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flismall
12-01-2005, 03:55 PM
It has been my understanding that lipo's should not be be discharged below 3 volts per cell and yet I have seen esc's for lipo's listing 5 volt and 7.4 volt advertised and I have seen these nunbers given in articles in the mags. Are these numbers safe for lipo's or are 3 and 9 volt overly conservative values.

Matt Kirsch
12-01-2005, 05:32 PM
There is a bit of a controversy as to what is meant by "3.0 Volts per cell." Some say that it's the cell under load. Some say that it's the cell at rest. There's quite a difference between the two, because a load will draw the voltage of a cell down quite signifigantly.

The problem with the "at rest" argument is that you can't possibly know how far a cell will "bounce back" after the load is removed. The higher the load, the deeper the voltage drop, so the more the cell bounces back. If you draw a cell down to 2.5V on a light load, it won't bounce back to nearly as high a voltage as it would if you were running a heavier load, and you could be doing damage.

That said, sometimes advertisers are unclear/unsure about what they're advertising. In the case of your quesiton, can you cite examples of ads where the ESC is said to cut off at 5 volts or 7.4 volts? You may also be misunderstanding the advertising.

In general, if the ESC says "LiPoly compatible" or better yet, if it has a programmable cutoff, that's always been good enough for me. Perhaps I should be a little more thorough and see what exactly the ESC manufacturer means when it makes the claim of LiPoly compatible, too?

Mike Parsons
12-01-2005, 05:34 PM
9 volts is conservative. It is a safe zone so to speak. Below 2.5v per cell is deadzone as I recall. Most of the manufactures had a variation of the formula that set the cut-off below 3v PC. Even Castle originally had a below 3v cut-off on their pheonix line prior to members storming the castle to have it changed (pun intended). Castle in their usual fashion of customer service, changed the code to 3v PC.

The Electrifly Silver Series's autodetect has a 67% of detected battery voltage (Vx .67= 2.6v pc). I have yet to hit cut-off as I can feel performance drop prior to that happening. I have however gotten close the the 2.6 on a 3 cell @ 8.1 volts (2.7v PC) and there have been no problems with my pack :)

-Mike

skid_68
12-03-2005, 01:55 AM
Just take your watt meter with you when you fly. I check my lipos a couple times per flight to make sure Im above the 3v per cell.

hoppy
12-03-2005, 02:08 AM
There is no argument on this issue...it is plain and simple. (To me at least:))

Set the ESC to at least 3V/cell or higher.

Look at the discharge graphs in this thread and then answer this question.
http://www.rcgroups.com/forums/showpost.php?p=3538164&postcount=1

How many mah were left in any of the cells tested at voltages less than 3V/cell? In other words, how many mah would you gain by setting your ESC to cut off at 2.5V instead of 3V. Look at this graph. See how steep the curve is even before 3V/cell is reached? There's no energy left in the cell to warrant risking damaging it. In fact, how much do you gain by setting it lower than 3.5V/cell?

Now if you have a marginal power system that's drawing max amps and is pulling down the voltage from throttle up on, that's another issue. As you can see in this graph, some cells drop voltage during high C discharge and could benefit from a lower LVC.
http://www.rcgroups.com/forums/attachment.php?attachmentid=538186

But most of the newer cells have the steep drop off from a voltage higher than 3V.

You can check more cells here: http://www.rcgroups.com/forums/showthread.php?t=342568 See if you can find your cells and what happens to the at the end of the discharge.

flismall
12-03-2005, 03:42 PM
Many thanks to all who responded to my LVC question, I received a lot of good info.
flismall

Spaceclam
12-13-2005, 01:58 AM
if you set your ESC to 3v/cell it will trip unless you use large batteries and dont draw much current. i routinely run my lipos down to 2.6v/cell on a heavy load , and have run them down as far as 2.4v/cell when the load is removed they will bounce back to around 3v/cell. this can be proven by the # of people who's escs cut off after 30 seconds of flight with a 3c pack, and a 9v cutoff. put in a bigger battery and that goes away. if the load is close to the maximum discharge, you can safely drop down to 2.6v/cell (7.8v on a 3c pack) but there is controversy as to how low resting voltage can be. i have heard as high as 3.2, and as low as 2.6. some people have stories of resting voltages of 2.4, and others swear that anything less than 3v will ruin it. in reality, it depends on the cell. 3v/cell is a good conservative number that will guarantee battery life, but when under load, 2.6v is a more realistic number.