• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

80% Rule v.s. Voltage Cutoff

jsut210

Expert Crash-Lander
#1
I am still a bit of a newb to the hobby right now so I have came across one question regarding LiPo discharge. I have read all around that a healthy voltage to discharge a Lipo to is about 3.7v, nominal voltage (Not under load). So as a beginner, I have a battery alarm on my tricopter that buzzes when by pack gets to 3.3v a cell (under load). When I use this alarm, my battery voltage equalizes to about 3.7v without a load. However, I just got an Accucel 6 and am now trying to follow the 80% rule. I'm running the common 3cell 2200mah and I figured that I should be putting about 1760mah back into the battery. But after flying until I previously have been, I have found that I'm putting close to 2000mah back into the battery. What should I do?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#2
I am still a bit of a newb to the hobby right now so I have came across one question regarding LiPo discharge. I have read all around that a healthy voltage to discharge a Lipo to is about 3.7v, nominal voltage (Not under load). So as a beginner, I have a battery alarm on my tricopter that buzzes when by pack gets to 3.3v a cell (under load). When I use this alarm, my battery voltage equalizes to about 3.7v without a load. However, I just got an Accucel 6 and am now trying to follow the 80% rule. I'm running the common 3cell 2200mah and I figured that I should be putting about 1760mah back into the battery. But after flying until I previously have been, I have found that I'm putting close to 2000mah back into the battery. What should I do?
Rejoice because your battery pack has more capacity than stated!

Seriously, the capacity injected at charge time is a bad measure -- I know what David says, but in this he's wrong (IMO). Here's why:

The capacity of a pack diminishes over it's lifespan, so as the pack gets older and older you have to drain the pack more and more to get to that target mAh. This doesn't prolong your battery life . . . it accelerates it's demise! Time-in-flight is just as bad for exactly the same reasons.

If you look at voltage/discharge curves folks have generated over time, (usually abusing the battery in the process) the accumulated mAh out vs. rest voltage has a sharp knee in it -- you'll generate a *LOT* of mAh going from 3.9-3.8v, same for 3.8-3.7, but hardly any mAh at all going from 3.7-3.6v.

What's that mean to us? At about 3.7v, the intended chemical reaction (which is reversible) starts to loose capacity and are slowly replaced by other reactions (which aren't reversible -- DAMAGE!).

And what % mAh capacity does the 3.7v resting correspond to? Yup! That's where we get the 20% from.

Now what you're doing calibrating "X load voltage" = "3.7v rest voltage" is good. It will however change based on the current draw. If you pull out a new airframe that needs more throttle to run, you may find that beeper fires off at 30-40%. If you switch to a lighter, gentler airframe that uses less current, you'll find that beeper won't sound until below 20%. The same can happen when switching between battery sizes, because of different current capacities.

My point? That set point can be different for each airframe, but should alert at the same capacity for that airframe/batery combo, almost every time
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#4
Pretty much. The chemistry is on your side all the way down to there (mostly reversible), but starts to turn against you as the voltage drops further, which it will do rapidly.
 

xuzme720

Dedicated foam bender
Mentor
#5
3.3v is a pretty good place for alarms on a multi since even hovering, you will be pulling a pretty hefty load. If you are using the alarm in a single motor plane, it's better to set higher, say around 3.7-3.8 per cell. If you are alarming at full throttle, no worries, but when you start to hear the alarm at low or no throttle, land and swap for a fresh pack.
 

jsut210

Expert Crash-Lander
#6
Yea, I have a timer on my Turnigy 9x that beeps after about 9 mins when my time's almost up at which point I just hover around and wait for my alarm to start blinking. It blinks when the battery is approaching low and then beeps when it is low.