Join Date: Sep 2002
LIPO-Use and Other Information
To start the thread here ia an article from Motorola, it covers a lot of information that has been posted in the last few weeks on other threads:
Motorola Energy Systems Group Testing Laboratories
With the advances in digital technology and the development of portable consumer devices that incorporate wireless Internet applications and services, the importance of having a reliable energy source has greatly increased. Cellular phones, personal digital assistants and other portable devices typically utilize rechargeable battery technologies such as nickel-metal hydride, lithium-ion or lithium-ion polymer chemistries. In general, batteries are able to withstand most common environmental conditions, however different rechargeable battery chemistries react to the environment differently. To ensure a long battery life, testing is performed to analyze the factors that determine the cycle life delivered by the rechargeable battery system. These factors include the battery storage conditions, charge and discharge control and the depth of discharge for the battery. In order to design an end product that will meet customer expectations, it is essential to understand the anticipated end use of the battery system and determine the effect the application may have on the life of the battery.
Battery Storage Conditions
Battery storage conditions can vary widely by product type and general area of use. A common mistake made by cellular phone users is to leave their battery pack in their vehicle during the heat of the day. A car's internal temperature can exceed 80°C, and the temperature of a dashboard with direct exposure to the sun can exceed 120°C. Portable power tools typically utilize rechargeable battery packs that are subjected to many different storage conditions, from an air-conditioned room to an outdoor workshop without climate control. Determining the affect of the various storage environments on a rechargeable battery system can be a difficult prospect.
Generally speaking, heat is often detrimental to the performance of all batteries regardless of the chemistry. Continuous exposure to high temperatures accelerates the deterioration of the battery's internal active materials. Battery cell suppliers guarantee a battery to 60°C for as long as three months. In order to achieve a one-year lifetime, the temperature that the battery is exposed to should not exceed 45°C. The lower the temperature that the battery is stored, the better the battery's chance to deliver 500 cycles.
Testing has shown that a battery's state of charge can also have a significant effect on battery life. Rechargeable batteries should not be stored in a fully charged condition.
For many end product applications, it is not feasible to suggest that the battery be stored in a controlled temperature environment, or at a 50 percent charge state. In these cases, design engineers determine how the anticipated storage environment will effect the life of the battery. The most accurate way to obtain this information is to perform a temperature storage test on the battery at the elevated or lowered temperature. Determining the temperature(s) for storage, the storage time and the initial capacity of the battery is the first step in determining the temperature effect. Once determined, the battery packs can be stored, at various states of charge, for the specified time and at the specified temperature. The battery packs are then cycled to determine the recoverable capacity. This is the available capacity of the battery pack after one charge and discharge cycle. From this information, the percent capacity loss caused by the storage at various temperatures can be determined and reconciled with any product or consumer specifications.
Nickel-metal hydride (Ni-MH) chemistry is more sensitive to high temperatures than the other rechargeable chemistries. Testing has shown that continuous exposure to 45°C will reduce the cycle life of a Ni-MH battery by 60 percent. Higher temperatures will do even more damage. In contrast, of all the chemistries considered, nickel-cadmium (Ni-Cd) cells are designed to be the most robust and can best handle high temperatures. Lithium-ion (Li-ion) cells can perform almost as well as Ni-Cd. Lithium-ion polymer cells have, for the most part, the same chemistry as lithium-ion cells. However since they lack a metallic container, they may show swelling if exposed to higher temperatures.
Selection of the proper cell chemistry for an application can be critical in providing a product that meets the customer expectations. The results of temperature storage testing can assist design engineers in determining the chemistry that is most suitable to the end product application and environment. A determination of the proper cell chemistry at an early design stage can eliminate the need for later product changes.
Charge and Discharge Control
The cycle life of a battery can be greatly affected by the current and voltage control to the battery during charge and discharge. Nickel-based battery chemistries will generate heat when overcharged, which will dry out the battery and degrade the active material of the electrodes. The Ni-MH chemistry is very sensitive to overcharge heating. Nickel-based cells can have charging terminated by 1) measuring the heating rate of the cell as it charges, 2) detecting the peak voltage, 3) detecting a certain temperature or 4) timing out. The first two methods involve costly circuitry but are the best at ensuring the maximum number of cycles. The last method is performed on lower cost battery systems and involves low charge currents. Testing has shown that with method 1 and 2, Ni-MH and Ni-Cd batteries can consistently deliver over 500 cycles. Method 3 is difficult to control but testing has shown that charging to 50°C consistently will reduce cycle life by 50 percent. Method 4 can result in cycle life reduction up to 30 percent since the batteries can be overcharged. Although the amount of cycle life reduction will depend on the level of charge current, the higher the current the fewer cycles delivered.
