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-   -   What Are Lithium Polymer (LiPo) Batteries, and Are They Better Than Nimh Batteries? (https://www.rc-monster.com/forum/showthread.php?t=2250)

Batfish 02.05.2006 02:56 PM

What Are Lithium Polymer (LiPo) Batteries, and Are They Better Than Nimh Batteries?
 
Lithium Polymer (LiPo) is a different chemical composition than standard Nickel-Metal Hydride (Nimh) sub-c batteries typically used in radio control vehicles.

LiPo has a higher energy density than Nimh and can therefore provide more power in less overall space. An example would be comparable battery packs: a 6-cell Nimh battery pack (3300mah) will provide 7.2V nominal at a size of approximately 140mm L, 51mm W, 19mm H, and a weight of approximately 400g.
A 2-cell LiPo battery pack (3200mah) will provide 7.4v nominal at a size of approximately 130mm L, 42mm W, 16mm H, and a weight of approximately 200g.

LiPo battery cells are packages in a thick, foil-like wrapping and typically held together with thick heat-shrink or a plastic case.

LiPo batteries are sensitive to over-charging and over-discharging. If over-charged, a LiPo battery may swell and catch fire. If over-discharged, the LiPo battery may become damaged and become incapable of being charged to full capacity again.

LiPo batteries MUST be charged on a LiPo-compatible charger at the manufacturer's recommended charge rate (typically 1C). A 1C charge means to charge the battery at 1x it's capacity. For example, a 3200mah battery would need a 3.2 Amp charge to charge at 1C; 3200 milli-amps = 3.2 amps. Cutoff for charging should be set at 4.2 volts per cell (if the charger does not have an automatic setting).

LiPo batteries have a much lower self-discharge rate than Nimh batteries have. This means that a fully-charged LiPo battery will keep it's full charge for hours, days, maybe even weeks, without having to be re-charged. Comparitively, a Nimh battery can lose up to 25% of it's full charge after being stored for just one week without usage.

LiPo batteries should never be discharged below 2.7 volts per cell. Over-discharging may damage the batteries and permanently decrease their capacity and performance. Several speed controllers have a voltage cut-off setting for LiPo batteries, and several manufacturers sell LiPo Low-Voltage Cut-Off devices.

Please note that this information is intended to provide you with basic information in regards to Lithium Polymer batteries and is NOT a replacement for manufacturers instructions and guidelines for usage of their LiPo batteries/chargers.

PLEASE FOLLOW THE MANUFACTURERS INSTRUCTIONS FOR CARE AND MAINTENANCE OF LITHIUM POLYMER BATTERIES.

More replies to will follow in this thread to provide more LiPo-specific information.

Batfish 02.05.2006 03:07 PM

What Do the "s" and "p" Mean When the Battery Reads "2s2p", "3s1p", etc...?
 
First, for 2s/3s/4s - the "s" means "series". This is the amount of cells the pack has that add to the total voltage. Each cell is 3.7 volts nominal, so 2s=7.4v, 3s=11.1v, 4s=14.8v, etc... Nimh packs are cells in series, so you could draw a comparison by calling a 6-cell nimh pack a 6s pack of nimh.

Next, 2s2p, the "p" means "parallel". Unlike traditional nimh packs, lipos are commonly run in parallel in addition to series. This "p" number is the amount of cells in parallel the pack has, which adds to the total capacity. Each "p" adds the capacity equal to one individual cell capacity in the pack. This concept can get a little tough to understand for the first time, so I'll try to explain more.
For example, let's take my FMA BalancePro 2s2p 7.4v 6400mah pack. The pack is made with cells that are 3.7v (same for all lipo) and 3200mah. 2s means TWO cells in SERIES, so 3.7v*2=7.4 volts. 2p means TWO MORE cells in PARALLEL with the first two, adding capacity equal to one cell's capacity in the pack, so 3200mah+3200mah=6400mah.
Therefore, 2s=7.4volts and 2p=6400mah.

Packs can be added together to increase voltage, similar to the way we add two nimh 6-cell packs together in an E-Maxx to make 14.4 volts. Two 2s1p LiPo packs connected in series would provide 14.8 volts. When attaching packs in series OR parallel, it is important to make sure they begin at the same voltage so neither pack is discharged more than the other. Remember; over-discharging can permanently damage the batteries' capacity and performance.

Please note that this information is intended to provide you with basic information in regards to Lithium Polymer batteries and is NOT a replacement for manufacturers instructions and guidelines for usage of their LiPo batteries/chargers.

PLEASE FOLLOW THE MANUFACTURERS INSTRUCTIONS FOR CARE AND MAINTENANCE OF LITHIUM POLYMER BATTERIES.


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