Bottom Based Balancing in Lithium Ion System

Abstract

A method comprises discharging each of a plurality of cells of a battery pack, stopping the discharging of a selected cell of the plurality of cells when the selected cell reaches a predetermined capacity, and charging the plurality of cells after the discharging has stopped for all of the plurality of cells.

What is claimed is: 
     
         1 . A battery system comprising:
 a battery comprising:
 N cells arranged in series and each having a positive terminal and a negative terminal; and 
 N+1 external electrical contacts corresponding to the positive terminals of each of the N cells and the negative terminal of one of the N cells, respectively; and 
   a battery charger comprising:
 a plurality of terminals that electrically connect to the N+1 external electrical contacts of the battery; 
 a capacity determination device that determines a capacity for each of the N cells; and 
 N discharge devices that are selectively applied to each of the N cells respectively via the plurality of terminals. 
   
     
     
         2 . The battery system of  claim 1  wherein the battery further comprises a charging protection device that stops conducting when a current through the charging protection device exceeds a first threshold, wherein the charging protection device is connected between one of the N+1 external electrical contacts and one of the positive terminal and the negative terminal of one of the N cells. 
     
     
         3 . The battery system of  claim 2  wherein the battery further comprises:
 an additional external electrical contact for connection to a tool; and 
 a short-circuit protection device that stops conducting when a current through the short-circuit protection device exceeds a second threshold, wherein the short-circuit protection device is connected between the additional external electrical contact and the one of the positive terminal and the negative terminal of the one of the N cells. 
 
     
     
         4 . The battery system of  claim 3  wherein the first threshold is greater than the second threshold. 
     
     
         5 . The battery system of  claim 1  wherein the battery further comprises a temperature measurement device that measures a temperature of the battery. 
     
     
         6 . The battery system of  claim 1  wherein the N discharge devices each selectively apply a first load at a first time and a second load at a second time. 
     
     
         7 . The battery system of  claim 1  wherein the capacity determination device comprises:
 a measurement device; and 
 a multiplexer that selectively connects two of the plurality of terminals to the measurement device. 
 
     
     
         8 . The battery system of  claim 7  wherein the measurement device comprises:
 a voltage measurement device; and 
 an analog-to-digital converter that converts a voltage measured by the voltage measurement device to a digital value. 
 
     
     
         9 . The battery system of  claim 7  wherein the battery charger further comprises a balancing capacitor that is selectively connected to an output of the multiplexer.