electric power for scale sailplane

[Pete Packer]

since there is no "electrified scale sailplane" heading, i think this might go here, and benefit others who may just be getting started. I am no newbie to R/C or electric, but I have not "electrified" a sailplane for a number of years. What is the current wisdom regarding use of lifepo4, lipo, or nimh cells as rx packs in small 3-4 meter scale electric sailplanes?
lets say for example 800-1000 watt or 4s lipo motor battery with 6 mini or micro servos.
1. to regulate or not
2. To use BEC or not
3.charging discharging issues.
3.ease of use and everyday reliability vs. maximum performance/weight saving

my specific example is a 3 meter Fox slope machine, so hand launch, short climb motor runs, or longer motor runs say 60 seconds + from flat field.

[Alex]

I personally prefer to have a separate Rx battery on my more expensive planes. I do not use lipos for Rx packs, since they don't like cold(er) temperatures (resulting in fast voltage drop), and they are not as crash resistant as other battery types.
I have started using A123's on my more expensive ships, but would not invest in that battery pack + regulator for cheaper planes. They also do not come in a variety of mAh sizes. A123's need to be closely monitored. They will hold their voltage for a long time, but suddenly drop then. However, they can be charged really quickly. As long as you have access to a power source for charging after so many flights, you should be fine.

For your Fox, I would recommend either a separate NiMH or A123 Rx battery. Or power your Rx with the motor lipo, but make sure you set cut off at least at 3.5V per cell in order to have enough juice left for your Rx

[sruelle]

"MY" way

OMG no BEC for me, after loosing a plane because of the BEC, put now a 4 cell 4/5A 2000mA or 2 cell A123 without regulator (just drain 100mA out of the battery to avoid the cell to be at 3.5+ V)

For the motor, I use 3mn total motor time, Kv 650, 6 cells A123, 13x8 giving 5.5m/s on a 4m LS4


Hope that help.

Stéphane

[Pete Packer]

Question: if your using a regulator why do you need to drain 100MAH out of a Lifpo4 cell before flying?

[Pete Packer]

Generally I have felt the same about not using BECs in the past. guess nothing has changed. The extra ounces won't hurt a fast Fox Sloper anyway

[sruelle]

Question: if your using a regulator why do you need to drain 100MAH out of a Lifpo4 cell before flying?

Sorry without regulator

Corrected

Stephane

[Pete Packer]

Stephane:
OK, thanks for clarifying, kind of thought that.

[Pete Packer]

I am a little knowledgeable about lifepo4 cells (from reading only). since they possess such a flat discharge curve how are folks effectively monitoring them, a traditional battery checker with load switch will obviously not work. measuring volts? a cut off no fly voltage? or is everyone savvy doing the math by calculating MAH by approximate servo load?
I have seen cute little onboard checker specifically for lipo and lifepo4 cells. they work?

[sruelle]

Usually, it is tricky... rule of thumb with safety is in general the way.

Fly for 30mn, charge back and get the mA drained, an deduct theoretical flight duration with battery capacity and cut it by half...

Have seen sweet devices from emcotec where it beep if consumption reach a peak or exceeding a limit, useful in slope soaring where model is not far but useless for aerotowing....

Stephane

[giovanni]

Can a HiTec Aurora 9 telemetry system be used to monitor the batteries?