AnalogTalk

In the previous post we talked about the charging and discharging behavior or a Li-ion battery.   On our way to a full fledged fuel gauge we need to first measure battery voltage.   Figure 1 shows the battery voltage measurement circuit using and ADC, in this case a MCP3421 18-bit ADC (U1).

Figure 1 Battery Voltage Measurement

Since the MCP3421 device has an internal reference voltage, the measurable maximum input voltage range is limited to the internal voltage reference voltage of up to 2.048V.  To measure the input voltage higher than the internal reference, a voltage divider is used, which is formed by R1, R2, and R3.  The R3 is optional and is used to calibrate the R1 and R2 component tolerance.  By choosing the series resistance value of the voltage divider to be very high (> 1 MΩ), the current losses due to the voltage divider is negligible.

In the example circuit as shown in Figure 1, the ADC is configured as single ended by connecting the positive input pin (VIN+) to the battery voltage, while the negative input pin (VIN-) to the VSS.   The ADC output is available to the MCU via the I2C bus line.
Figure 2 shows the discharge curve of a 3.7V Li-Polymer battery (3.7V, 170 mAH).  The curve shows that the battery voltage reduces linearly until it reaches about 80% of its full capacity.

Figure 2 Li-Polymer Battery Voltage Discharging Curve
Since the battery discharging characteristics are very linear until the point where the curve falls off sharply, measuring only the battery voltage is an alternative low-cost method to estimate the current status of the battery. In this case, the measured battery voltage can be compared with the fuel values in the lookup table in the MCU firmware.
The circuit shown can be used for measuring the battery voltage of any battery type. When the circuit is used, the voltage divider (R1, R2, R3) must be properly adjusted in order to keep the maximum input voltage (or the voltage at VIN+ pin when the battery is fully charged) to the ADC device is less than the ADC internal reference voltage (2.048V).
Although using the voltage alone is not sufficient to represent the battery fuel status, this method is widely used for simple and cost-sensitive applications because of its straightforward implementation.

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Battery Management

Delta-Sigma Analog-to-Digital Converters

The battery discharging behavior changes with various parameters, such as battery chemical type, load current, temperature, and aging. The figure below shows the general behavior of battery discharging curves of several battery chemical types.  The battery discharging curve of most batteries is almost flat until it reaches about 80% of its full range, and then falls off sharply after that. Intelligent battery chargers, or delta V chargers look for this drop off.
Since the battery’s internal chemical reaction is largely governed by voltage and temperature, the battery discharging behavior is greatly affected by the temperature.  The low temperature limit is determined by the freezing temperature of the electrolyte.  Most batteries do not work well below -40°C.  The battery performs better at higher temperatures because the chemical reaction processing is accelerated.  However, the rate of undesirable chemical reactions increases and results in a decrease of battery life.  At extremely high temperatures, the active chemicals become unstable and can destroy the battery and your laptop.

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Battery Management

Sony has announced a new type of lithium ion rechargeable battery that greatly improves power and life performance (better stability for fewer exploding laptops of course).

The Olivine-type lithium iron phosphate used is a perfect cathode material due to its robust crystal structure and stable performance, even at high temperatures. These batteries have a high power density of 1800W/kg, and extended life span of approximately 2,000 charge-discharge cycles.

It is claimed that these new batteries will provide 80% capacity retention after those 2,000 charge-discharge cycles is also able to charge in a half hour. Power tools is an ideal use,where then it will gradually infiltrate its way to consumer electronic electronic devices.

More here

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MCP73213 LiFePO4 Battery Charger

If you have interest in the previous post on overvoltage protection (OVP) let us suggest the recently introduced family of battery chargers, the MCP73113, MCP73114 and MCP73213 Lithium-Ion (Li-Ion); and MCP73123, MCP73223 Lithium Iron Phosphate (LiFePO4) 1.1A Chargers, all have built-in Overvoltage Protection. The devices have a maximum input voltage of 18V and come with various OVP set points—5.8V and 6.5V for the single-cell MCP73114 and MCP73113/23 chargers; or 13V for the dual-cell MCP73213 and MCP73123 chargers.

To provide more flexibility, the MCP73113, MCP73114 and MCP73213 devices provide a variety of charging-voltage options for Li-Ion batteries—4.1 to 4.4V for the single-cell and 8.2 to 8.8V for the dual-cell devices.  The MCP73123 and MCP73223 devices target LiFePO4 batteries, and offer charging-voltage options of 3.6 and 7.2V, respectively.

With the option for various charging voltages, it is possible to target different batteries depending on their requirements. The LiFePO4 option provides designers the flexibility to try out one of the up-and-coming battery technologies with built-in smoke release resilience and higher current capabilities.

For more on these chargers, head on over here

We have all heard the horror stories of laptop batteries blowing up or favorite tunes players burning a hole through someone’s jeans. The reasons for the magic smoke escapes vary from poor battery cell construction to overheating and poor charging mechanisms, yet all agree: magic smoke should stay inside our electronics.  While it is up to battery manufacturers to cage the magic smoke inside the batteries, plenty can be done outside the battery to prevent smoke escapes from other parts of the electronic device.

Case in point: Overvoltage Protection (OVP) on a battery charger circuit. OVP allows the battery charger to shut down in case the input voltage goes too high and prevents the circuit from overheating. And, as we know, overheating is smokes’ most notorious escape accomplice.  The value added by battery chargers with this feature is fast making it a standard so in your next design make sure OVP is on the feature list.

The MCP73871 USB/AC load-sharing Li-Ion/Polymer charger was reviewed recently.  Check it out if you’re into portable applications.

Dual-source single-chip battery chargers are not unique and we have seen a number of ac/USB versions over the last two years. Microchip, however, comes in with a fresh mindset and offers some feature combination that I have not seen elsewhere.

The full EN-Genious review here:
http://www.en-genius.net/site/zones/lowpowerZONE/product_reviews/lpwrp_090108

Brief Description:
The  MCP73871 charge-management controller—a Li-Ion/Li-Polymer charger with an intelligent charge management feature that enables simultaneous AC-DC-adapter or USB-port charging and powering of devices. The single-chip charger features an integrated pass transistor, and numerous battery and termination-voltage options—making it ideal for complex, space-constrained portable applications.