Part 2- Lipo for Dummies... by a Dummy- How to pick a battery for a new project

[Frank v B]

How do I pick a battery for a new project?
If someone gave you a model and it came without instructions?

Considerations in order of importance:
1) Safety- the battery must be large enough that it can handle the amperage and voltage required.
2) Power requirements- will it properly fly the airplane (take-off, speed, landing)?
3) Balance- is the plane tail heavy or nose heavy? A bigger, more powerful battery usually shifts the CG forward.
4) Fit- will it fit in the battery space of the plane? Can you enlarge the space?
5) Flight time required.  A 3 minute flight with a V-900 at 150 km/h is fine.  A 5 minute flight with an Apprentice is very limiting.
6) ventilation (cooling).  The higher the amperage, the better your cooling system needs to be.

Guiding principle: "Always overpower an airplane.  You can always throttle back..... if you have to"
Flying an underpowered airplane is very difficult.  High risk especially in the first turn after take-off on a windy day.

Overall Approach
1)  Check the stats on the motor and ESC on the internet and never exceed them... ever.  The limits include:
- Voltage- some are 2 cell (S), some 2S-4S, some 4S-6S
- Continuous amps- the maximum amperage it can draw for the entire flight.
- Burst amps- a surge in power for a limited time.  Note: approaching this limit creates damaging heat in the motor, speed control and battery.

      Note: when in combination, the motor and ESC are limited by the lowest value of each rating.  Example- If you have a motor that can handle 2-4S and an ESC that is 2-3S, your max is 3S or the ESC will blow.  If a motor can handle up to 30 amps continuous but the ESC can handle 40 amps, you are limited to 30 amps or the motor will blow.

2)Be conservative-  I will prop a motor, ESC and battery to about 80% of the limit.  Feel free to test the limit on the voltage but not amperage.  It is safe to put 4S through a 4S motor and ESC.
The closer you get to the AMP limit on a motor or ESC the more heat will be generated.  Heat is bad.  High heat leads up to the white smoke of surrender.

Example: the plane has an E-power 46 and an E-Flite 60 amp ESC.  Stats from the web:
E-Flite Power 46 4-5S Lipo 40 amps continuous, 55  Amps 30 seconds
E-Flite 60 amp ESC- 3-6 S 60 amp continuous, 75 amp peak.

Between the two, the limits are 4-5S and 40 amps.

Recommendation on the battery for this power package:
5 S battery if you have one and prop it to a safe 35 amps (35 amps gives a safety margin to avoid heat).
Battery amperage determines duration.  For a 6 minute flight (1/10hr) you would need a 3.5 amp battery (6/60 x35).  That is also the minimum C rating (C=10).  Most modern batteries are a minimum of 20 C so it is fine.

If you are serious about the project and have to buy batteries, I would strongly recommend buying only 1 battery and test the performance.  If it is a good combination, then buy more.  If it needs tweeking you can increase it to 4.0 amps if the CG needs to move forward or you need more flight time.  Once you are happy, invest in more batteries.

Frank