|Some Do's, Don'ts & The Technology of LiPo's|
Much confusion exists about Lithium batteries in model flying circles, and this article is an attempt to explain some of the do’s and don’ts and what some of the terminology means, to make it simpler for those new to flying, new to electric flying, or new to LiPo batteries. At the same time, it includes information that may be of benefit to the experienced LiPo user.
One of the things that confused me most to start with was terms like 3S1P. This refers to the pack configuration, and simply means the number of cells in Series and in Parallel. Connecting cells in Series (+ to -, like in a torch) increases the voltage, so with a single LiPo cell having a nominal (It’s voltage at half charged) voltage of 3.7v, two in series is 7.4v and 3 is 11.1v. (A single NiCad/NiMh cell is 1.2v, so a 3cell LiPo is the equivalent of a 9 or 10 cell NiCad). Connecting in series does not increase the amperage of the battery pack, but connecting them in parallel (+ to +, - to -) does. So if you connect 3 1000mah cells in series (3S1P) you will have an 11.1v 1000mah pack. If you add another similar pack in parallel (3S2P) you will have a 6-cell pack, still with 11.1v, but now with 2000mah.
The effect of this on a model plane is that it will fly for twice the time. (It will of course weigh a little more.)
Another confusing aspect was the C rating on the batteries. All good LiPo’s have a phrase like “12C continuous, 20C burst” on the label. This refers to the rate at which it is safe to pull power from the battery, something that we did not have to worry too much about with NiCad’s. They would just get hot! But Lipo’s can explode in a chemical fire that cannot be extinguished! They can also be damaged to the point that they are useless. But it is fairly simple to work out. Multiply the C figure by the amps in your pack.( 1000ma =1amp) If our two battery packs above are marked “12C continuous, 20C burst”, this translates to (12 x 1000mah [1 amp] ) = 12 amps continuous or (20 x 1000mah ) 20 amps for a few seconds. Our second pack, the 3S2P, you have doubled the mah capacity of the pack, so you can now pull 24 amps continuously and briefly 40amps.
A LiPo battery will not recover if it goes below 2.5 volts per cell in series. Hook the battery and meter to a motor sufficiently powerful to over-use it. Then gradually increase the throttle until the meter shows approximately 3.2v per cell (i.e. 6.4v or 9.6v for 2 and 3 cell packs respectively) Look at the amperage at this point, and this is the maximum you can pull with this battery continuously. You can probably go 40% higher briefly, but only for 5 seconds or so. Since there is an inevitable loss of power over time, with both motor and battery wearing, 0.7v is a reasonable safety margin.
When considering whether to use LiPo batteries in any given model, or which battery will suit, it is first necessary to determine what amperage your motor and prop will pull. The only way to do this for yourself is by means of the meter used above. Most modern brushless motors will have maximum amperage it is safe to use them up to, and many manufacturers publish the current draw of their motors with props of a various sizes, either on their website or in the instructions with the motor. This is a satisfactory rough guide, but as individual components vary, and a prop by one manufacturer may give a different reading than one by another maker, it is always wise to check! And remember that a freshly charged battery will push a lot more current than one that has had a few seconds run. Just when you need full power to take off with a newly charged battery is when you are most likely to over-stress your batteries.
LiPo’s should only be used with ESC’s that have a means of cutting the power to the motor when the battery reaches a predetermined voltage. Some ESC’s are programmed via your transmitter and some have a cable which you can connect to a computer. You have to make sure you have set this up correctly for the number of cells you are using. This is very important. The other fact to bear in mind is that most of the ESC’s programmed via your transmitter will only be set up on a cell count and the manufacturers will probably have set the voltage of this to 2.8v per cell. This will prevent the cell from being destroyed but probably won’t help the long term life of the cell. When you are flying you will notice a drop in performance and this is the time to land. This will keep your cells in good condition. With the P/C programmed ones you can usually set the cut off voltage yourself with a soft ramp down which will give you time to land on reduced power (the current is reduced so the voltage goes up).You must not over discharge your LiPo. Anything under about 2.5 volts per cell is critical and the LiPo is probably destroyed and could cause a fault on charging.
Charging LiPo batteries is another area of confusion. Because they need special consideration when being charged, it is ABSOLUTELY ESSENTIAL to use a lithium compatible charger, and to TAKE CARE when setting it. Although faster charging batteries are being developed, currently it is necessary to charge at 1 C. So for our first pack, set the charger for a 1amp charge. Because the charger uses this as a maximum figure, charging a LiPo normally takes 11/2 hours or more. Another difference with LiPo’s is that balancing the cells is advisable. Often, all the cells in a battery will have a minute difference in voltage, and many manufacturers fit a small plug to attach a balancing unit.
This reduces the voltage to the same level in all the cells. But unfortunately as yet there is no standard type of balancing socket, so all the different manufacturers sell their own balancers. You can use some of the new multi cell balancers which will do two to four cells or perhaps two to eight cells and make up leads to fit your packs. Most manufactures sell leads for their cell, which you can connect to your balancer plug. Make sure the balancer is suitable for the cells you are using. DO NOT DO THIS UNLESS YOU ARE SURE OF WHAT YOU ARE DOING-GET SOMEONE WHO DOES OR ASK THE MANUFACTURE IF THEY DO THE LEAD.
If you have connected two packs in parallel to increase their capacity make sure they are in exactly the same state of charge and measure their voltage (with the new meter you have just bought) before connecting them. This is most important if you are going to charge them in this configuration. The safest thing to do is to separate them and charge them as two separate batteries. This will then overcome the problem of the two packs not being balanced to each other exactly.
It is very important that the connections to the cells cannot short out under any circumstances so only use a female deans type connector or gold connectors with a female on the + side and a male on the _ side and shorten one lead so the ends cannot come into contact . Use good quality heat shrink to protect them. You can also slide a piece of tube over the male plug when not being used.
Storage of LiPo batteries is relatively easy. Unlike Nickel batteries, they lose a very small amount of power and so can be charged and stored ready for use. But research has proved that they last best if stored in a cool place at a 40% charge. Since very few of us have the means to test the charge percentage, 3.5v per cell would be close, measured on a voltmeter