Looking After Your Lipo's
By Heather Mardon
Step 1. Choosing It|
The first step on the road to LiPo Nirvana is choosing the right size battery for your model. There is a bewildering array of products and the market also suffers from a degree of manufactures "Hype". A Lipo battery has three main variables, Cell Count, Cell Capacity and "C" rating.
Cell Count is how many cells are wired in series. This determines the packs voltage. Most common are 2 & 3 cell configurations. These are often referred to as 7.4v & 11.1v packs (Based on a nominal cell voltage of 3.7v).
Cell Capacity is how much energy the cell stores and is measured in milliamp hours, abbreviated to mAh.
The "C" rating is the tricky one. Different manufactures seem to have varying views on the definition of this, and so there are some pretty bold claims about on how well some cells perform. In general "C" is a measure of how much current you can draw from a battery before its voltage OR capacity falls below accepted limits. In the case of voltage the widely accepted limit is 3v for Lipo's. The capacity limit is around 80%. The "C" number is multiplied by the packs capacity to give you the current.
Example: A 1200mah battery rated at 10C should be able to deliver 10x1200mah = 12A
"C" ratings are often quoted as "Continuous" and "burst". Continuous means just that i.e. for the above example it means 12A until the battery reaches 3v per cell. Burst is a bit harder to define, but in general is rather short i.e. 5-6 seconds with a reasonable gap inbetween bursts.
The main thing to keep in mind is that these ratings are the MAXIMUM for the cells, it is not recommended that they be run at this level for extended periods or you will suffer degradation in performance. An analogy would be your cars engine may be rated at a maximum rpm of 7000 but it is not recommended that you drive around at that rate all day!
With this is mind lets choose a battery for an example model.
|The model I have chosen is a Pelikan Models Dingo, the motor I will be using is a Kontronik Kora 15/14 Outrunner. From the Motor test database I see it will draw about 31A at 10v on a 14x7 prop. Being a 3D style model it's likely that it will see quiet a few bursts at this current level. The average current is likely to be around the 20-25A region. I would chose a 3300mah 15C 3s pack as it would easily handle the current draw, give very good flight times. You could get away with a 15C 2500mah pack but unless you are scraping for the very last gram of weight you are better investing in a bit of "overhead". The larger pack will also hold its voltage higher during the flight giving a little bit more power than the smaller battery.|
Now you have chosen the right size battery it is also a good idea to select a battery that has "taps". This is an extra plug or set of connectors that allow connecting to the individual cells in the battery pack. This is required for balancing which is covered later in this article.
As in most cases you get what you pay for, beware of cheap internet auction site deals, there are some horror stories of what you may end up with. So unless you are prepared to strip the pack down to confirm what you have actually bought it pays to stick with reputable brands and suppliers.
Step 2. Charging It
Now you have your battery pack, how do you charge it?
Lithium Polymer batteries require a purpose built charger. Your old NiCad/NiMh only charger will not work and there is a real danger of fire if you try and an use it.
You can chose a dedicated lithium only charger or a more universal charger that will also be able to charge your NiCad's or NiMh cells.
Charge Rate: LiPo's should be charged at 1C or less. This means if you have a 3s 3300mah pack you can charge it at 3.3A. If your charger is not capable of this current, then just chose the closet value to it, for example the Apache 2500 has a maximum charge rate of 2.5A so you would select that. The only disadvantage of a lower charge rate is that it will take longer.
My recommendation is to get a charger that is microprocessor controlled with a display to show how many mAh have been put into the battery.
Lipo's are very efficient at charging so what you put back in is very close to what you used.
This way you can tell if you can extend your flights further or should cut them shorter.|
Step 3. Using It
Now we get to go and fly! Well not quite. It is vital that your battery gets sufficient cooling in your model. Excessive heat is a killer for Lipo's, anything over 60 degrees C is going to shorten your batteries life span. Make sure your battery gets even cooling all around otherwise some cells may heat up more than others risking imbalance.
Next up is to ensure your Speed Controller is programmed correctly for LiPo operation. Most controllers differ in there programming so you are going to have to read the manual. On some controllers you must specify the actual cell count used. If this is the case beware when moving the ESC to another model that you reset it accordingly. If your controller auto detects the number of cells ensure that you only ever use fully charged packs otherwise it may get it wrong.
Running a battery right out of juice to the ESC cut off every time you use it will shorten the packs life. It is better on your battery (and safer) to use a timer and land at about 2/3 - ¾ discharged. From a safety perspective this gives you a bit of spare "gas" in the tank should the need arise to go around and delay landing. We have already said that high temperatures are bad for Lipo lifespan, but what about the cold? Cold temperatures will not harm the batteries but they do affect the performance. The voltage that the battery puts out will be lower at cold temperatures. The ideal is around 25 degrees Celsius. So if you have noticed a drop in performance as the winter temperatures set in its no surprise. If you pre-warm your packs say in the car on the way to the field or for a few minutes in front of the fan heater before going to the park you will notice the difference.
Note: users of Futaba 9C radios can use the handy feature of linking the throttle channel to a timer, this means you accurately time your flights and don't have to remember to start the timer.
The final but very important last step when using your battery is NEVER leave it plugged in to your model. Even speed controllers with switches will still draw a small amount of power when turned off. So if left unplugged your battery will completely discharge and will either be stuffed or seriously degraded.
Step 4. Maintaining It
The last stage of maintenance of your Lipo's is to keep them balanced. All the cells in a pack should be kept at identical voltages. If they start to drift apart they are then going to risk either going over 4.2v when charging or under 3v while in use. Either scenario is bad news. These conditions are most likely to occur if the pack has been pushed hard or over discharged.
If your battery is equipped with Taps then you can chose from several balancer units on the market to keep things nice and even. It's best to use the balancer during the charging process.
If you do not abuse your cells then a usage of the balancer once in every 5 cycles should be sufficient to keep things in check although using it more frequently cannot hurt.|
For those that skipped straight to the end here are a set of summary bullet points.