Setting your flight timer, the 80% rule

This rule protects both the lipo and the heli. Lipo voltage under load is fairly flat during hard discharges until you reach about 10% remaining capacity, then it drops very sharply. If this voltage drops too far it can damage the lipo. Likewise since voltage is directly related to motor speed, this sharp voltage drop will also cause an equal sharp drop in rpm and that can effect the helis flight.

The 80% rule is applied by monitoring the mAh used in flight and keeping it below 80% of the rated capacity of the pack. That 80% is calculated by

80% of pack mAh = pack mAh * .8

For example here is 80% for the following packs

3s 850mAh 15C lipo
850mAh * .8 = 680mAh

3s 2200mAh 45C lipo
2200mAh * .8 = 1760mAh

6s 2200mAh 20C lipo
2200mAh * .8 = 1760mAh

6s 5000mAh 25C lipo
5000mAh * .8 = 4000mAh

Notice how the number of cells and other ratings of the pack make no difference for the 80% rule. Only the capacity of the packs is used.

How to calculate % mAh used after a flight
So how do you figure out what % of a lipo has been used after a flight? You charge the lipo with a smart charger capable of displaying the total mAh put in during the charge. That number represents the mAh taken out during the flight. To find the percentage

mAh used / lipo capacity = % used

For example take a 2200mAh lipo. Say you use this pack and recharge it to find you put in 1550mAh. The % used is

1550mAh / 2200mAh = 70.5%

Determining flight time
Now that you know what the 80% rule is and how to calculate the % used during flight, now lets put it to use. All pilots, especially those flying electric models, should use a flight timer. So how do you know what time to use? Well you could ask around to see how long people fly but you would likely get answers ranging from 3min to 10min. In reality every heli has its own flight time and is is determined by a whole host of things from weight to setup to flight style. So you are going to have to determine the flight times of your helis yourself.

The good news is that it is very simple. You start by flying for a set amount of time. Next you recharge the flight pack to learn how much mAh you used. Then you adjust from there. It is best to start small with a say a 3min flight. Then recharge and see what it used. Then you can start adding minutes until you approach 80%.

Once you know how long you can fly for, you can decide what time to use and fly for that time until something changes. Remember that each pack can give different results, so you may need to run different times for different packs. Also things always change over time. So always watch what your charger puts back in after each flight, and adjust as needed.

WARNING: Flying in the cold weather with Lipos. Flying in cold weather will reduce the flight time of the battery. So make sure you keep your batteries warm and cut your flight times short until you know what you are taking out of the battery. THen you can adjust your timers. Otherwise you'll be doing some autos and shortening the life of your batteries.