Lithium Power Solutions

by Greg Covey

Introduction:

As we push the envelope of electric flight technology further, we learn from our mishaps how to improve things. In the past few years, we have learned that Lithium battery technology can be dangerous if not respected properly. By stressing our Lithium packs in applications that draw excessive current, create high temperatures, over-discharge, and over charge the cells, we have learned that this increases the damage to the pack, causes a risk of fire, and, decreases the lifespan of our usage from that pack which can be very costly.

Today's Lithium Power Solutions help to increase safety and longevity of Lithium battery packs. By automating charge current, fixing cell counts, and, monitoring each cell during both charge and discharge cycles, the Lithium Power Solutions available in today's market provide a safer application that is much more cost effective for R/C electric power.

In the Past - ESC LV Cutoff:

Until recently, Lithium packs were treated in a similar fashion to NiCd and NiMH packs during both charge and discharge cycles. They were treated at the pack voltage level and not the individual cell level. The Electronic Speed Controller (ESC) would monitor the pack voltage and shut the motor down when the voltage was reduced to a certain level. Over time, the ESCs became smarter and needed to see the low voltage level for a certain period of time before cutting off the motor, and, they also became Lithium-aware so that different cutoff voltages could be used to better suit each battery technology or flight performance.

The big shortcoming of this approach was for Lithium packs that had a weak cell or became unbalanced. By the time the low voltage cut-off was detected for the pack voltage, one or more cells may have been discharged too far.

              

        

Depending upon the cell manufacturing technology and charger design, a 3C charge rate is achievable for charging a pack to 90% full capacity in only 20 minutes!

When Lithium packs age, through handling or insertion stress into the plane, crashes, etc., the outer cells lose their capacity early. This can happen from small dents in the surface through rough handling, warping of the cells, or by tossing the packs back in your flight box. Further, the inner cells can be stressed by higher temperatures if the packs are pushed even to rated discharge limits for too long. The result is that each cell starts to have a slightly different capacity in the pack. By monitoring the cells during both charging and discharging, you can safely use the pack at the capacity of the weakest cell without further damage.

 

Other Criteria for Longevity:

The two most common criteria left for the user to maintain the health of their Lithium pack are discharge current levels and cell temperature. When treated properly, a Lithium pack can achieve 500 cycles before losing only 20% capacity. Lithium packs that have been stressed by over current draw, which creates high temperatures in the cells, have their lifespan reduced to 50 cycles or less.

How do we know the true cell limitations?

Cell Rating Spec.

As Lithium cells become more of a commodity, the Lithium Power Solutions will differentiate the pack performance and longevity. We'll see the same cells being sold by multiple vendors under different names (e.g. ThunderPower, Cellpro Slimline). The cell ratings will vary and a user must select the most trusted source of information.

The Cell Rating Spec. is the primary guideline for properly using LiPo cells and packs. The discharge curves below are based on a test of a statistically valid sample of production cells. All tests are performed at room temperature with no cooling to the cells so that all ratings are conservative.

There are four curves in the set that define cell rating:

bulletC-Rate 1 Capacity rating: Cell capacity is set by international standard at a discharge rate of 0.5 times the specification capacity of the cell. For example, 0.5C for a 3200 mAh cell is 1.6 amps. Capacity for any cell of any chemistry will be reduced more or less as discharge rate is increased. The rate for this cell is reduced as seen at 3V cut-off such that capacity is 2950 mAh at a discharge rate of 64 amps.
bulletC-Rate 2: Maximum continuous rate. The discharge current that the cell can deliver continuously without failing. Recommended only for applications where it is absolutely demanded. Cycle life is not more than 50 cycles at this rate; about the same as for Ni Cd or Ni MH at extreme high rates. This is the "advertised" C-Rate.
bulletC-Rate 3: Maximum Life rate, continuous. Maximum continuous discharge rate for cell temperature to stay below 140 deg F at an ambient temperature of 68 Deg F. This rate will produce no degradation in life cycle as measured at 0.5C in statistical tests.
bulletC-Rate 4: Maximum Life rate, cyclic. Maximum average discharge rate that maintains cell temperature at or below 140 Deg F. This is the rate the cell will be operated at most times. As a rule of thumb, this may be estimated at 70% of C-Rate 2. The maximum "burst rate" may be much higher than C-Rate 2 for very brief periods (less than a second) so long as there are periods of discharge rate less than average that allow the average to be held.

HIGH CELL TEMPERATURE BRINGS SHORT CYCLE LIFE.
USE TEMPERATURE SENSORS TO KNOW
: TMP-86/140-R or TMP-140-N

 

Summary:

Monitoring Lithium packs at the cell level during both charge and discharge cycles are needed to protect the weakest cell in the pack because they age independently due to handling and temperature stress. Cell level monitoring protects the longevity of your Lithium pack and your investment. Understanding true cell rating specifications from a trusted source empowers you to be aware of marketing hype.

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