As portable devices increase, battery-powered systems become more common. Battery defects are also occurring frequently, so it has become important to identify and replace battery defects early. Any battery defect, from individual batteries used in mobile phones to battery banks used to store renewable energy, can lead to system downtime.

The battery analysis system that can check the battery health status uses a precision analog-to-digital converter (ADC) as a key component. Depending on the key specifications of the ADC, such as speed, resolution, and delay time, the battery health status can be analyzed more accurately. Looking at the Randles lead-acid battery model in Figure 1, we can see how important the performance of the ADC is.


In Figure 1, R1 is the active electrolytic resistance, R2 is the charge transfer resistance, and C is the double layer capacitance. These are combined to form a schematic equivalent circuit of a lead-acid battery. By measuring these three components and comparing them to the expected and known values, we can get an approximation of the battery's health, which includes Cold Cranking Amps (CCA), State of Charge, and Capacity.

There are many ways to test a battery, such as discharge/charge cycles, DC loads, and AC tests, but the most accurate method is electrochemical impedance shunting (EIS). EIS can quickly measure CCA, SOC, and battery capacity. The process involves extracting a series of small low-frequency signals from the battery and measuring the corresponding current across a shunt resistor and the battery DC voltage. From these measurements, R1, R2, and C can be calculated, and compared to the expected value, the battery health can be calculated.

Depending on the health and type of battery being tested, the current and voltage measured range from very low to very high levels. The ADC selected to convert the measurements must be able to accurately measure even small changes in the input signal over a wide input range.

Successive approximation register (SAR) ADCs are the preferred converters due to their high dynamic range, speed, resolution, and low latency. A high-resolution SAR ADC can accurately measure low-speed signals (from DC to several megahertz) and then use a host processor (FPGA) to oversample and digitally filter them to improve system accuracy. Alternatives include delta-sigma ADCs (not so suitable for measuring a range of input frequencies) and pipeline ADCs (which offer high speed but poor resolution). SAR ADCs also have low latency, which reduces the time required to make measurements without compromising accuracy.

Battery analysis systems can struggle to measure current (from milliamps to several amps) or voltage (from several volts to tens of volts) with great accuracy across the full range. For more accurate measurements, a high-resolution SAR ADC with a wide dynamic range (input range) and at least several hundred kSPS (kilo samples per second) should be used to take multiple measurements for each input signal, which should then be digitally filtered by the host processor. Figure 2 is a schematic diagram of a battery tester system.


As shown in Figure 2, as the load changes, AC current is drawn from the battery, which in turn produces an AC voltage across a small, high-accuracy sense resistor. The voltage developed across this resistor is then amplified and measured using a high-precision data acquisition system designed to minimize signal distortion. In the case of measuring the battery DC voltage, this input is often scaled using an amplifier, allowing the ADC to measure a wide voltage range. In both cases, the ADC chosen to digitize the signal must have sufficient resolution to detect even small changes in the input signal.

To measure this voltage, you can choose from a variety of SAR ADCs. The TI ADS8900B family in Table 1 is a good choice in many ways, including high resolution, fast sampling rate, and excellent AC and DC performance. These features are critical to measuring the wide dynamic range of signals encountered during battery health analysis and maintaining accuracy across the entire input range. th:213px;height:18px;"> Gain

	ADS890xB	ADS891xB	ADS892xB
Resolution	20bit	18bit	16bit
speed	Up to 1MSPS	Up to 1MSPS	Up to 1MSPS
Input range	± 5V</td>	± 5V	± 5V
INL (Integral Nonlinearity)	± 1.1 LSB	± 0.5 LSB	± 0.3 LSB
signal to noise ratio	104.5dB	102.5dB	96.8dB
Total harmonic distortion	-125dB	-125dB	-125dB
± 0.005%FSR	± 0.005%FSR	± 0.005%FSR
Offset (-40 ℃ ~ -125 ℃ )	± 10.5 LSB	± 3 LSB	± 3 LSB
Package	4mm x 4mm QFN	4mm x 4mm QFN	4mm x 4mm QFN

These products also adopt internal reference buffers to further improve system accuracy and reduce size. This feature is especially important for portable battery analysis systems. Figure 3 compares external and internal reference buffers in a data acquisition system.

In precision data acquisition systems, the reference voltage circuit is important because it provides a reference point for the data converter to compare the input signal. Any errors in the reference voltage will result in inaccurate measurements of the input signal. During each conversion cycle, the ADC draws significant current from the reference due to the converter’s internal switched capacitor architecture. The reference buffer minimizes the voltage droop that occurs during conversion. The ADS8900B family maximizes AC and DC performance by optimizing the internal reference buffer to drive the ADC’s reference pin, providing a more accurate system than using an external reference buffer.

We have previously demonstrated that the ADS8900B can be used to more accurately measure battery health in battery analysis systems. However, any system that wants to precisely measure small or dynamic signals can benefit from the benefits offered by these products, in addition to battery analysis systems.