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.

Today - Cell Monitoring / Increased Safety: Today, we have the first Lithium systems available that can monitor a pack at the cell level on both charge and discharge cycles. Systems like Cellpro and BalancePro HD for R/C airplanes and Scorpion for R/C cars are protecting each cell in the Lithium pack which helps to protect your investment and dramatically increases the safety of charging.

By monitoring each cell in the pack, we can continue to operate safely at the performance level of the weakest cell. The stronger cells are protected from unbalanced operation during a charge cycle by balancing as we charge the pack. This prevents a cell from overcharging. The introduction of specialized node connectors and operation connectors eliminates the guesswork for the number of cells in a pack. This is a hardwired protection that cannot be fooled like the auto-detect chargers. In the past, problems have occurred due either to user jumper settings or automated guesswork by the charger that could only see the pack voltage. By using a balancing charger, there is no need to remember or take time to rebalance the pack every 5 or 10 flights. The end result is much greater safety and longer pack life, thus lower life cycle cost.

High Performance Lithium Solution - BalancePro HD:

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! The first product to achieve this high level of performance without any cycle life degradation is the BalancePro HD system from FMA Direct. One key element of the 3C charge rate is the cell balancing BalancePro HD 6s charger. The charger delivers a fast, safe, 20-minute charge and balances every cell for maximum pack performance. The operation is automatic and any setting of the current knob will not create an unsafe charge rate. Another key element of the 3C charge rate is in the quality Kokam SHD cells that provide the highest current delivery and lowest internal temperature during discharge on the market.

Discharge Protection Modules:

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.

The new Cellpro 4s Discharge Protection Module (DPM) plugs in-line with any ESC control cable going to the receiver throttle channel and also plugs into to the Cellpro battery pack node connector. It monitors each cell during flight and cuts off the motor when the weakest cell is depleted. Additionally, the DPM provides a 30 or 60 second (user selectable) early warning by pulsing the motor to tell the pilot that it is time to land. No more dead stick landings! No more damaged packs by ESCs guessing the cut-off voltage.

 

Compatibility:

	BalancePro Adapters: The BalancePro HD-to-Cellpro adapter (shown on the left) allows any Cellpro brand battery pack to plug into the BalancePro HD 6s charger. Now with a simple adapter you can keep your Cellpro batteries in perfect balance with every charge using the BalancePro HD 6s charger! Watch for other BalancePro Adapters coming soon!
ThunderPower and PolyQuest Adapters: Adapters for charging ThunderPower and PolyQuest packs on either the Cellpro 4s Balancing Charger or BalancePro HD 6s Balancing Charger will be available soon. The Cellpro Node Connector pigtails can also be purchased separately to convert older packs for balanced charging.

 

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. Experimental data shows that Li Po packs decline very rapidly when the 80% capacity level is reached as salts form in the cells and the cell then deteriorates as the crystals grow even when not in use.

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:

• C-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.
• C-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.
• C-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.
• C-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:

The long-used technique of monitoring just the pack voltage did not provide proper safety and protection against hazards when using Lithium technology. Monitoring Lithium packs at the cell level, during both charge and discharge cycles, is 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. Selecting a vendor that provides market-wide compatibility is often a better choice for value and flexibility. Added support like free Viewer Software demonstrates a true commitment to the customer.

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