LiPo Battery info, characteristics, advantages, and disadvantages
Characteristics
Lithium is the lightest of metals and it floats on water. It also has the greatest electrochemical potential which makes it one of the most reactive of metals. These properties give Lithium the potential to achieve very high energy and power densities in high power battery applications; therefore, they are GREAT for r/c cars.
Lithium batteries have now taken their place as the rechargeable battery of choice for portable consumer electronics equipment. Though they were expensive when introduced, volume production has brought the prices down. In the near future, we will see the Lithium batteries overtake the r/c market.
Some Advantages
In many ways Lithium is almost the perfect cell chemistry. They have many attractive performance advantages which make them ideal for high power applications. Just what we need for r/c cars...
High cell voltage of 3.6 Volts means fewer cells and associated connections.
Flat discharge characteristics
Very high energy density.
Very high power density.
Low weight
Can be optimized for capacity or rate.
Individual cells up to 1000Ahr capacity available (not milliamp hour).
Can be discharged at rates up to 40C. The high discharge rate means that we can get by with lower capacity batteries that will deliver high energy rates.
Fast charge is possible.
Can be deep cycled.
The cell maintains a constant voltage for over 80% of its discharge curve. Thus, it delivers full power down to 80% DOD. This means that the battery will deliver a consistent punch (and voltage) before becoming effectively discharged.
Very low self discharge rate.
Very high coulombic efficiency (Capacity discharged over Capacity charged) of almost 100%. Thus very little power is lost during the charge/discharge cycles. NiMh batteries fall in the 80-90% range.
No memory effect.
No reconditioning needed.
Will tolerates micro cycles without affecting performance. This means that the battery does not need to be fully discharged before charging again. If we run 5 minute races, you can recharge without cycling the cell.
Long cycle life. Cycle life can be extended significantly by using protective circuits to limit the permissible DOD of the battery. This mitigates against the high initial costs of the battery.
Does not need reconditioning as do nickel based batteries (NiCd, NiMh).
Some Shortcomings
Internal impedance higher than equivalent NiCd or NiMh (contrary of what has been posted)
Stability of the chemicals has been a concern in the past. Because Lithium is more chemically reactive special safety precautions are needed to prevent physical or electrical abuse and to maintain the cell within its design operating limits. Lithium polymer cells with their solid electrolyte overcome some of these problems.
We will see stricter regulations on shipping methods than for other cell chemistries.
Degrades at high temperatures.
Capacity loss or thermal runaway when overcharged.
Degradation when discharged below 2 Volts/cell.
Venting and possible thermal runaway when crushed.
Need for protective circuitry.
Although Lithium cell technology has been used in low power applications for some time now, there is still not a lot of field data available about long term performance in high power applications (like an r/c car). Reliability predictions based on accelerated life testing however shows that the cycle life matches or exceeds that of the most common technologies currently in use.
These drawbacks are far out weighed by the advantages of Lithium cells and are now being used in an ever widening range of applications.
Charging
Should be charged regularly.
Battery lasts longer with partial charges rather than full charges.
Can not tolerate overcharging and hence should not be trickle charged.
Constant voltage – Constant current
Chargers have to operate in one of these modes or use both of them.
Hope this will help.
Isaac