I am wondering how much voltage sag should be expected with a battery under load? I am running a YKS 5000mha 25C 3S battery on a Flamewheel 450 quad. The quad hovers at 120 watts ( about 11 amps). The battery voltage drops about 0.44 volts under load which would suggest an internal resistance of about .04 ohms. I get up to 20 minute hover times which seems to track with the total wattage at 80% of the battery capacity. Here is what concerns me - I set the battery fail safe warning limit to 10.7 volts and the alarm/RTL limit to 10.5. The 0.44 volt sag under load seems to take a big chunk out of the usable voltage range for the battery. The battery starts out at 12.45 volts and drops to 12 or so when I hit the throttle.
Do these numbers sound reasonable? Are there different battery types that offer lower internal resistance? Is there a relationship between the C number and battery performance?
Thanks,
Doug
Replies
Dax, (and others),
Thanks for the quick replies. I have observed all of the things you've talked about - Rapid initial drop, flat discharge curve in the middle, and Very rapid drop toward the end of the discharge. When I was first starting to test and fly my quad, I had set the RTL fail safe to around 10.1 or 10.2 volts. I had the copter right in front of me doing a hover endurance test. When the fail safe triggered, it rose to the default RTL altitude (20m), then started it's decent to land. I watched as the battery voltage started to plummet below 10, then 9.6.... The battery died at about 4 feet above the ground and the copter tumbled the rest of the way but did land upright on all four. I learned my lesson that day.
I appreciate the advice about lower limit on discharge voltage. I currently have my RTL fail safe set to 10.5v.
I'm also happy to hear that my battery seems to be performing as advertised. Thanks again for the quick responses and good advice.
Happy flying,
Doug
The voltage lag will be present to some extent. My conclusions after burning thru some very expensive batteries:
First, Use a battery with double the required C rating. Using only the lower 50% of the rated current capacity will reduce the voltage lag. Also understand that the C rating provided by the manufacturer is more of an estimate, not guaranteed.
Second, Accept this very important fact, If ANY individual LIPO cell go's below 3.0v UNDER LOAD, it will never perform the same, likely 30% of the original capacity, it will get worse from then on. That one week cell will result in the end of that LIPO pack. I had to push it, set my fail safe at 3.3v per cell, and that's to low to complete the RTL without killing my batteries. A very expensive lesson. Be conservative when pushing your batteries for more flight time.
Hi
A 3S battery will have 12.6 Volt (4.2 V/cell) fully charged at no load. The normal voltage is 11.1 Volt (3.7 Volt/cell).
Every battery has a discharge curve and the voltage drops until minimum voltage where you are supposed to charge it again.
Normally you can discharge a LiPo to 3.0 Volt/cell but many users do not want to go below 3.2 Volt.
Yes! You can expect a lower internal resistance on batteries with higher C. Otherwise the battery would get too hot at this higher current.
You should also check your cables. You WILL drop voltage but with increased area and shorter length you can reduce it.
Anders