Evolving from Lithium-ion batteries, Lithium polymer (LiPo) batteries offer a distinct advantage over their predecessor in terms of cost, utility, durability and flexibility. A LiPo battery has a very high power-to-size ratio, can be shaped to fit in an array of devices. Another advantage of lithium polymer cells over nickel-cadmium (NiCad) and nickel metal hydride (NiMH) cells is that the rate of self-discharge is much lower, so they hold a charge longer while in storage. These characteristics are making LiPo very popular in a number of applications, including radio-controlled models.
Individual LiPo cells are rated at 3.7V, and so multiple cells are usually combined in series to create either 7.4V or 11.1V “packs”, or connected to a “step-up regulator” for model train use. They may also be wired in “series-parallel” if both voltage and current need to be increased. When LiPo cells are combined to create a larger pack, it is crucial that all the cells be identical. For more information on how LiPo packs are wired, click here. To see how I make my own 2-cell LiPo packs, click here.
It is also very important that cells be charged properly to avoid catastrophic failure. Packs are wired with special balancing connectors, and charged with special chargers, which monitor the charging of each cell individually — this process is known as “balance charging.” Balance charging prevents one or more cells in a pack from being over charged, as in this video:
The pack shown is a 2-cell (2S), 2200mAh pack, and for the purpose of the demonstration, it has been connected without the balance connection to the charger. It is also being purposely over-charged. Obviously, this is a much larger pack than we’ll be using in an On30 model, but it would be an appropriate LiPo pack for use in large-scale garden railroads.
For charging my own batteries, I initially selected the iMAX b6AC, as it offers good performance at an affordable price. The b6AC can be powered from your AC mains, or from a 12V adapter. The iMAX b6 is a little less expensive, and does not have the built-in power supply. Instead, it uses a 12V adapter. Otherwise, the two chargers are identical, and offer modes for charging, balancing, discharging/conditioning, and storage charging of a variety of battery types and configurations. When the charger detects that the pack is completely charged, it stops charging and audibly signals that it’s time to disconnect the pack.
I have subsequently selected the Turnigy P403 as my primary charger. It offers simplified, fully automatic operation, and is well matched to the Turnigy 2-cell and 3-cell LiPo battery packs.
Some other important points to bear in mind are the charging and discharging rates. Unless specifically noted by the battery manufacture, LiPo cells should never be charged at a rate higher than their “normal” rating. In other words, a 240mAh battery pack should not be charged at a charging current higher than 240mA. The maximum discharge current is generally higher. That’s the battery’s “C” rating. For example, a 2-cell, 240mAh 30C LiPo battery pack can supply 7.4V at up to 7.2A, but only for a few minutes. That’s a lot of juice!
LiPo cells should never be charged to a voltage of more than 4.2v. Charging to 4.2v will achieve the maximum capacity, and therefore, run time. But the greater capacity comes at the expense of overall battery longevity. Charging to only 4.1v can extend cell longevity by up to 100%. In other words, a cell that would last for 300-500 cycles when charged to 4.2v will last for 600-1,000 cycles. The resultant loss of run time, however, is only about 10%. Charging to only 4v doubles longevity again while dropping the capacity by only another 10%. Charging to greater than 4.2v stresses the cells unduly, and, as shown in the video about, can lead to catastrophic failure.
Batteryuniversity.com has an excellent, highly-detailed article on prolonging the life of lithium-based batteries. While the article is specifically about lithium ion, the information is directly applicable to LiPo cells.
It is also important to inspect LiPo batteries periodically for signs of deterioration. Visible signs that a LiPo are reaching the end of their life include swelling or bubbling. If you notice this, or any other sign of physical damage, you should remove the cell from service immediately. Other signs that a LiPo is wearing out are reduced run times, inability to balance charge properly, and excessive warmth during running or charging.
See the Dead Rail Wiring Diagrams page to see how the wiring works.