Analog Devices’ Battery State of Health Monitor Includes a Coulomb Counter

The LTC3337 from Analog Devices (ADI) tallies the charge going from a battery to a load, or vice versa. It also monitors the battery’s voltage, impedance and temperature.

The LTC3337. Image courtesy of ADI

The coulomb counter tallies the accumulated charge flow in either direction and delivers the result to an internal register, where it is accessible through an I2C interface. The SOH monitor’s other measured parameters, including voltage, temperature, and impedance, are also communicated via the I2C interface, which switches at 400 kHz.

The device also features a programmable discharge alarm, and when the chosen charge level is reached, an interrupt is generated at the IRQ pin. 

 

What is a Coulomb?

A coulomb is the unit of electrical charge, or electrical energy. A coulomb is equal to one ampere of current flowing for one second.

As such, the number of coulombs drawn from a battery is the amount of energy the battery has supplied. Conversely, the number of coulombs delivered to a battery is the additional energy it now has stored, ready for use.

The amount of charge in a battery is analogous to the amount of gasoline in a fuel tank. So measuring the amount and direction of coulomb flow indicates how much “gas” has been used, or how much has been “pumped” in.

 

Basic Features

The LTC3337 can work voltage inputs ranging from 1.8 to 5.5 volts and draws a quiescent current of 100nA. The user can select from eight different peak current levels, ranging from 5 to 100mA. The selection is made via the three IPK pins. The device also includes an integrated ±10mA supercapacitor balancer.

 

Counting Coulombs

The AVcc pin is the source of power for all of the LTC3337’s internal circuitry. The pin is connected to either the BAT_IN or BAT_OUT pin. If the former is chosen, coulomb flow is calculated for charge going into the battery, while the choice of BAT_OUT allows for calculating the charge coming out. 

When the highest of the eight peak level current settings, 100mA, is chosen, the unit can measure charge of up to a total of 977.3 amp-hours (typ). Over the SOH’s full temperature range, maximum coulomb count error is ±5%

 

Voltage and Impedance

The SOH monitor includes a 12-bit analog-to-digital converter (ADC), which is employed to digitize the unit’s die temperature and make the output available to the I2C bus. 

The voltage is measured when the coulomb counter is delivering no current and measured again when the coulomb counter is delivering a known IPEAK pulse. The difference between the two is divided by the current, yielding the impedance.

 

Applications

• Low power battery powered systems
• Battery Backup
• IoT
• Applications using ADI’s SmartMesh
• Asset Trackers 
• Electronic Door Locks

Physical Considerations

• The SOH Monitor is available in a 12-lead, 2 x 2mm LFCSP package.
• The unit operates over a -40 to +125℃ temperature range

 

 

Environmental

• The LTC3337 is RoHS compliant