Lithium Polymer Battery Guide
Lithium Polymer Battery Guide
The following is a brief guide to lithium polymer (Li-Po) batteries. It is by no means an exhaustive guide.
What makes lithium-polymer batteries special?
In short, their density. Lithium Polymer (often abbreviated Li-Po or Li-Poly) batteries pack more capacity (mAh) into less space, which means lighter battery packs. For model aeroplanes, RC or otherwise, less weight generally means more airtime! Not only that, but a lower wing-loading. Always remember the golden rule of lithium polymer batteries – SAFETY.
What is C?
'C' refers to the capacity of your battery and is usually prefixed with a whole number indicating the continuous discharge, burst discharge and maximum charge currents supported by the battery (more on this later). This is an important attribute of a battery to understand, because it is used by manufacturers to indicate the various performance metrics of your battery, such as the charge, discharge and burst discharge limits. Although 'C' is measured in amps, the capacity of batteries within the modelling fraternity are commonly measured in milli-amps (or more specifically, milliamperes). 1 amp is the equivalent of 1000 milliamps. For a 2500mAh battery then C would be equal to 2.5 (2500/1000).
What does continuous discharge mean?
The continuous discharge rating of a battery indicates how much work the battery can do. If we take an average C rating of 20C, this means that the battery can deliver its capacity 20 times faster. Or, if we take the battery from the previous example, we could continuously draw 20 times 'C' from the battery pack, i.e. 20 x (2500/1000) or 50 amps. Unfortunately we could not draw this current for the full hour, only 1/20th of the hour (i.e. 3 minutes), after which the battery capacity would be exhausted.
How about "burst discharge"?
The burst discharge rating of a battery indicates how much work the battery can do for a short period of time. This is usually no more than 30 seconds, but should be checked with the manufacturer of your battery to be certain. The calculation for the maximum current draw, given the capacity of the battery pack and the burst discharge C is the same as for continuous discharge. For example, given a burst discharge rating of 30C, this gives 30 x (2500/1000) or 75 amps. Were we to continuously draw 75 amps from this battery pack, the pack would be exhausted in 1/30th of the hour (i.e. 2 minutes)
How should I store my Li-Po battery?
Lithium-Polymer batteries should be stored in a cool dry place at approximately 50% of their capacity. Storing a LiPo battery near its minimum voltage or near its maximum voltage will severely reduce the lifetime of the pack. Most modern battery balance chargers have a storage programme which will charge or discharge your Li-Po battery to the appropriate storage voltage.
What does 3s2p mean?
First some background: Lithium Polymer battery packs are usually made from a number of Lithium Polymer cells. A single cell has a nominal voltage of 3.7 volts and a maximum voltage of 4.2 volts. To create a complete pack, these cells are joined together either in series or in parallel. Cells wired in series result in a pack which has a voltage equal to the sum of the voltage of each cell. The total maximum voltage quoted for a lipo battery is usually always the sum of the nominal voltage of each cell connected in series. For example a 3s lipo is usually quoted as having a voltage of 11.1V, although the maximum voltage of a 3s lipo is 12.6V (4.2V multiplied by 3) and the absolute minimum voltage is generally considered to be 9V. To maximise the service life of your lithium polymer battery it is recommended not to permit any cell to be discharged below 3.2V. Higher than 3.2V is better still!
Cells wired in parallel result in a pack with a capacity equal to the sum of the capacity of each cell. 3s2p therefore means that there are 3 cells wired in series (3s and thus 11.1V) and that the resulting 3 cell pack is connected to another 3s pack of exactly the same specification, in parallel. The resulting pack contains 6 cells in total. If the capacity of one of the 3s packs is 2500mAh, then the capacity of the 3s2p pack is thus 5000mAh. Whilst being discharged, each of the 3s packs is effectively performing 50% of the work.
How does cold weather affect my LiPo battery?
Like most batteries, the temperature of a Li-Po has an impact on its performance. Consider that batteries rely on a chemical reaction in order to provide a current – a low temperature reduces the speed of this reaction and a reduction in overall pack performance may be observed on a cold day. Allowing a Li-Po pack to freeze is certainly not recommended, as this would almost certainly damage the pack.
Why do I need a balancer (What is balancing)?
Lipo battery packs usually contain more than one cell. Drawing current (discharging) a pack involves the pack being discharged as a complete unit. For various reasons, especially in lower cost battery packs, cells do not always discharge equally, which results in a pack which may be out of balance. An unbalanced pack is a pack where one or more cells does not have the same voltage as the other cells within the same pack. The risk in this situation is that a simple Li-Po battery charger with no balancing capability only sees the overall voltage of the pack. Consider a 3s pack with two cells at 4.0 volts and one cell at 4.2 volts. The charger is aiming for a final voltage equal to 4.2V per cell – which it considers should be 12.6V in total. The total voltage of such a pack would be 4+4+4.2 = 12.2V therefore the charger may continue to charge the pack until it sees 12.6V. Given that one cell is already at 4.2V. This is a potentially dangerous situation which will surely result in one of the cells being over charged. This can result in a fire or an explosion. A balancer or balancing charger ensures that each cell in your pack maintains a consistent voltage. So, a balancing charger or stand-alone balancer is a worthy addition to your workshop, not least to ensure a level of safety but also to keep your batteries in good condition and get the best performance and efficiency from your power system.