Balancing LiPo battery packs
When talking about balancing LiPo battery packs, we're not talking about finding their Center of Gravity! Instead, the balance refers to the voltage of each cell in respect to one another.
LiPo (aka li-po, lipoly & lithium polymer) battery packs are relatively new to the radio control flying scene, and have helped change the face of the hobby. A good quality LiPo pack in conjunction with a powerful outrunner brushless motor can give an electric rc airplane incredible performance and flight times; indeed the more expensive set-ups and competition models can give a nitro powered plane a good run for its money!
LiPo packs can be single cell or multi cell; a single cell has a nominal voltage of 3.7V and so packs can be bought in increments of 3.7 ie a single cell (1S) 3.7 volt pack, a two cell (2S) 7.4 volt pack, a three cell (3S) 11.1 volt pack - and so on.
Above, a 1S, 2S and 3S LiPo pack
Obviously a 1S LiPo pack can't be balanced, but any pack with two or more cells should be balanced - if not with every charge then certainly on a regular basis. Balancing LiPo battery packs ensures that each cell matches the voltage of the other one, or as closely as possible, to prevent irregular current flow and hence give your rc airplane or helicopter optimum performance.
It's generally accepted that a difference of 30 - 50mV (.03 - .05v) between cells is acceptable, but different manufacturers might state different amounts and the specifications for your particular pack should be read.
How to balance the LiPo battery pack
You'll be relieved to hear that there's no great secret to balancing your LiPo pack!
Your pack should have two sets of leads coming out of it; one goes to the ESC and will more than likely be made up of a red (positive) and black (negative) lead and the other will me made up of thinner leads that all go into a single white connector. This is the balancing connector, and the number of leads going in to it will be determined by how many cells the battery pack has. A 2S pack will have 3 leads, one for each cell plus a negative; a 3S pack will have four leads, one for each cell and a negative - you get the idea.
To balance the pack you need a balancing charger, or a separate LiPo balancer used in conjunction with a charger. The charger, incidentally, needs to be specific for lithium polymer batteries, you cannot use an NiMH or NiCD charger on LiPo packs!
Above, a quality charger with separate balancing board and a
cheap LiPo specific charger (top right)
There are many many different chargers available, some very basic and some very complex. The most basic ones, often sold with RTF aircraft, may only feature a socket for the balancing connector and will charge through that, while others will charge the pack through the main ESC leads and balance the cells via the white connector at the same time.
Follow your charger instruction manual carefully to charge and balance your pack correctly.
Is the pack balanced?
There are a couple of ways to check whether your LiPo pack is balanced.
The first is to buy a small electronic gadget called a LiPo voltage checker, such as the one pictured right. You simply plug it in to the white balance connector of the pack and let it do it's thing - it will cycle through each cell and tell you the voltage of each one.
The second method requires the use of a volt meter and a bit of simple arithmetic!
With the negative lead of the volt meter touching the negative terminal of the white balance connector of the pack, touch the positive lead of the meter on to each of the other terminals of the connector, one at a time. Determine the highest reading (ie the total voltage of the pack) and then move the meter lead to the next terminal, this should give you a lower reading. Calculate the difference between the two readings, this gives you the voltage of the first cell. Now touch the meter lead on the next terminal and subtract this voltage from the second terminal - this will give you the voltage of the second cell. Work your way through each terminal until you arrive at the last one, the reading of which will be the voltage of the last cell in the pack.
By calculating the voltages differences between each cell, you'll be able to see how well the pack is balanced.
Above: step 1, get the battery pack total voltage - here it's 12.79V
Above: step 2, get the next voltage level down and subtract
from the first - here it's 12.79 - 8.53 = 4.26V
Above: step 3, repeat for each cell of the pack - 8.53 - 4.27 = 4.26V
So, for the above example we have three cells nicely balanced at 4.26v, 4.26, and 4.27v. Huh? Personally I think I need a new voltmeter, because those values are actually higher than the proper voltage checker gave, and a LiPo cell should never be over 4.2V! But the principle is the same...
Balancing LiPo battery packs should be done regularly and you should get in to the habit of checking the balance often. A poorly balanced pack will give poor performance, and in the worst case scenario will be damaged beyond repair.
It's also important to understand that lithium polymer battery packs vary greatly in quality between brands, and it stands to reason that a better quality pack will probably balance better and stay balanced longer than a cheaper, lower quality one. Similarly, better chargers will balance better than cheaper ones.
