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BATTERY CAPACITY AND THERMISTOR IDENTIFICATION SCHEME WITH A TWO CONTACT BATTERY

IP.com Disclosure Number: IPCOM000007604D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2002-Apr-09
Document File: 3 page(s) / 130K

Publishing Venue

Motorola

Related People

Joseph Patiiio: AUTHOR

Abstract

This proposal eliminates two of the four battery contacts presently necessary on the battery to deter- mine the battery's capacity and battery's tempera- ture or thermistor value. The proposal uses the capa- bility of the charger's pp to read an A/D voltage to calculate a corresponding resistance. In the existing scheme, two independent A/Ds are used to deter- mine the capacity resistor (Rc) and the thermistor (Rt) value. Since the pull up resistor value is a known constant, the tip can calculate the resistance presented to the A/D for either Rc or Rt. The proposal elimi- nates the need for both of these lines, their corresponding contacts, and 1 of the 2 A/Ds. This is accomplished through the use of3 switches (Ql, 42, and Q3). Refer to block diagram attached. When determining the capacity of the battery, the charge current circuit will be disabled and the switches will be configured in the following manner.

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MOIVROLA Technical Developments

BATTERY CAPACITY AND THERMISTOR IDENTIFICATION SCHEME

WITH A TWO CONTACT BATTERY

by Joseph Patiiio

  This proposal eliminates two of the four battery contacts presently necessary on the battery to deter- mine the battery's capacity and battery's tempera- ture or thermistor value. The proposal uses the capa- bility of the charger's pp to read an A/D voltage to calculate a corresponding resistance. In the existing scheme, two independent A/Ds are used to deter- mine the capacity resistor (Rc) and the thermistor (Rt) value. Since the pull up resistor value is a known constant, the tip can calculate the resistance presented to the A/D for either Rc or Rt. The proposal elimi- nates the need for both of these lines, their corresponding contacts, and 1 of the 2 A/Ds. This is accomplished through the use of3 switches (Ql, 42, and Q3). Refer to block diagram attached. When determining the capacity of the battery, the charge current circuit will be disabled and the switches will be configured in the following manner.

battery (defined by the Rt), and 2- The capacity of the battery (defined by the VR2). Once the capac- ity is known, you will only need to check for tem- perature during charging.

  The calculation is determined as follows. When the Q3 and Q2 transistors are off (step 2 above), selecting the zener diode VRl as a 5 Volt zener, the comprised voltage at pull up resistor R4 and VRl is 7 volts (12-5). Now select zener diode VR2 between 1 volt and 6.8 volts so that the 7 volts being supplied will be sufficient to latch the VR2 zener diode for any zener value selected within that range. When this occurs, the voltage being read by the A/D input 2 is the resistor divider voltage between 7 volts and the latched zener diode VR2 through resistor R4 and Rt.

V' = VA/D2 (42 08) = VR2 + Rt(7-VR2)

Rt + R4

  When the same measurement is done but this time with Q2 on, the supplied voltage is now 12 volts, so the new A/D reading is now the resistor divider voltage between I2 volts and the latched zener diode VR2 through resistor R4 and Rt.

v2 = %,D 2 (42 on) = VR2 + Rt(12-VR2)

Rt + R4

  Since the voltages read by the A/D are known and since the supplied voltages and pull up resistor are known, you now ,,have 2 equations and 2 unknowns (Rt and VR2) which can now be solved for. Solving for VvR2 from equation #l and substi- tuting into equation #2, V,, and VVRZ can be cal- culated by using the quadratic equation formula.

Va, = (S*R4+Vl*R4-V2*R4+7) +/- [(S*R4+Vl*R4-V2*R4+7)2-(4)*(-7)((V1)(R4)2-(V2)(R4)2)lo.5

TO DETERMINE BATTERY CAPACITY

AND BATTERY TEMPERATURE

1. Turn current circuit off (should be off since haven't started charging yet).
2. Turn...