Helping batteries last longer
How long should things last? I think it depends on what you are willing to tolerate. My daughters had this little Sesame Street cassette tape player with a series of large colored buttons. The instructions were never to push the red button, which occasionally our kids would. This little player was used nearly every day for more than 6 years. Eventually things began to wear out.
The first thing to go was the cassette loading door. After a couple of years, it would not always stay closed. My middle daughter asked if it was broke. I told her it was just wearing out. She asked me what the difference was. I told her that we could still use it; but that we would just have to set “Big Bird” on the door.
I had a similar experience with my first notebook computer. I wanted to keep it and use it until the purchase price, divided by the number of days I had owned it, was about a dollar. That ended up being about 2,000 days. Toward the end of its days, it was the plastics and mechanical parts that were giving me issues. For example, the hinges for the display began to stick, then break. I repaired them to the level I could tolerate the functionality.
The charger jack became very loose and I repaired it to the point it was almost reliable. Yes, I had to bring it up to tolerable functionality. I since have changed my goal to somewhere between $3 and $5 per day of ownership for replacing my notebook computers. Luckily the price of notebooks has come down appreciably.
This brings me to our topic. While I used a dollar-per-day of ownership to target the replacement of my laptop, there are other targets. You may want to keep the equipment until the embedded battery no longer can support the run time expectations. I do appreciate the embedded battery strategy as long as the battery life meets my level of tolerance for truly portable operation. As a side note, I have an electric razor that has had the same battery for 8 years. I have gone through 5 sets of blades, but the battery still meets my expectations of 4 shaves per week before recharging is required.
Battery degradation is a popular topic or complaint. Most portable users just accept the phenomena. They live with it by purchasing a replacement battery, if their battery is replaceable, or replace the entire device. A colleague of mine, Yevgen Barsukov, presented on this topic of battery degradation at this year’s International Battery Seminar and Exhibit.1
Just knowing the kinetics of battery degradation can help determine actions that can reduce the rate of the phenomena. For example, temperature accelerates degradation and higher state charge further increases the reaction rate. So charging your device to full and then leaving it in your car in the summer, can take weeks off your battery’s life.
Battery degradation occurs in two areas: Coulomb losses and impedance increase. Coulomb losses are due to loss off active materials such as Lithium or cathode material. This loss is due to the material not being able to participate in energy cycling. Impedance increase reduces the rate capability of the battery, causing the battery to produce lower terminal voltages during discharge and slower recharge cycles.
Barsukov addressed Lithium plating as one component of capacity degradation. This component results in the loss of Coulomb capacity. In his paper he presented a method of charging that reduces this degradation component. He applied a multi-level charging (MLC) profile based on information from the capacity gauge. In particular capacity states lower charge rates were used. However, at other capacity states higher charge rates could be used without resulting in Lithium plating. The idea was not to cause an overall increase in total charge time to full.
Figure 1 shows the results from Barsukov’s paper using multi-level charging compared to the traditional constant-current, constant-voltage charging (CCCV). In the first 96 cycles, there is a marked decrease in capacity degradation when using MLC. The MLC approach uses 5 different levels of current during charging. Further tests are required to determine if the benefit is primarily in the early cycles or just in the first 25 or so cycles. These types of kinetic studies help investigate how capacity information may be used beyond just letting the system know the remaining energy.
Figure 1: Multi-level charging versus CCCV
Although you may not be able to recover the degradation once it occurs, it seems that you can keep some of the components from happening. In doing so, the life of the portable equipment with embedded batteries can be extended. In the end, batteries do wear out. You replace them when they are removable, or replace them along with the entire portable device when they are not. Unfortunately, it is not just as simple as reworking a hinge or a little plastic. Even sitting Big Bird on top of the portable device does not help battery performance.
Let me know your experience with your embedded battery systems, particularly the ones where the battery is not replaceable by you.
For more information about this and other power topics, visit TI’s Power House blog.Also see:
Buck regulator platform boosts battery life and reduces standby power
SolMate by GreenSimian Signals New Era in Smartphone Battery Life
What's your battery shelf-life experience?
Maximizing battery life on embedded platforms - Part 4. Turning off peripherals and subsystems
Measurements optimize battery run time
External Cell Balancing in bq3060
VARTA Microbattery Introduces CoinPower Family of Rechargeable Li-Ion Cells Featuring High Energy Density and Extended Battery Life
- Y. Barsukov, “Minimizing the Degradation of Li/Ion Batteries with Globally Optimized Charge Control from the Fuel Gauge,” 30th International Battery Seminar & Exhibit, March 11-14, 2013.