Backwoods R/C's Since 2001

Charging RC LiPo Batteries is a topic in itself. LiPo, and Li-Ion batteries obviously have some very different characteristics from conventional RC rechargeable battery types. Therefore, charging them correctly with a charger specifically designed for LiPo batteries is critical to both the life span of the RC LiPo battery pack, and your safety.

Maximum Charge Voltage and Current

A 3.7 volt RC LiPo battery cell is 100% charged when it reaches 4.2 volts. Charging it past that will destroy the battery cell and possibly cause it to catch fire. This is important to understand once I start talking about Balancing RC LiPo batteries, so keep that in the back of your head for right now.

It is critical that you use a charger specified for Li-Ion or LiPo and select the correct voltage or cell count when charging your RC LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 11.1V (a 3S pack) by mistake and tried to charge your 2S pack, the pack will be destroyed and most likely catch fire.

Most good RC LiPo battery chargers will use the constant current / constant voltage charging method (cc/cv). All this means is that a constant current is applied to the battery during the first part of the charge cycle. As the battery voltage closes in on the 100% charge voltage, the charger will automatically start reducing the charge current and then apply a constant voltage. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with chargers constant voltage setting (4.2 volts per cell) at this time, the charge cycle is completed. Going past that, even to 4.21 volts will shorten battery life.

RC LiPo Battery Charging Current

Selecting the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here use to be "never charge a LiPo or Li-Ion pack greater than 1 times its capacity (1C)."

For example a 2000 mAh pack, would be charged at a maximum charge current of 2000 mA or 2.0 amps. Never higher or the life of the pack would be greatly reduced. If you choose a charge rate significantly higher than the 1C value, the battery will heat up and could swell, vent, or catch fire.

Times are a changing...

Most LiPo experts now feel however you can safely charge at a 2C or even 3C rate on quality packs that have a discharge rating of at least 20C or more safely, with little effect on the overall life expectancy of the pack as long as you have a good charger with a good balancing system. There are more and more LiPo packs showing up stating 2C and 3C charge rates, with even a couple manufactures indicating 5C rates. The day of the 10 minute charge is not far off (assuming you have a high power charger and power source capable of delivering that many watts and amps).

Once again, the three main things that shorten LiPo battery life are over-discharging, heat, and inadequate balancing.

Balancing

Finally onto RC LiPo battery balancing – what is balancing and why is it important?

Remember me telling you to keep the 100% charged voltage value of 4.2 volts per cell in the back of your head? Well, here is where that number comes into play. For a single cell (3.7 volt LiPo battery) you don’t have to worry about balancing since the battery charger will automatically stop charging when the 100% charge voltage of 4.2 volts is reached.

Balancing is required however on any RC LiPo battery pack that has more than one cell since the charger can’t identify from different cells and know if one might be overcharged even though the total voltage of the pack indicates otherwise. For example let’s look at a 3 cell LiPo battery pack (three LiPo cells hooked in series or 3S).

This would be an 11.1 volt battery pack (3.7 volts per cell x 3 = 11.1 volts). The 100% charge voltage of this LiPo pack = 12.6 volts (4.2 volts x 3 = 12.6 volts). Our trusty charger set up for a 11.1 volt RC LiPo battery pack will then stop charging at 12.6 volts – simple right.

Well what would happen if one of those three cells is charging a bit faster than the other two? There could be two cells at only 4.1 volts and the one that is charging at bit faster could be getting overcharged up to 4.4 volts before the charger stops charging at 12.6 volts. That would certainly cause damage to that one cell, perhaps even a fire.

This is an extreme example and that kind of voltage difference between cells is unlikely with a healthy pack, but even a 0.1 (100 mV) voltage difference between cells can cause issues and damage over time.

On the other end of the spectrum is if there is one cell in the pack that is not reaching full charge when the pack is charged and then gets discharged below 3.0 volts even though the 3 cell battery pack is indicating a voltage of 9 volts or higher.

Balancing ensures all cells are always within about 0.01-0.03 volts per cell so over charging or discharging of one or more cells won’t ruin your battery pack, or worse become a safety issue from overcharging a cell.

