Battery pack discharge recovery circuit

Electricity: battery or capacitor charging or discharging – One cell or battery charges another

Reexamination Certificate

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Details

C320S162000, C307S048000

Reexamination Certificate

active

06262560

ABSTRACT:

BACKGROUND
This application relates to battery packs of the type used for powering cordless power hand tools and the like, and to recharge circuits for such battery packs. In particular, the application relates to battery packs incorporating lead-acid battery cells.
Lead-acid battery cells of the type used for powering cordless power tools, such as thin metal film cells, do not tolerate deep discharges. Indeed, such cells are prone to permanent damage if left in a state of deep discharge for an extended period of time.
It is known to provide recharging devices for restoring or maintaining the charge on a lead-acid battery. Some such devices incorporate an auxiliary battery which typically is of a size and capacity comparable to the battery being recharged. Such devices are not suitable for use with battery packs of the type used for powering cordless power hand tools, since such battery packs must be of a relatively small size and weight. Furthermore, during normal operation of a cordless power tool, the battery pack terminal voltage can undergo significant momentary terminal voltage drops, depending on the load on the battery. For example, when the tool is turned on, its motor may draw approximately 20 amps for a fraction of a second until it comes up to operating speed. Furthermore, in the case of an impact tool, such as an impact wrench or the like, during the impact phase the motor may draw approximately 15 amps for about five seconds, a typical fastener installation period. Similar heavy loads will be drawn during the stall phase of a nut driver or screw driver tool. During such heavy current loads, the battery terminal voltage may dip below the voltage level which would be indicative of deep discharge, even though the battery is not, in fact, in deep discharge. Thus, a deep discharge recovery circuit which relies on battery terminal voltage to indicate a deep discharge condition must be able to distinguish from these normal, momentary voltage drops.
SUMMARY
This application describes a system which avoids the disadvantages of prior battery pack recharge circuits, while affording additional structural and operating advantages.
An important feature is the provision of a circuit which can recharge a battery pack from deep discharge with the use of a recharging cell which has nominal output voltage substantially less than that of the battery pack being recharged.
Another feature is the provision of a circuit of the type set forth, which is adapted to be connected to a cordless power tool of the type powered by the battery pack and which ignores momentary battery pack voltage drops during normal operation of the tool.
A further feature is the provision of a circuit of the type set forth which is of simple and economical construction.
Certain ones of these and other features may be attained by providing a charging circuit for a lead-acid battery of a battery pack having positive and negative terminals, the circuit comprising: a processor connected across the battery and operating under control of a store program for monitoring battery voltage and generating a control signal at a control output, a power converter having power input terminals and a control input connectable to the control output and power output terminals respectively connected to the battery terminals, and a sacrificial battery cell connected across the power input terminals and having a cell voltage substantially less than that required to charge the battery, the processor being responsive to the battery voltage dropping below a predetermined level for generating the control signal, the converter being responsive to the control signal for increasing the cell voltage to an output voltage sufficient to charge the battery and for providing the output voltage at the power output terminals.


REFERENCES:
patent: 5084664 (1992-01-01), Gali
patent: 5276393 (1994-01-01), Gali
patent: 5677612 (1997-10-01), Campagnuolo et al.
patent: 5686809 (1997-11-01), Kimura et al.
patent: 5738919 (1998-04-01), Thomas et al.
patent: 6118253 (2000-09-01), Mukainakano et al.

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