Wired For Balancing

Lithium polymer battery packs are superb sources of energy. When managed properly, they can yield hundreds of charge/discharge cycles throughout their lifespan. But when proper management is ignored, pack degradation or failure will rear its ugly head. Cell balancing is a crucial part of this maintenance. However, in order to balance a pack, it must be wired for it. The good news is that these days more and more packs are "prewired" for balancing. But what if you already own one that isn't? As usual, RC Heli magazine comes to the rescue.

A Li-Po Battery Pack Primer
For those of you already using Li-Po battery packs, let's begin with what I hope is a review. For those who are just venturing into electrics, let this serve as an introduction to Li-Po fundamentals.

Li-Po packs are built from individual Li-Po cells wired together in such a way as to increase voltage, capacity, or both. Each individual cell produces a nominal voltage of 3.7 volts and is available in a wide range of capacities. To power a popular heli such as the Align T-REX 450, you may use a pack consisting of three 2200 mAh cells wired in series (referred to as a 3S pack). This configuration results in an 11.1 volt / 2200 mAh pack.

The "C" rating of the pack will specify its maximum discharge rate. The pack described above with a 20C rating lets you know that you can discharge that pack at 44 amps (20 x 2200 milliamps).

The two most common ways of misusing Li-Po battery packs are by exceeding their C rating and/or depleting them beyond their rated capacity. A good rule of thumb is to only deplete a pack to within 80% of its rated capacity. Using the pack above once again as an example, it should only be discharged to 80% of its 2200 mAh rating (or 1760 mAh). This is a much safer practice than depending on the auto cut-off feature of an ESC to let you know when it's time to land.


Balancing
So where does "balancing" figure into the equation? Consider this...

Li-Po cell manufacturers inform us that running an individual cell below 3.0 volts may damage it. That would equate to 9.0 volts for a 3S pack. Without regular balancing, cells become "unbalanced."


An unbalanced pack can lead to a scenario where you might run the pack as an entity down to 9.2 volts. However, two cells could be at 3.2 volts each, while the third cell is down at 2.8 volts. By just measuring the pack, you might believe that you're safe because it measures out at 9.2 volts. In reality, however, one cell haze already ventured into the danger zone.

How they are charged without balancing
Basic Li-Po battery charging is a two-step process. During the beginning of the charge cycle, a constant current charge is utilized while the pack voltage is monitored. When the pack's voltage reaches approximately 4.2 volts per cell, the charger switches to a constant voltage charge. During this phase of the charge cycle, the current going into the pack is monitored and the charge is terminated when the current decreases to just above zero amps.

As you can no doubt see, unbalanced charging can lead you into the same situation as I just described. This can result in the overcharging of individual cells.
What balancing accomplishes
Packs that are wired for balancing facilitate access to each cell in the pack.* The cells are not charged via the high current discharge leads, but rather through a separate set of leads that allow each cell to be monitored and charged individually. The charger can then ensure that each cell is charged to its maximum capacity.

Balance chargers, as well as in-line balancers, can do their thing during the discharge and/or charge cycle. When discharge balancing, higher voltage cells are discharged until all the cells are at the same voltage as the lowest cell in the pack. Discharging continues in this manner until all the cells reach a pre-set cut-off voltage. The opposite occurs during balance charging. However, in reality it isn't quite that simple. During charging, a pulse-width modulation scheme is used to control the current delivered to individual cells. But for the purpose of this discussion, you have the essence of what's going on.

* Balance packs that contain cells wired in parallel will expose those cells wired in parallel with each other as one cell.

How the batteries are wired
The following diagram details the wiring schematic for a standard non-balance 3S pack:


Only the negative terminal of the first cell and the positive terminal of the last cell are brought out of the pack. Both charging and discharging are performed through these leads.


The schematic for the same pack with balancing leads would look like this:

As you can see, by virtue of the balance leads, you are now capable of accessing each cell individually. Looking at the balance leads, the black and white leads are the negative and positive terminals, respectively, of the first cell. The white and blue leads are the negative and positive terminals of the second cell. Finally, the blue and orange (or red) leads are the terminals of the last cell.

Exception to the rule
Be aware that there are battery packs on the market that should not be modified for balancing without extensive modification. These packs are recognized by the presence of a separate two-lead charge plug. Packs of this style contain internal circuitry designed to safeguard the cells during the charge process.


Wiring a battery for balance charging/discharging
IMPORTANT: Before attempting to modify any Li-Po battery pack, one must be aware of two things. First and most important: Be very careful that you do not short any of the wires or cell tabs. Shorting will generate a very high current, which in turn produces extremely high heat. Cell damage and fire may result. Second: Performing a modification on a pack will almost certainly void its warranty.


To begin the modification, you'll have to remove the pack's protective heat shrink covering. Use a sharp razor knife and be very careful not to cut into the cells or their associated wiring. Save the label that contains the battery specifications in order to transfer them on to the pack when it's re-covered.


Once the covering is removed, the individual cells will be exposed. The inter-connections between the cells and leads are on one end of the pack. You may have to remove an insulator to expose the solder pads.

The balance leads must be soldered to the individual cell tabs. There are different style connectors available and the one you use is a matter of personal choice. For this example, I'll use a Thunder Power balance connector.


Looking at the following photo, I've labeled where we'll attach the balance leads.

Solder the balance leads onto their respective pads. Again, be very careful not to short anything. When complete, it should look like this:

Once the leads are attached, connect the pack to your balancer and test. At this point all that's left to do is enclose the pack with some fresh heat shrink covering. Make sure you reinstall any insulating material that you may have removed.

You have now successfully added a balance connector to the pack.
Conclusion
Adding a balance connector to a Li-Po battery pack is a relatively simple modification that will undoubtedly extend its service life and improve its performance. Your battery will be much happier for it.

Safe flying!