You don’t have to balance your RC LiPo battery pack each time you charge it. Most will agree every 10th to 20th time is fine with a healthy battery pack. The problem is knowing if your pack is healthy, cells in older packs may become unstable? As far as I am concerned, if you have a good balancer or balancing charger, use it at every charge, or at least at every 2nd charge. That might be overkill, but if it prevents a damaged battery or fire just once, well, you decide.

Balancing Taps & Charging

Ok, so now you know why a RC LiPo battery has to be balanced, the question now is how do you do it?

Every muti celled RC LiPo battery will have what is called a balance tap or balance plug. This plug allows individual charging or discharging of each cell in the battery pack. Here are the four main ways to balance a LiPo pack.

Lipo’s can be balanced while charging the pack through the balance plug with a balancing charger. This method uses the charger to individually charge each cell and ensure the voltages are the same in each cell as they charge. The above picture shows a dedicated 3 cell charger charging a 3 cell RC LiPo battery through the balancing plug/tap.The limitation here is the maximum charge rate. Since the gauge of balance plug wiring is small, this method only works on smaller LiPo's or charge rates not much higher then 2.5 amps maximum. A good clue if you are pushing too many amps through the balance leads would be a warm/hot balance plug/wiring.

LiPo’s can be balanced with a stand alone balancer such as a Blinky Balancer while the pack is being charged through the main power plug. Shown here is a computerized charger charging a 3 cell LiPo pack through the main power plug with the Blinky balancer hooked up to the balance plug.The Blinky will monitor the voltage of each cell in the pack and apply a small load to discharge any cell that is indicating a charge voltage higher than the other cells in the pack keeping all cells within about 0.02 volts (20 mV) of each other.

A RC LiPo pack can also be balanced with a stand alone balancer after charging the pack through the main power plug. Again in this picture, the Blinky balancer is hooked up to the balancing plug, but this time after the pack was charged.Obviously this method of balancing is theoretically not as safe for the LiPo pack since one or more cells could be overcharged during charging, but it will balance all the cells and keeps them in check for the discharge cycle and subsequent charge cycles.

Finally the very best way to balance and charge a LiPo battery is by using a computerized charger with built-in balance circuitry.With this set-up, the battery is charged through the main power plug and the balance plug/tap is plunged into what is called a balance board which is in-turn plugged into the computerized charger in most cases; however, some chargers will have the different balance ports built into the charger eliminating the need for a separate balance board.

Good computerized chargers (like that little TP-610C pictured above) with built-in balance circuitry, will confirm correct cell count, alter the charge & balance rates, and when balancing actually occurs in the charge cycle to ensure a "stress free" and safe charge/balance cycle that extends the useful life of the LiPo pack.

This is by far the safest way to charge higher capacity multi celled LiPo's and opens up a whole new world to more advanced charging methods such as multiple pack parallel charging (the way I charge my LiPo's most of the time now).

Balancing Plugs/Taps

Balancing plugs/taps currently come in four different configurations (for the most part) and it is important to know which one your balancing charger, stand alone balancer, or balance board supports so you choose the correct plug type when purchasing your RC LiPo battery.

JST-XH Plug

This is the most common balancing plug type. Used On: Align, E-Flite, Common Sense RC (V2), Great Planes, Dualsky, Esky, Electrifly, Losi, Rhino, Team Great Hobbies, Trinity, Turnigy, Venom, Zippy - just to name a few.

Thunder Power Plugs

Used On: Thunder Power, FlightPower, Apex, EVO, MPX, Outrage, and a few other battery brands.

Polyquest Plugs

Used On: Polyquest, E-tec, True RC, Extreme Power, Impulse, Enermax, Hyperion, Poly RC, Xcite, Fliton, and a few others.

JST-EH Plugs

These are probably the least common type of balancing plug, but you will find them on a few big name battery brands such as: Kokam, Graupner, Core, and older Vampower battery packs.

You can get converters/adapters to use with different balancing plug configurations, but it is much easier and less costly if you just make sure you get the correct plug/tap that works with your charger when you purchase your LiPo battery.

Main Power Plugs

Once again there are several out there depending on your power handling requirements and own personal preference. Getting one type and sticking with it is the easiest way to go since all your connectors will be the same as will your charging plug/s.