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Performance Gains in Lithium Battery Charging 2500 Eisenhower Avenue, PO Box 890, Valley Forge, PA 19482-0890, 610-676-0188, FAX 610-676-0189, www.galaxypower.com, E-mail: [email protected]Galaxy Power, Inc. 2500 Eisenhower Avenue. PO Box 890. Valley Forge, PA 19482-0890. (610)676-0188. FAX (610)676-0189 Performance Gains in Lithium Battery Charging Technology using the Galaxy Power QuickLithium™ Charging Algorithm Lithium Battery Technology Lithium-based (Lithium-Ion and Lithium-Polymer) batteries have become the de facto standard for battery powered portable electronics equipment. Currently, devices such as Laptop PCs, Cell Phones, and PDAs utilize this battery chemistry almost exclusively. Lithium-based battery technology possesses many technological advantages when used in these applications. Lithium-based batteries offer the highest volumetric efficiency and mass efficiency of any of the currently available battery chemistries. For a given weight and volume, they are capable of storing more electrical energy than any of the other readily available cell chemistries. In addition, Lithium-based batteries have a low rate of self-discharge; a Lithium-based battery retains its charge when stored far longer than other battery technologies. Although Lithium-based batteries are more costly than Nickel-Cadmium or Nickel-Metal-Hydride batteries, they are not prohibitively expensive for use in high-end products. As Lithium-based technology advances, expect to see this battery technology migrate into many other applications. Charging Lithium-based batteries requires different technologies from those used for other cell chemistries. Attempting to charge Lithium-based batteries beyond 4.2 Volts per cell permanently damages the battery and may result in the battery exploding. Over discharging Lithium-based batteries also results in irreversible changes to the batteries. For this reason, manufacturers incorporate semiconductor-based protection circuitry in the Lithium battery pack to prevent this from happening. In addition, Lithium-based batteries must be limited in the amount of current they supply; significant over-current discharge can cause this type of battery to explode. Manufacturers incorporate a PTC fuse device within the battery pack to limit current. A PTC fuse is actually a thermistor type device that exhibits a relatively low resistance at its rated current, but increases resistance exponentially if this current is exceeded. Charging Methods Contemporary charging methods utilized for charging Lithium-based batteries charge the batteries at a constant current, typically at a 1C to 1 ½ C charging rate, therefore, a 1 Ampere/hour battery would be charged at 1 to 1 ½ Amp rate. When a cell voltage of 4.2 Volts is reached, the current is decreased to maintain this voltage constant and current is monitored until it reaches 1/10th of its initial value. At this point, the battery is considered to be fully charged. If a Lithium-based battery is discharged to its low voltage limit, then fully charged in this manner, it takes 2 ½ to 3 ½ hours to be fully charged. Compared to Nickel-Cadmium (Ni-Cd) or Nickel-Metal-Hydride (Ni-MH) batteries, this is a long charging time. Ni-MH batteries can be charged in approximately 1 hour. The limitation on charging time for Ni-MH batteries is a result of their exothermic charging characteristics. Attempts to charge them at a faster rate results in excessive heating of the battery pack which is detrimental to battery life. Ni-Cd batteries can be charged at a much faster rate as they possess an endothermic charging characteristic. Assuming that the charge is terminated before they enter an overcharge region, they can be easily charged in 15 minutes without adverse effects. This relatively long charging cycle compared to that of other battery chemistries is one of the less desirable characteristics of Lithium-based batteries. Another undesirable characteristic is “capacity fade”. Each time a Lithium-based battery is charged it loses a little of its storage capacity. All battery chemistries exhibit some capacity fade, however, Lithium-based batteries fade more rapidly than Ni-Cd orNi-MH batteries. The battery loses a little capacity each time it is charged. Cell resistance increases with each charge cycle until the battery is no longer usable. It appears as if the act of charging the battery damages it, almost as if the size of the battery plates decreases with each charging cycle. 2500 Eisenhower Avenue, PO Box 890, Valley Forge, PA 19482-0890, 610-676-0188, FAX 610-676-0189, www.galaxypower.com, E-mail: [email protected] Galaxy Power, Inc. 2500 Eisenhower Avenue. PO Box 890. Valley Forge, PA 19482-0890. (610)676-0188. FAX (610)676-0189 If we look at an electrical model of a Lithium-based battery pack we can see some of the reasons for these problems. Terminal and Lead Resistance Terminal and Lead Resistance Overcurrent Protection Cell Protection Device Internal Cell Resistance Ideal Lithium- Ion Cell PTC Fuse Lithium-Ion Battery Model As can be seen from this schematic, there are a number of resistances in series with the battery. The Lithium-based battery, as we already have stated is a voltage sensitive device. All of these external resistances along with the battery’s own internal cell resistance make it difficult to actually measure battery voltage and to control the charging cycle. Many of these resistances are a result of the protection devices required when using a Lithium-based battery. However, terminal and lead resistance, and cell resistance are present with any type of battery. In the case of Ni-Cd or Ni-MH batteries, these resistances are unimportant as we typically charge with a constant current source until the charge cycle is terminated. With Lithium-based batteries we cannot do this due to the limitation on maximum cell voltage. Resistance Source Magnitude Voltage Drop @ 2.5Amps Terminal and