During the design phase of a consumer product, an evaluation of how to terminate charge can be addressed. In order to reduce the cost of the product, the designer might risk losing some performance of the battery pack. Performing cycle life testing on a small sample of batteries or battery packs with different types of charge termination can help designers. With this information, the designer can determine the cost in reduced cycle life of the battery pack in comparison to the production cost savings.
As a rule it is best to stay within the cell suppliers recommended voltage ranges, but in some cases the battery pack may be exposed to conditions outside this range. This can become not only a performance issue, but also a safety one. Overcharge and overdischarge can cause cells to become volatile, as well as reduce their cycle life. Understanding the effect of exposing a cell, or pack, to voltages outside the manufacturers' limits can be done by performing two sets of tests. First, the cells or packs can be cycled to above or below the suggested limit to determine the effect over the life of the battery. Additionally, for safety concerns, a battery overcharge test can be performed by placing a fully charged battery in a charge mode for an extended period of time. This test program should expose potential performance and safety issues, which can be especially important with lithium-ion and lithium-ion polymer cells.
Overcharging lithium-ion cells can result in safety concerns if the voltage is allowed to go above 4.5 volts per cell. Generally lithium-ion cells will deliver the most cycles when charged to 4.1 volts. Cell manufacturers usually suggest charging to 4.2 volts to get the maximum capacity from the cell and still insuring 500+ cycles. Testing has shown that overcharging by 0.1 V or 0.25 volts will not result in safety issues but can reduce cycle life by up to 80 percent.
Overdischarging the batteries, or taking the cells below the suppliers' recommended voltage (1.0 V for nickel-based cells, and between 2.7 and 3.0 volts for lithium-ion cells) will also result in reduced cycle life. Ni-Cd cells are designed to be very robust and will tolerate overdischarge conditions better than most chemistries. They are well suited for the power tool industry because of this attribute. Circuitry to insure proper voltage cut-off should be designed into the end device. Continuously overdischarging Ni-MH cells by 0.2 V can result in a 40 percent loss of cycle life; and 0.3 V overdischarge of lithium-ion chemistry can result in 66 percent loss of capacity.
Depth of Discharge (DOD)
Depth of discharge (DOD) is defined as the level to which battery voltage is taken during discharge. For instance, 100 percent DOD means that the battery voltage has been taken down to the lowest level recommended by suppliers. Twenty percent DOD means that 20 percent of the battery capacity has been removed. This level of DOD is often referred to as a shallow discharge. Discharging to less than the recommended voltage is known as overdischarge. The shallower the discharge, the more cycles the battery will provide. This is true for all battery chemistries.
The relationship between DOD and cycle life is logarithmic. In other words, the number of cycles yielded by a battery goes up exponentially the lower the DOD. Research studies have shown that the typical cellular phone user depletes their battery about 25 to 30 percent before recharging. Testing has shown that at this low level of DOD a lithium-ion battery can expect between 5 and 6 times the cycle numbers of a battery discharged to the one hundred percent DOD level continuously.
Different products have different usage profiles, and therefore different typical DOD. As a result, what is good for a cell phone may not be good for a camcorder or power tool. By knowing the typical DOD a product will see during normal use, a better understanding of cycle life can be gained. If a product will typically see a 10 to 20 percent DOD in normal use, it may not be necessary to require a battery pack to sustain a cycle life of 500+ cycles based on 100 percent DOD. Most cell supplier data is based on a standard charge and discharge rate to one hundred percent DOD. To determine how a lower DOD might affect the life of a product, a cycle life test can be performed without completely discharging the cells. The actual cycle life of a battery may be beyond what was anticipated based on supplier data. This information can be used in determining how to market the end product in terms of performance.
In summary, the three main areas that affect the cycle life of a battery pack are storage conditions, charge and discharge control and depth of discharge. Understanding the anticipated end use of a battery system, and determining the affect the application may have on the life of the battery are essential in the design of a product to meet customer expectations. Every battery application has its own unique requirements. If the expectation is for a rechargeable battery to last the life of its host product then the environment and end user habits must be understood. Testing to these unique requirements is necessary to assure that the battery will have a long life.
At Motorola Energy Systems Group Testing Laboratories, Ed Decker is Staff Engineer and Cindy Millsaps is Lead Product Safety Engineer. They have more than 24 years of combined experience in working with product safety testing.