It is hard to believe, but just a few short years ago, Lithium-ion polymer (LiPo) batteries were new to the industry. Since then, not only have LiPo’s come to the forefront, but they are partially responsible for resurrecting electric RC altogether. Li-Po’s, compared to the NiMH technology they replaced, are lighter, deliver more linear current, have higher capacity, last through many more cycles, and are a lot more user-friendly. If you are choosing your first LiPo pack—or are thinking about investing in another—here are some important variables that you will find among packs on the market.
When selecting a Li-Po pack, the most important variable is voltage, which determined by the number of cells in series. The most common LiPo application is a 7.4V (2S) pack, which most closely mimics the voltage used in an old-school 6-cell NiMH pack. If you are trying to achieve insane speeds, however, or run a large-scale electric vehicle, an 11.1V (3S) or greater may be a better fit. Most of the time, you are limited by the ESC capabilities and room on the chassis. Do some research and chose to correct voltage for your vehicle.
Simply put, capacity—expressed in mAH—deterimes the potential runtime of a LiPo. Capacity may be determined by increasing the number of cells in parallel. Assuming the cells are identical, a 3P battery should have a higher capacity than a 2P battery. Higher capacity batteries are generally larger and heavier than lower capacity, so a 8000mAh pack may not necessarily fit in your vehicle. In general, chose the highest capacity battery that will fit comfortably on the chassis.
Since a LiPo pack is made of multiple, smaller LiPo cells, they all must work together in harmony. Although their synchronization is pretty consistent, the cells do not necessarily charge and discharge equally. Some packs have balancing ports which allow for the use of a balancer while charging. The balancer addresses each cell individually and ‘balances’ the charge in each cell. There are various balancers and ports on the market, so make sure to purchase the correct type for the battery, although not all batteries are balancer-compatible.
LiPo cells themselves are flexible and delicate. They are not rigid like sub-c type batteries and require supplementary protection. Some packs offer more protection than others. Most packs on the market use a rigid plastic case to protect the internal cells—where some use a thinner more flexible case. If given the choice, use the most rigid protection possible. Also, some cases are designed for use in certain vehicles. Most cases are rectangular and have corners, but some cases—designed for cars that use stick-type sub-c packs—have more rounded edges that fit better in the chassis.
Many LiPo packs use wires connected to Zero Loss plugs, which then plug into the battery. These are generally a better option than a pack which doesn’t use Zero Loss plugs, because unlike sub-c cells, LiPo’s are NOT rebuildable. So if a wire frays, it needs to be replaced, and if you don’t have replaceable wires the pack is essentially ruined.
It wasn’t long ago that there were only two options for battery connectors: Tamiya-style and Deans Plugs. Tamiya-style connectors are notoriously terrible, and could never handle high-current applications—leaving Deans plugs to pick up the slack. Although Deans Plugs are still among the best on the market, Losi and Traxxas also produce high-end connectors that can handle high-draw LiPo applications. When selecting your LiPo pack, chose whichever plugs that will work best—as long as they aren’t Tamiya plugs.
A pack’s “C-rating” refers to continuous discharge potential. The higher the C-rating, the more Amps are available for discharge, and in turn, acceleration improves. But as tempting as it is, do not assume off the bat that a 30C battery will perform better than a 25C. Unless you are running a high RPM (kV) motor, you will never need anything more than 25C to begin with. And even if you are running a hot setup, as most racers know, more speed does not necessarily mean lower lap times. Only focus on C-rating if you are looking for additional ways to make your already fast car a little faster at high RPM.
Although LiPo technology is relatively new to our industry, it spawned from the need for better cell phone and laptop batteries. The RC car industry has always done a great job in adapting new technologies for the RC realm. Now, there are a number of different Li-Po batteries on the market, but they all share the same lingo. If you are looking to purchase a new LiPo pack, do some basic research, and chose the best pack for job.