Is the runtime of a portable device directly related to the energy stored in a battery? The answer should be "yes" but in reality, the runtime is often governed by other deficiencies than depleting capacity alone. Truth be told, declining capacity, increasing internal resistance, elevated self-discharge, and premature voltage cut-of on discharge play a significant role in capacity a battery provides and the overall runtime of a portable system design.
In this article, we'll examine the impact that declining capacity, increasing internal resistance, elevated self-discharge and premature voltage cut-off on discharge have on the overall battery design of a mobile device. We'll then examine how battery analyzers can be used to help track down these problems during the design phase.
Capacity on the Decline
The amount of charge a battery can hold gradually decreases due to usage and aging. Specified to deliver a capacity of 100% when new, the battery requires replacement when the capacity drops to below 80% of the nominal rating.
The energy storage of a battery can be divided into three imaginary sections consisting of: available energy, the empty zone that can be refilled, and the unusable part (rock content) that increases with aging (Figure 1).
Figure 1: The three imaginary sections of a battery available energy, empty zone and rock content.
In nickel-based batteries, the so-called rock content is commonly present in the form of crystalline formation, also known as memory. To prevent memory, nickel-based batteries should be deep-cycled once every one or two months. If no full discharge is applied for four months and longer, a restoration becomes increasingly more difficult the longer service is withheld. The two nickel-based chemistries used in mobile communications systems are the rugged nickel cadmium (NiCd), a battery that has been around for the last 50 years, and the higher energy-dense but more delicate nickel metal hydride (NiMH).
Performance degradation of the lead-acid battery is caused by sulfation and grid corrosion. Sulfation is a thin layer that forms on the negative cell plate if the battery is being denied a fully saturated charge. Sulfation can, in part, be corrected with cycling and/or topping charge. The grid corrosion, which occurs on the positive plate, is caused by overcharge.
Lithium-ion (Li-ion) battery loses capacity through cell oxidation, a process that occurs naturally during use and aging. The typical life span of a Li-ion battery is two to three years, whether used or not. Storing the battery in a cool place at a 40% charge minimizes aging. An aged Li-ion cannot be restored with cycling or any other external means.
Internal Resistance Increases
The capacity of a battery defines the stored energy. Internal resistance, on the other hand, governs how much energy can be delivered at any given time. While a good battery is able to provide high current on demand, the voltage of a battery with elevated resistance collapses under a heavy load. Although the battery may hold sufficient capacity, the resulting voltage drop triggers the "low battery" indicator and the equipment stops functioning. Heating the battery will momentarily increase the output by lowering the resistance.
A battery with high internal resistance may still perform adequately on a low-current appliance such as a flashlight, portable CD player or wall clock. Cell phones and PDAs, on the other hand, require heavy current bursts. Figure 2 simulates low and high internal battery resistance with a free-flowing and restricted tap.
Figure 2: A battery with low internal resistance is able to provide high current on demand. With elevated resistance, the battery voltage collapses and the equipment cuts off.
NiCd batteries offer very low internal resistance and deliver high current on demand. In comparison, NiMH batteries start with a slightly higher resistance and then have their resistance readings increase rapidly after 300 to 400 cycles.
The internal resistance of lead-acid batteries is very low. The battery responds well to short current bursts but has difficulty providing a sustained high load. Over time, the internal resistance increases through sulfation and grid corrosion.
Li-ion batteries have a slightly higher internal resistance than nickel-based batteries. Aging gradually increases its cell resistance and Li-ion loses its performance due to elevated resistance rather than capacity loss.
All batteries suffer from self-discharge, of which nickel-based batteries are among the highest. The loss is asymptotically; the self-discharge is highest right after charge and then levels off. Nickel-based batteries lose 10% to 15% of their capacity in the first 24 hours after charge, then 10% to 15% per month afterwards.
Lead-acid batteries are one of the best batteries in terms of self-discharge. These batteries only self-discharge 5% per month. Unfortunately, this chemistry has the lowest energy density and is ill suited for portable applications.
Li-ion self-discharges about 5% in the first 24 hours and 1% to 2% afterwards. Adding the protection circuit increases the discharge by another 3% per month.
The self-discharge on all battery chemistries increase at higher temperatures. Typically, the rate doubles with every 10 deg. C. A noticeable energy loss occurs if a battery is left in a hot vehicle.
Usage and aging also affect self-discharge. NiMH is good for 300 to 400 cycles, whereas NiCd may last over 1000 cycles before high self-discharge affects the performance. An older nickel-based battery may lose its energy during the day through self-discharge rather than actual use. The battery gets flat at the end of the day, even if not used.
Nothing can be done to reverse self-discharge. Factors that accelerate self-discharge in nickel-based batteries are damaged separators induced by excess crystalline formation, allowing the packs to cook during charging, and high cycle count, which promotes swelling in the cell. Lead-acid and lithium-based batteries do not increase the self-discharge with use in the same manner as their nickel-based cousins do.
Not all stored battery power can be fully utilized. Some batteries cut off before the designated end-of-discharge voltage is reached, leaving precious battery energy remains unused. Applications demanding high current spikes push the battery voltage to an early cutoff. This is especially visible on batteries with elevated internal resistance. The voltage recovers when the load is removed and the battery appears normal. Discharging such a battery with a moderate load on a battery analyzer will sometimes produce residual capacity readings of 30% and higher (Figure 3).
Figure 3: Some batteries fully use their battery resources, leaving valuable energy unused.
Premature voltage cut-off is not caused by the equipment and high internal battery resistance alone, warm temperatures also play a role. Heat lowers the battery voltage and initiates an early voltage cutoff. A multi-cell battery pack may contain a cell with an electrical short. Memory on nickel-based batteries causes a further decrease in voltage, contributing to an early cutoff.
Battery Analyzers Needed
As the issues above point out, designers can simply not rely on the green light as an indication that they have a strong battery pack working in their system design. The green "ready" light does not verify battery performance but simply reveals that the pack is fully charged.
To get a more detailed understanding of true battery performance, designers need to turn to a battery analysis instrument, such as the one shown in Figure 4, which provide accurate state-of-health information. Additionally, designers can use these instruments to prolong battery life and restore lost capacity in a battery pack design packs. Future replacements of the batteries can be predicted, improving system reliability and cutting costs.
Figure 4: Photo of a typical battery analyzer.
When choosing a battery analyzer, designers should look for a product that includes adapters that allow convenient interface of different battery types. The adapters should contain the battery parameters and configure the analyzer to the correct setting. The user should be able to change the settings with a few strokes on the analyzer's keypad.
It's also worth looking for an instrument that offers a variety of programs. For example, some battery analyzers provide an accelerate test feature that allows the instrument to check the battery state-of-health in three minutes by gathering data from six variables and combining them to derive the test results. Some instruments also include features that allow designers to measure internal battery resistance as well as measure self-discharge capabilities.
Rechargeable batteries do not die suddenly but gradually get weaker over time. By using the analyzer, designers can better predict how a battery will perform in the field. In the end, this will lead to better overall system performance and better runtimes for end users.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. He has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Isidor can be reached at firstname.lastname@example.org