AN-1217 A Boost Converter Design for Energy Harvesting Applications (Part 2)

It is useful to compare Li-Ion charging with Lead-Acid charging. The traditional lead-acid battery charge cycle has Bulk (often but not necessarily Constant Current), Absorb (Constant Voltage) and Float (Constant Voltage) phases. It is important to complete the charge cycle by running it though all three phases. Li-Ion battery charging differs from lead-acid charging in the following aspects:

i) There is no Float phase – charging current must drop to zero once Absorption is complete; completion of Absorption is signaled by the charge current dropping to below 0.1C.

ii) If charging is stopped before Absorption is complete, the battery life does not worsen as with lead acid batteries; in fact cutting short the Absorption phase or eliminating it altogether actually improves the battery life [1].

iii) Deeply discharged Li-Ion batteries need a trickle charge of 0.1C to make them ready for the Bulk phase

iv) Slow charging simplifies matters. From [2]: “the quicker the charge, the lower the capacity when the battery switches to the relatively slow constant-voltage part of the charging regime. For example, charging at 0.7 C results in a capacity of 50 to 70 percent when 4.1 or 4.2 V is reached, whereas charging at less than 0.2 C can result in a full battery as soon as the voltage reaches 4.1 or 4.2 V.”

A general-purpose Li-Ion charger therefore needs a fair amount of logic to take care of these different battery states and properties. However, things become interesting in the specific context of energy harvesting. Application Note AN-1202 [3] presents a boost converter for energy harvesting applications. This Note features a battery that is used to support the system when harvested energy runs low. What if we wanted a rechargeable Li-Ion battery here? Since we are dealing with loads that are not power hungry, it is more important to have extended battery life rather than have a battery charged to exactly 100% of capacity. This allows us to exploit property (ii) above.

Schematic-level addition for Li-Ion charging

Figure 1. Schematic-level addition for Li-Ion charging

Further, charging currents being low, we can also exploit property (iv). These effectively imply that the Absorption phase offers no advantage and can be dispensed with. As long as we make a reasonable assumption that the Li-Ion battery will not be over-discharged (or design specifically for it), then (iii) is also rendered largely irrelevant. Consequently, we are left with only the Bulk phase to contend with. Therefore we can quite easily incorporate Li-Ion battery charging capability into the design presented in AN-1202 with the addition of a single resistor R4 and a GreenFET Q2b as shown in Figure 1, which will be controlled by the GreenPAK™ via the GPIO line marked Battery Charge. The additional piece conveniently operates off the 5V output!

(The single GreenFET in AN-1202 has been replaced by a dual GreenFET here.) When Q2b is on, the battery charges via the 5V output of the system. The value of R4 must chosen so that the overall load is small enough to not cause excessive output ripple; it should at any rate be no more than 0.1C. Q2b is turned on via the Battery Charge GPIO pin only when the supercap is fully charged, and the battery is not fully charged. If the battery as well as supercap are fully charged, harvested energy is bled away as before through R1. The change in the bleed logic is implemented by modifying the associated GreenPAK Designer diagram as shown.

GreenPAK Designer changes for Li-Ion charging

Figure 2. GreenPAK Designer changes for Li-Ion charging

We now need an additional ACMP to monitor the battery voltage. In Figure 2 this is ACMP1 which is configured with a hysteresis of 200mV so that when the battery voltage is above 4V, it is deemed full enough and when it falls below 3.2V it flags the need for charging. The supercap voltage is monitored by ACMP0. NAND gate L0 bleeds the supercap when the supercap and battery are both charged, whereas LUT14 outputs a high to turn on Q2b to charge the battery when the battery voltage is below 3.2V and the supercap is fully charged.

(Note: In comparing Figure 2 of AN-1202 and Figure 2 in this Addendum, it will be evident that some ACMP labels have changed; this is because some ACMPs cannot be configured independently of others. For this reason a few GPIO assignments are also different.)

References

1. “BU-409: Charging Lithium-ion”, http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

2. “A Designer's Guide to Lithium (Li-ion) Battery Charging” https://www.digikey.com/en/articles/techzone/2016/sep/a-designer-guide-fast-lithium-ion-battery-charging

3. Ramaswamy R., “A Boost Converter Design for Energy Harvesting Applications” AN-1202

About the Author

Name: Ramkumar Ramaswamy

Background: Ramkumar Ramaswamy (Ram) is a freelance electronics engineer based in Bangalore, India. He received his MSc in Physics from the University of Delhi and his PhD in Operations Research from the Indian Institute of Management Calcutta. He has authored several technical articles.

Contact: appnotes@silego.com

 

Files



See full list of Application Notes