Electricity: battery or capacitor charging or discharging – Battery or cell charging – With thermal condition detection
Reexamination Certificate
2002-04-19
2002-12-03
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
With thermal condition detection
Reexamination Certificate
active
06489752
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for charging rechargeable batteries.
BACKGROUND OF THE INVENTION
The several advantages of cordless power for portable power tools and certain kitchen and domestic appliances have led to the development of a wide range of sizes of power- or battery-packs, that is, a contained group of power cells. These power cells may include nickel cadmium (NiCd), nickel metal hydride (Nix), lithium, or lead-acid cells, etc.
Referring to
FIGS. 1-2
, a typcial battery pack
10
is connected to a charger
20
. Battery pack
10
comprises a plurality of battery cells
11
connected in series, which dictate the voltage and storage capacity for battery pack
10
. Battery pack
10
includes three battery contacts: first battery contact
12
, second battery contact
14
, and third battery contact
13
. Battery contact
12
is the B+ (positive) terminal for battery pack
10
. Battery contact
14
is the B− or negative/common terminal. Battery contact
13
is the S or sensing terminal. Battery contacts
12
and
14
receive the charging current sent from the charger
20
(preferably from current source
22
, as discussed below) for charging the battery pack
10
.
As shown in
FIG. 2
, the battery cells
11
are coupled between the battery contacts
12
and
14
. In addition, a temperature sensing device
15
, such as a negative temperature co-efficient (NTC) resistor, or thermistor, RT is typically coupled between battery contacts
13
and
14
. The temperature sensing device is preferably in closer proximity to the cells
11
for monitoring of the battery temperature. Other components, such as capacitors, etc., or circuits can be used to provide a signal representative of the battery temperature.
The charger
20
preferably comprises a controller
21
, which in turn includes positive terminal (B+)
16
and negative (B−) terminal
17
, which are coupled to battery pack
10
via battery contacts
12
and
14
, respectively. The positive terminal may also act as an input, preferably an analog/digital input, in order for the controller
21
to detect the battery voltage. In addition, the controller
21
may include another input T, preferably an analog/digital input, which is coupled to the temperature sensing device
15
via the third battery contact
13
(S). This allows the controller
21
to monitor the battery temperature. Controller
21
includes a microprocessor
23
for controlling the charging and monitoring operations. Controller
21
may control a current source
22
that provides current to battery pack
10
. This current may be a fast charging current and/or an equalization current. Current source
22
may be integrated within controller
21
.
Referring to
FIG. 3
, the battery temperature and voltage varies during the charging process. for example, battery temperature decreases as the battery is charged. The battery temperature then quickly increases as the battery becomes fully charged. However, if the charging process is not stopped when the battery is fully charged, the battery could be overcharged and thus damaged by the rising temperature. Accordingly, battery temperature or battery voltage are usually monitored as indicators of the full charge condition.
Among the voltage monitoring methods, the Saar double inflection termination method described in U.S. Pat. Nos. 4,388,582 and 4,392,101, is preferred to detect a battery reaching full charge. Other voltage monitoring methods more typically employed are (1) the minus-delta-voltage method, (2) the peak detect method, and (3) the voltage slope detect method. In the minus-delta-voltage method, a sample of the battery peak voltage is stored and compared to the most recent voltage. Termination occurs when the most recent voltage falls below a set point, usually within between 0.5% and 1.0% of the stored peak, or about 10 to 20 millivolts per cell for a NiCd battery.
The peak detect method is more modem version of the minus-delta-voltage method. Basically, the same method is used, except the set point can be set closer to the peak by using more accurate instrumentation.
The slope detect method is another voltage monitoring method. According to this method, the voltage peak B is detected by calculating the slope of the voltage curve V, or voltage change rate (dV/dt). Termination occurs when the voltage change rate is 0 or negative.
Temperature monitoring methods typically employed are (1) absolute temperature termination and (2) temperature change rate (slope) termination. Absolute temperature termination relies on the temperature rise that occurs when the battery is fully charged. Under this method, the charging process will be stopped when the battery temperature reaches and/or exceeds a certain temperature.
The temperature change rate (slope) termination method requires monitoring the slope of the battery temperature over time, or temperature change rate (dT/dt), during the charging process. Termination occurs when the temperature change rate reaches and/or exceeds a predetermined rate. In other words, termination occurs when a trip point is reached and/or exceeded.
Referring to
FIGS. 1-2
, the charger
20
may accept different battery packs
10
,
10
′,
10
″, where like numerals refer to like parts. Battery packs
10
,
10
′,
10
″ are similar, but differ in several respects. First, both battery packs
10
,
10
′ receive air blown from charger fan
24
to cool cells
11
. In battery pack
10
, the temperature sensing device
15
is covered and/or disposed outside of the airflow, so that the airflow does not affect the temperature sensing. On the other hand, in battery pack
10
′, the temperature sensing device
15
is not covered and/or disposed in of the airflow, so that the airflow affects the temperature sensing. Battery pack
10
″ does not receive any air blown from charger fan
24
. Accordingly, the temperature sensing device
15
cannot be affected by blown air.
FIG. 4
shows the temperature/voltage curves T and V, respectively, when battery pack
10
′ is charged, as opposed to the temperature/voltage curves for battery packs
10
,
10
″, shown in FIG.
3
. If the temperature curves T of
FIGS. 3-4
are compared, it is obvious that the temperature curve of battery pack
10
″ is not as smooth, with many peaks and valleys. This is because the airflow affects the sensed temperature. However, having such peaks and valleys can cause termination of the charging process under the temperature change rate termination scheme prior to fully charging battery pack
10
′. In other words, battery pack
10
′ may be undercharged because of the airflow.
It is an object of the invention to provide a charging and monitoring method that will not result in undercharged batteries.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for charging a rechargeable battery pack is proposed. The charging method includes providing a current to the battery pack, sensing first and second battery temperatures, determining a first temperature change rate between the first and second battery temperatures, sensing a third battery temperature, determining a second temperature change between the second and third battery temperatures, and disabling termination of the charging method based on a temperature-based scheme if the first temperature change rate is equal to or exceeds a first predetermined threshold and the second temperature change rate is equal to or exceeds a second predetermined threshold.
Also disclosed herein is a battery charging method, which method includes providing a current to the battery pack, sensing first and second battery temperatures, determining a first temperature change rate between the first and second battery temperatures, sensing a third battery temperature, determining a second temperature change between the second and third battery temperatures, and disabling termination of the charging method based on a temperature-based scheme if the
Trinh Danh T.
Watts Fred S.
Ayala Adan
Tso Edward H.
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