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Low voltage

mrmund

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Maybe kind of a dumb question.
On my 5" quads I'm often getting a low voltage alert when I get on the throttle hard(pulling out of a dive, punch out). This even seems to happen sometimes with a fresh battery.
Is this normal?
 
I think this is normal for hard throttle moves, that is my test for whether a battery is still extremely strong, they can take a 5 second full throttle punch from full and it doesn't sag into low voltage range. It is almost impossible for any 4s battery to stand up to these newer high current motor/esc/prop combos. That is my experience over the last 5 years or so.
 
I don't think my new batteries do that, charged up on a balance charger.
Just to be clear, on battery charging and care I've been:
Charging on a balance charger up to 4.20 volts per cell
Running down to about 3.8-3.7 per cell
Is that right?
 
Yes, that is correct, but I run mine down to 3.4 or so, all batteries get "tired" and just don't have the punch of a good new battery, but whether you stop at 3.7 or 3.4, they all start to have reduced punch after a while, some aren't great when new. Like I said, I grade my batteries by a good hard punch out when freshly charged, if they don't sag out to low voltage too quick I know it is one of my stronger batts, but they all fly fine since we rarely run full throttle for long. I just let them sag and they recover a bit while I'm floating, and then I punch it again. Hard accelerations are always going to be hard on the battery, and it is another benefit of 5 or 6s, less battery sag comparatively.
 
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@mrmund You don't need to stop at 3.8v per cell. 3.8V is the storage voltage. The LiPo batteries are designed to run from 4.2v down to 3.4v or 3.5v. Check your settings in Betaflight as your low voltage threshold should be around 3.5v per cell. If it's set any higher, you will see low voltage alerts every time you punch it. For a 4S you will see 16.8V on a full charge down to 14v. You should probably be thinking about landing when you reach 14v or 3.5v per cell as the voltage tends to drops quickly after that and you could get into trouble in a hurry and drop dangerously close to 3v. Voltage sag under heavy load (ie. punch outs or long throttle pulls) is very normal. You can pull 20-25A easily with peaks nearing 30-35A deepening on motor/prop combinations. What you should see is the voltage bounce back once you law off the throttle. It's amazing what these batteries can do but there are limits.

Balance charging is important in order for all cells to have the same capacity and current is drawn equally across the cells, otherwise some cells will wear much quicker leading to a damaged cell and useless battery. On a good charger (like the ISDT Q6 Pro) you can also see the impedance of each cell. As they wear out the impedance will go up and they will sag more under heavy loads. Also if the impedance varies too much between cells, more current will be drawn from lower impedance cells resulting in more sag and more cell damage over time as the cells are intended to equally carry the load.

A good habit is to label your batteries #1, #2, #3, etc. and keep a quick log of how many cycles you have run them through while documenting the impedance from time to time in the log. If that's too much work for you, just keep an eye on the impedance and make sure the cell stay close in impedance. (15-25 mOhms is typical of a healthy cell). Once they get really high or vary too much between cells, you shouldn't trust that battery anymore and don't fly over water in a punch out. Ask me how I know this?

Lastly, if you use a parallel charging board, to charge multiple batteries at once, impedance becomes much more important. Always make sure your batteries are all within 0.3v of each other to parallel charge otherwise you will have a large current dump from the highest capacity to the lowest capacity battery instantly and you could have a bomb on your hands. always plug in the main leads before the balance leads to prevent an accidental high current discharge through the balance leads. Also, if one battery ends up with a bad cell, you could damage all the batteries that are charging in parallel at the same time so keep an eye on the health of your batteries.
 

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