I’m slowly progressing with my little drone project and I’m really enjoying it.
One thing that is difficult for me at the moment is my battery management. I have align s3 2200mah packs.
My first question is to what voltage people set their failsafe on these. I’ve had them at 10.6 but when I then charge I don’t get near to 2200mah charging. I also notice that when the load is off the packs, the voltage goes up. I guess this is normal, but what do I set failsafe to then?
Is it maybe a better idea to make sure the current sensor is calibrated correctly and use the current measurement to know how full/empty a pack is?
- Each cell has a range of 3.0 to 4.2 volts (empty to full).
- A 3S battery has three cells. So full charge is 4.2 volts x 3 cells = 12.6 volts. Empty is 3.0 volts x 3 cells = 9.0 volts.
- Below 3.3 volts per cell, you’re probably going to crash because there isn’t enough energy to arrest a descent. So figure on falling out of the sky actually at 3.3 x 3 cells = 10 volts
- So you need to make a point of coming back home to land long before you get down to 10 volts. At which point you do that is up to you. It depends on how much power your copter uses, to determine how much time you need to get back to land.
- 3.5 volts per cell is probably a good starting point, which would be 10.5 volts as a
BAT_LOW_VOLT parameter setting.
As general rule you don’t want to be on the ground disarmed with no less than 15-20% of the battery capacity left. You never run it down to zero. 15% of your 2200 mah battery is 330mah. So on top of that you need some time to actually fly home and land. How much MAH you need to come home an land depends on your copter and how much power it consumes. As a starting point for
BATT_LOW_MAH is probably double that at 660. See where that gets you coming home and adjust up or down i needed.
Your charger will never put 2200mah back into the battery, because you should never actually use all 2200mah. It will put back in what you take out of it. If you land at 15%, you’ve consumed 1870mah, and that’s about how much the charger will put back in.
Check out the Wiki page on battery monitoring and failsafes, because there is more too it than just this.
Many thanks for the response!
I knew about the 15-20% rule of discharge and i understand that i have to figure out how to handle the remaining power and when to land.
I’ll configure it at 10.5V on voltage failsafe. and change teh BATT_LOW_MAH to 400mah.
The failsafe itself would just be a ‘Land’. Thanks!
There is some subtle details you might want to consider before finding a way to safely manage battery packs:
- Battery voltage at which it is considered dead depends on the load(current drawn) from the pack. https://ksr-ugc.imgix.net/assets/027/474/479/4cf6dc96365c9c6dcd4e782e98933168_original.gif?ixlib=rb-2.1.0&w=680&fit=max&v=1576028781&auto=format&gif-q=50&q=92&s=2dfd20c59a99ccd7f0d5f3b814116c2c
- Battery voltage has a ‘lag’, which means it takes some time to rise up if a load is removed https://ksr-ugc.imgix.net/assets/027/474/450/ea7a4b44ae4a1504301ec2f09f50f858_original.gif?ixlib=rb-2.1.0&w=680&fit=max&v=1576028561&auto=format&gif-q=50&q=92&s=de9c50480502a0896b6217982c163fec
These two behavior depend on the battery chemistry, internal resistance etc. @Pedals2Paddles provided a good usual range on the behavior, but note that it varies a lot with load, battery etc.
PS: Check out our kickstarter page for more reading on battery behavior in load: https://www.kickstarter.com/projects/batmon/batmon-smartify-your-drone-battery?ref=b8esq5
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