lead resistance .0536? .134 Volts PTC Fuse resistance .0372? .093 Volts Protection Device resistance .042? .105 Volts Cell resistance Varies ˜ .036? .09 Volts TOTAL .1688? .422 Volts As can be seen from this data, if this battery was charged at a constant current until the voltage at the charger measured 4.2 Volts, then decrease current to hold 4.2 Volts AT THE CHARGER, the constant voltage phase of charging would start nearly ½ Volt too early. This would contribute to a long charge cycle. This would be worse if cell resistance was higher. The QuickLithium™ Charging Algorithm The Galaxy Power, Inc. QuickLithium™ charging algorithm utilizes several techniques to minimize charging time. QuickLithium™ measures battery voltage at a time when no current is flowing into or out of the battery. This quiet time sensing was pioneered in our patented QuickSaver® charging method. It permits us to measure the actual cell voltage without the inaccuracies introduced by all of the series resistances. We also measure the current flowing into the battery and the voltage at the output of the charger during charge. With this information we can calculate the actual voltage drop across these series resistances and compensate for it. As a result, QuickLithium™ delays the onset of the constant voltage phase of charging until the actual battery voltage reaches 4.2 Volts/cell. As cell resistance is dynamically changing during the charging cycle, we recompute this once per second. Note: All resistances measured on a BYD Type LP103463S Lithium-Ion Battery (1700mAh capacity). Cell resistance was measured when the battery reached 4.2 Volts, and will vary considerably depending on the state of charge of the battery, and the number of times it has been charged. 2500 Eisenhower Avenue, PO Box 890, Valley Forge, PA 19482-0890, 610-676-0188, FAX 610-676-0189, www.galaxypower.com, E-mail: [email protected] Galaxy Power, Inc. 2500 Eisenhower Avenue. PO Box 890. Valley Forge, PA 19482-0890. (610)676-0188. FAX (610)676-0189 QuickLithium™ Charging Algorithm Flow Chart Start Load Preset Period & Duty Cycle. Soft Start Measure Battery Voltage Under No Charge Condition (Vbo) Is Voltage = 4.2 Volts Cell? Yes No Measure Voltage (Vbc) & Current Under Charge (Ichg) Subtract Vbo From (Vbc) =To Vr Is Vbc – Vr = 4.2 Volts Cell? Yes No Is Ichg > Preset Value? Yes No Increase Charging Current Decrease Charging Current Decrease Charging Current Measure Vbc & Vbo & I Under Charge Is Vbc – Vr >4.2 Volts Cell ? Yes No Increase Charging Current Is Ichg= Preset End Value ? No Yes End Fast Charge Go To Topping 2500 Eisenhower Avenue, PO Box 890, Valley Forge, PA 19482-0890, 610-676-0188, FAX 610-676-0189, www.galaxypower.com, E-mail: [email protected] Galaxy Power, Inc. 2500 Eisenhower Avenue. PO Box 890. Valley Forge, PA 19482-0890. (610)676-0188. FAX (610)676-0189 The TI MSP430 microcontroller based development board used to develop the QuickLithium™ Charging Algorithm SmartPulse™ To minimize the effects of cell resistance, we borrow another technique from our QuickSaver® technology. Once per second, we introduce a short (5 Millisecond) but deep (2 ½ C) discharge pulse to the battery. This technique which Galaxy Power, Inc. has trademarked as SmartPulse™ has the beneficial effect of redistributing the charge more uniformly over the surface of the battery plates, preventing the formation of bubbles, and minimizing cell resistance. The plates of the battery are not a flat sheet of metal, rather, they are highly irregular to maximize their surface area and keep cell resistance low. However, as charge tends to reside on the peaks of this surface, the peaks enter an overcharge state long before the battery is fully charged. As this is an electrolytic process, these overcharged peaks tend to evolve oxygen bubbles that shield the rest of the plates from being charged. The discharge pulse discharges this excessive charge at the peaks and results in the battery being more uniformly charged. As no bubbles are present, cell resistance remains low and the battery will accept charge at a higher rate. Since Lithium-based batteries are highly susceptible to damage from overcharge, this discharge pulse minimizes this effect and results in a much lower rate of capacity fade. Results Testing was conducted on the same BYD Lithium-Ion battery that was used to measure resistances. The battery was discharged to the low-voltage trip point of its internal protection circuitry. The battery was then charged using contemporary constantcurrent constant-voltage charging methods at a 2 ½ Amp charging rate until it measured 4.2 Volts. Charging was continued until the current dropped to 170 mA. The battery capacity was measured using an Alexander Batteries Optimizer 2003 battery analyzer. Charging time using this method was 2 Hours, 20 Minutes. The battery was discharged again to its low voltage trip point, and then recharged using the Galaxy Power, Inc. QuickLithium™ method. Charging time was measured at 55 Minutes. The battery was again discharged using the Optimizer 2003 analyzer. The battery when charged with the QuickLithium™ method exhibited a 5% increase in charge as compared to the constantcurrent constant-voltage charging method. 2500 Eisenhower Avenue, PO Box 890, Valley Forge, PA 19482-0890, 610-676-0188, FAX 610-676-0189, www.galaxypower.com, E-mail: [email protected] Galaxy Power, Inc. 2500 Eisenhower Avenue. PO Box 890. Valley Forge, PA 19482-0890. (610)676-0188. FAX (610)676-0189 Conclusion QuickLithium™ technology appears to overcome most of the objectionable characteristics of Lithium-based batteries in realworld applications. Charging times with the QuickLithium™ charging technique are comparable with Ni-MH, and capacity fade is greatly minimized. QuickLithium™ should extend the utilization of Lithium-based batteries to many new applications. Bruce Rogers Sr. Staff Engineer Galaxy Power, Inc. Phone: (610)676-0188 FAX: (610)676-0189 E-Mail: [email protected] |