PSAS/ capstone2009/ design/ aps/ batterysense
  1. Battery Sensor Requirements
    1. Battery Sensor Research
  2. Battery Sensor Design
      1. ISL9208 battery charge leveling and cell voltage level shifting
      2. DS2788
      3. Features:
    1. Component Selection
      1. D3001-D3005
      2. D3006-D3009 and R3006,R3017,R3022 and R3023
      3. R3007
      4. Q1,Q2, R3008-R3011, C3001
      5. F3001
      6. R3012-R3015
      7. R3016
      8. R3018 and TR3001
      9. T1, C3002 and R3019
      10. R3020 and R3021

Battery Sensor Requirements

Battery Sensor Research

Initial research into battery sensor IC on Digi-key website shows that there are three companies making the chips: Texas Instruments, Dallas Semiconductor/Maxim-ic, and Microchip Technology. We also looked at Fairchild and Intersil. TI and Microchip technology don't have any chip that monitor a 4-cell Li-ion Battery. The only chip that works with 1-10 cell Li-ion Battery is DS2788 from Maxim. Intersil has three chips that might work for monitoring individual cell voltage, which are ISL9208, ISL9216/9717, and ISL94200. All of them use I2C interface:

In terms of a fuel gage, we chose the Maxim (was Dallas) DS2788. It's the only fuel gauge chip that made sense, we wish we remember why.

Battery Sensor Design

By using the ISL9208 for charge balancing and voltage level shifting and the DS2788 as a charge counter, we get a two chip solution that meets all of our requirements. The only downside is the DS2788 requires a low side shunt (ugh) and has a one wire bus interface (double ugh: what was Maxim thinking?).

This design is thus pretty chip centric, so we'll go over the design by really going over the chip.

ISL9208 battery charge leveling and cell voltage level shifting

The ISL9208 is almost the perfect chip for us, although it is a bit crusty. Here's an application note that's worth reading about it. A few notes on how we're using it:





Component Selection


LEDs for Fuel-Gauge Display

D3006-D3009 and R3006,R3017,R3022 and R3023

Protection circuits for the input/output data pins.


Sense resistor for the DS2788. With R3007 = 1mOhm, the current resolution is 1.5626uV/1mOhm = 1.5mA,which is good enough for our test.

The resistor I chose is CSNL10.001FRCT-ND(Digi-Key number).

Q1,Q2, R3008-R3011, C3001

Q1 is a P-channel FET, and Q2 is a N-channel FET. R3008 and R3009 form a voltage divider circuit to provide the right voltage level for VIN pin. R3010 and R3011 forms another voltage divider to provide the right voltage level for Vds of Q1. Before the start of a voltage conversion, VMA is driven high. Then Q2 turns on. As a result, Q1 turns on. Therefore, VIN (the Voltage sense input) will input the voltage, which is one fourth of the total battery pack voltage. At the end of the conversion cycle, the VMA pin is driven low. Then Q1 and Q2 are both off. As a result, VIN doesn't input any voltage data. VMA is the voltage measure active. The capacitor C3001 is used to stabilize voltage change.


Fuse to protect the circuit when the current is too high. The one I chose is 0456030.ER from Littelfuse Inc. The Digi-Key Part Number is 0456030.ER-ND.


Series resistors on each of the cell inputs to reduce the initial current surge through the ISL9208 inputs. From the application node, a series resistance of 15Ohms will add about 1mV of error to the cell voltage reading, which is acceptable.


Sense resistor of ISL9208 to monitor the change and discharge current. From example 1 of the application note, if the desired over-current level = 8A, and desired short circuit current level = 17A, then the ratio = 17/8 = 2.125. From the table 3, the short circuit threshold is 0.2V and the over-current threshold is 0.10V. The value of the sense resistor = 0.1V/8A = 12.5mOhm.

The resistor I chose is ERJ-B1CJR012U-ND(Digit-Key part number).

R3018 and TR3001

The fixed resistor and the thermistor forms a voltage divider. The TEMPI pin inputs the voltage across the thermistor to determine the temperature of the cells.Then the TEMPI pin drops below TEMP3V/13, an external over-temperature condition exists. Therefore, the resister value needs to be 12 times as the resistance of the thermistor. Since the thermistor we chose has resistance of 10k at 25 Celsius, the value of R3018 is 120Kohm.

The thermistor I chose is NTCS0805E3103JMT by Vishay/BC Components. The Digi-key part number is BC2292CT-ND.

T1, C3002 and R3019

T1 is an NPN transistor. The RGO pin connects the emitter of T1 and works in conjunction with the RGC pin to provide a regulated 3.3V. The RGC connects to the base of T1 and provides the control signal for the external transistor to provide the 3.3V regulated voltage on the RGO pin. R3019 is a pull-up resistor, and C3002 is used to stable the voltage changes.

R3020 and R3021

These two resistors control the wake-up threshold of the ISL9208. Their values are calculated using the EQ.1 in the application node. If the wake-up threshold is 3.8V, and the maximum voltage of each cell is 5 and we have a four-cell battery, then R2/(R1+R2) < 0.19. Since R1 determines the current consumption of the circuit, first choose the R1 value as the highest value that is reasonable to use. Let R1 = 1.2Mohm, then R2 = 281.5Kohm. Therefore, I chose R2 = 280Kohm.