LiPo Battery Charging Techniques
The choice of LiPo batteries and battery chargers can be overwhelming. In the past, with older RC cars and NiCd batteries, you could just take the battery, plug it into a wall charger, and leave it. After several hours, or preferably overnight, your battery would be ready for use again. LiPo batteries are a completely different animal.
If you don’t select a higher-end charging station, they will require a lot of your time and attention. And if you don’t treat them right, they can be downright dangerous.
Brand New LiPo Batteries
LiPo batteries are always partially charged with a ‘storage’ voltage. When you purchase a LiPo online and have it shipped, it will be charged at an appropriate storage voltage and will arrive in that state.
Because LiPos always have a partial charge, they are significantly more volatile than other RC battery type, such as alkaline, NiCd, or NiMH. The typical result from a mishandled, over-charged, or abused LiPo battery is smoke and fire. This YouTube video shows what can happen to a mishandled LiPo battery.
If you’re looking for the real thing, check out the bag below. I use this, it works great, and I trust it:
LiPo RC Battery Safe Guard Bag-Grey(11.5×8 Inch)
- New 2 S (7.4 V nominal) ideally between 7.0 V and 7.8 V
- New 3 S (11.1 V nominal) ideally between 10.5 V and 11.7 V
- New 4 S (14.8 V nominal) ideally between 14.0 V and 15.6 V
Digital Charging Stations
Digital charging stations have many built in safety features. Some of these safety features include automatic shutoff, short circuit prevention, wrong cell configuration alarm, overheat shutdown, and capacity shutoff. I personally recommend using an iCharger. The iCharger 206B, pictured to the right, has worked well for all my battery charging needs.
This charger requires an external power supply, however. Many of the LiPo chargers that come with an internal power supply will not have enough wattage to charge larger batteries in parallel.
I recommend a separate charger / power supply combo. The price may be a bit higher in the beginning, but you won’t have to order another setup when you decide to move on to parallel charging and higher capacity batteries.
I have simultaneously (parallel) charged four 5000 mAh LiPos at 1 C, for a total charge current of 20 amps, with the i206B. This unit can handle up to 6 cell LiPo batteries and 20 amps, allowing you to charge batteries used in large electric helicopters or FunJets with no problem.
LiPo Battery Charge Rate
The charging rate of a LiPo battery is expressed in terms of a ‘C’ rate. The ‘C’ stands for capacity, and is the mAh capacity of the battery, but with units of just amps. The mAh unit is current (mA) * time (hours) which is a unit of electric charge, but it’s more commonly thought of as capacity. The recommended charge rate for most LiPo batteries is 1 C, which is 1 times the capacity rating of the battery, with the units changed to amps. For example, a 5000 mAh capacity battery charged at 1 C would make for a charge rate of 5 amps.
When a discharged LiPo is first connected to a charger and the charging cycle begins, the charger maintains a constant current (CC).
After the battery is back up to about 80-90% capacity, the charger lowers the current to maintain a constant voltage (CV).
These two phases of the charge cycle make up what is termed the CC/CV curves. The outcome of this relationship is that there exists a point of diminishing return as far as fast charging is concerned.
The diminishing return situation is reached at about a 5 C charge rate. Any charge rate in excess of 5 C will not reduce the overall charge time of a LiPo battery due to the constant current, constant voltage charging process.
Charging LiPo Batteries in Parallel
TJinTech has a great tutorial on parallel charging LiPo batteries. This first item you’ll need in addition to your charger and power supply, is a parallel charging board. It’s possible to make these yourself, but you can save a lot of time and frustration by purchasing a ready-made unit, such as this XT-60 version from HobbyKing.
Many of the battery manufacturers specifically state to only charge LiPo batteries individually. With the proper equipment and enough knowledge, however, charging LiPo batteries in parallel can be a safe and efficient practice. If you follow TJinTech’s article, you’ll be parallel charging LiPo batteries in no time.
LiPo Battery Balancing
LiPo batteries are made up of several cells, ranging from just two to over twelve.
Fully charged, each cell is 4.2 V. To operate effectively, each of the battery’s cells must be identical in voltage. To achieve this ‘balance’ among the cell voltages, LiPo batteries are configured with a balance plug, which can be attached to the battery charger or parallel charging board.
The balance plug will adjust the current flowing to each cell. This ensures that during the charging process the battery will not only reach it’s design voltage, but each cell will reach 4.2 V.
LiPo batteries can be charged without balancing, but it’s recommended to balance each time.
LiPo Lifetime and Number of Charge Cycles
The lifetime of a LiPo battery is measured in charge cycles. The number of charge cycles that a LiPo can tolerate depends on several factors. Quality of construction, design, discharge rate during use, charge rate, and general wear and tear all affect the lifetime of a LiPo battery. Many batteries will maintain performance after 75-100 charge/discharge cycles. One manufacturer, Hyperion, claims their batteries have a useful life of up to 500 charge cycles.
RC Transmitter Batteries
The battery that comes with an RC transmitter is typically not a LiPo. Many transmitters ship with a NiCd battery pack which allow for maybe a few hours of use. This may sound like a long time, but in between testing, preflight checks, and actual flight, the NiCd transmitter battery will run low fairly quickly.
These batteries also take a long time to recharge, usually over night.
Instead of using the stock NiCd transmitter battery, a better practice is to take advantage of LiPo technology and purchase a LiPo specifically designed for use in a transmitter. These transmitter LiPo batteries have slower self-discharge rates, and require charging much less often than a traditional NiCd.
Although they are frequently out of stock at online hobby stores, the HobbyKing ZIPPY Flightmax 2500 mAh transmitter battery is a good choice for Futaba transmitters.
LiPos are unlike other types of rechargeable batteries, such as NiCd and NiMH, in that they will be permanently damaged if discharged below a minimum voltage. The voltage on a LiPo battery must be maintained above 3 V, otherwise the battery will not accept a future charge. The number of cells per battery doesn’t have an effect on this discharge limit, as the limit is per cell. Typically LiPos should only be discharged to 20% of their capacity to prevent the battery from suddenly dropping voltage and sustaining irreparable damage.
LiPo Battery Charging Safety
The following list provides some basic safety precautions for charging LiPo batteries:
- Don’t charge LiPos unattended
- Don’t puncture a LiPo cell
- Take care not to short circuit the battery by plugging in connections backwards. This is particularly important if you soldered on a connector (XT-60, bullet, etc.) backwards and forgot to label the reversed orientation
- LiPos that have been involved in crashes, dropped, or sustained some kind of physical trauma may be damaged internally, and should be watched closely at the next charging
- Store and charge batteries in a well-ventilated area
- Charge LiPos on a metal surface or inside of a LiPo storage pouch
- Use the balance plugs
- Don’t be aggressive with charge rate. In addition to shortening the battery’s lifespan, this can cause overheating and a possible fire
E-flite’s LiPo Battery Guide has a lot of useful and important information regarding general safety while charging LiPo batteries.