bars, or 3) voltage <4.5 volts.
Towards this end, rechargeable lithium ion battery packs are conventionally provided with a “smart” electronic circuit in series with the batteries to provide protection against exposure to an excessive voltage or current. Such smart protection circuits may also guard against an undervoltage condition caused by overdischarge of the battery pack.
By way of example, a conventional “smart” protection circuit
21
for a rechargeable lithium ion battery pack is shown in FIG.
2
. In particular, first and second MOSFET switches
20
and
22
are placed in series with one or more battery cells
24
. The MOSFET switches
20
and
22
are switched ON or OFF by control circuitry
26
, which monitors the voltage and current across the battery cell(s)
24
. In normal operation, the MOSFET switches
20
and
22
are switched “ON” by the control circuitry
26
to allow current to pass through in either direction for charging or discharging of the battery cell(s)
24
. However, if either the voltage or current across the battery cell(s)
24
exceeds a respective threshold level, the control circuitry
26
switches OFF the MOSFETs
20
and
22
, thereby opening the circuit
21
. The control circuitry
26
also monitors the voltage and current levels across a charging source
28
to determine when it is safe to switch back ON the respective MOSFETs
20
and
22
.
As will be appreciated by those skilled in the art, the smart protection circuit
21
is relatively complex and expensive to implement with respect to the overall expense of a conventional battery pack. Further, the series resistance across the MOSFETs
20
and
22
is relatively high, thereby decreasing the efficiency of both the charging source
28
and the battery cells
24
. Notably, both MOSFETs
20
and
22
are needed to prevent current from passing in either direction when the circuit is open,—i.e., by way of respective body diodes
23
and
25
biased in opposite directions—, which increases the complexity, cost and total in-series resistance of the protection circuit
21
. Also, because the MOSFETS
20
and
22
are subject to failure if exposed to a sudden high voltage (or use of an improper high voltage charger), secondary protection of the battery cell(s)
24
is still needed, such as, e.g., a positive temperature coefficient (“PTC”) resettable fuse employed in series with each cell.
By way of background information, devices exhibiting a positive temperature coefficient of resistance effect are well known and may be based on ceramic materials, e.g., barium titanate, or conductive polymer compositions. Such conductive polymer compositions comprise a polymeric component and, dispersed therein, a particulate conductive filler. At low temperatures, the composition has a relatively low resistivity. However, when the composition is exposed to a high temperature due, for example, to ohmic heating from a high current condition, the resistivity of the composition increases, or “switches,” often by several orders of magnitude. The temperature at which this transition from low resistivity to high resistivity occurs is called the switching temperature, Ts. When the device cools back below its switching temperature Ts, it returns to a low resistivity state. Thus, when used as an in-series current limiter, a PTC device is referred to as being “resettable,” in that it “trips” to high resistivity when heated to its switching temperature , Ts, thereby decreasing current flow through the circuit, and then automatically “resets” to low resistivity when it cools back below Ts, thereby restoring full current flow through the circuit after an overcurrent condition has subsided.
In this application, the term “PTC” is used to mean a composition which has an R14 value of at least 2.5 and/or an R100 value of at least 10, and it is preferred that the composition should have an R30 value of at least 6, where R14 is the ratio of the resistivities at the end and the beginning of a 14° C. range, R100 is the ratio of the resistivities at the end and the beginning of a 100° C. range, and R30 is the ratio of the resistivities at the end and the beginning of a 30° C. range. Generally the compositions used in devices of the present inventions show increases in resistivity, which are much greater than those minimum values.
Suitable conductive polymer compositions are disclosed in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), 5,049,850 (Evans et al), 5,250,228 (Baigrie et al), 5,378,407 (Chandler et al), 5,451,919 (Chu et al), 5,582,770 (Chu et al), 5,701,285 (Chandler et al), and 5,747,147 (Wartenberg et al), and 6,130,597 (Toth et al). The disclosure of each of these patents is incorporated herein by reference for all that it discloses.
Referring to
FIG. 3A
, a crowbar type protection circuit
31
is also well known. In particular, a switch element
30
is placed in parallel across the battery cell(s)
24
. The switch
30
is opened or closed by control circuitry
36
, which monitors the voltage and current across the battery cell(s)
24
. In normal operation, the switch
30
is left open. However, if either the voltage or current across the battery cell(s)
24
exceeds a respective threshold, the control circuitry
36
closes the switch
30
, thereby shorting the circuit across the battery cell(s)
24
.
FIG. 3B
illustrates the current versus voltage curve
35
through the switch element
30
, when it is closed. Notably, the current can quickly reach relatively high levels, depending on the characteristics and duration of a particular power surge. Towards this end, a first overcurrent element
32
may be provided between the switch element
30
and the charging element
28
to help protect the switch element
30
from continuous current from the charging element
28
(shown in FIG.
2
). Similarly, a second overcurrent element
34
may be provided between the switc

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