LiPo: How low can you go? (voltage per cell) - RC Groups
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Oct 12, 2013, 10:01 PM
Registered User
Discussion

LiPo: How low can you go? (voltage per cell)


I've been reading a lot and talking to various people at the airfield about how low I can take the voltage (per cell) on my LiPo battery pack and I have gotten a wide range of opinions. The sticky on this site says do not go below 3 volts (I've read this a few other places too), while someone (who I have reason to believe) at my local airfield said that I should stop flying when I hit 3.9 volts, with anything below 3.6 volts causing permanent damage to the battery. Then it seems the ESC does a soft cutoff around 3.3 and my charger discharges to 3.2 volts.

So, with all this information, when do you consider the battery too low to fly? Should I stop around 3.9 or can I go longer? My original target was around 3.5 volts per cell.

For what it is worth: From multiple tests, I seem to get about 2 minutes of flight per .1 volt on my plane (I don't know if that is good/bad/normal).
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Oct 12, 2013, 10:11 PM
Frankenstein recycled packs
rampman's Avatar
I fly till around 3.8 per cell, static. Depending your pack and amp draw...it could get down to 3.3 to 3.5 to 3.6 per cell under a load. Who knows unless you have telemetery. Important thing is the 3.7 to 3.8 static.

Rick
Oct 12, 2013, 11:31 PM
Registered User
I consider 3.7/cell as a "dead" battery, and time my flights according to the plane and battery being used, according to past testing and experience.
Oct 13, 2013, 04:23 AM
Registered User
What voltage you're looking for depends entirely on WHEN you're measuring the voltage :

The battery voltage drops when power is being taken out of it. 3V (or 3.2V or maybe 3.3V) is the minimum voltage the battery should ever see UNDER LOAD to avoid damage. I.e. when it's actually running the motor at full power. But you'll actually only know this voltage if you have some telemetry or similar in the plane.

When the load comes off, the battery voltage bounces back up. So 3.7V (or 3.8V or sometimes 3.9V) is the voltage OFF LOAD. I.e. what you should see AFTER a flight when you check the battery. That's considered a safe voltage off load to ensure that the battery never went as too low when it was under load.

Steve
Oct 13, 2013, 08:51 AM
Space Coast USA
hoppy's Avatar
Like Steve said, a battery voltage of 3.7V/cell (unloaded) tells you that you have used 80% of the batteries capacity. Most manufacturers recommend not discharging below 80% so your answer is any flying time that results in the battery being above 3.7V/cell after landing is OK.

Here is a voltage chart vs remaining capacity.

By cell
4.00V--84%
3.96---77%
3.93---70%
3.90---63%
3.86---56%
3.83---48%
3.80---43%
3.76---35%
3.73---27%
3.70---21%
3.67---14%

The % can vary by as much as 5% depending on the brand/age/condition of the battery.

You can see that 3.9V only uses 37% of the batteries capacity.
Oct 13, 2013, 10:47 AM
222 km/hr Parkjet flyer
solentlife's Avatar
I would like to comment on the 'voltage after land' ... you should wait a few minutes for pack to literally stabilise before testing ... immediately after land will still show a lower voltage than true rested.

Generally thyey reckon a pack should sit at about 3.8 - 3.85v for storage ... shoot me if wrong ! But a pack will often show much lower than that straight after flight. Leave it a couple of minutes and it bounces back up.

I have a discharger I built that I watch the voltage drop during discharge. I stop it when I see about 3.6v on the screen ... 1 minute later - I see all cells at about 3.8 - 3.85v ...

My way to deciding the flight time of a battery is brutal but works :

I test the pack in the model at home ... run up to full throttle for a set time ... watch the Wattmeter and get a reading of amp / volts under load ...

I then mathematically derive the maximum time for that pack size to 100% discharged. I then calculate 80% of that and use that as maximum time for flight - setting Tx timer to just under that. I know my timer will sound 59 secs before its zero ... giving me ample time to land. The time calculated there I know is shorter also than I can really use as it is calculated at full throttle continuous without allowance for use of reduced throttle in flight.

I test the battery with Wattmeter after flight and I take note of whats left in pack. If I find that all flights are leaving a larger amount in ... I adjust timer to give a bit longer ...

It's worked for me for years ... when glow engines ... to todays electrics ...

Nigel
Oct 13, 2013, 02:44 PM
Space Coast USA
hoppy's Avatar
Right on Nigel, I figure it takes a couple minutes to walk back to the car and hook up the voltage monitor.
Oct 14, 2013, 04:52 PM
Frankenstein recycled packs
rampman's Avatar
Quote:
Originally Posted by slipstick
...The battery voltage drops when power is being taken out of it. 3V (or 3.2V or maybe 3.3V) is the minimum voltage the battery should ever see UNDER LOAD to avoid damage. I.e. when it's actually running the motor at full power. But you'll actually only know this voltage if you have some telemetry or similar in the plane.

When the load comes off, the battery voltage bounces back up. So 3.7V (or 3.8V or sometimes 3.9V) is the voltage OFF LOAD...

Steve
IMO, new batteries better hold much higher under load than 3.2, even 3.4. If not, time to upgrade your packs or reduce your load.

I did some tests on an AGA Power 2200 3S 65C at slightly over 100A for a buddy.
This pack held around 3.54 per cell the whole, though short, discharge range. Very impressive, especially for a pack that showed around mid 20C real C ratings by my PL8 charger and the online IR tool.
Mee thinks it was actually >40C but the IR tool is not totally accurate, just one of many tools we can use.
If a cell drops to the low 3's the user is losing a LOT of power and, by today's standards, losing a LOT of life from his/her packs by stressing them too hard.
I think these numbers are about 5 or more years old. Sorta like the infamous salt water soaking to discharge a bad or worn out lipo.

Rick
Oct 14, 2013, 05:37 PM
A&P in training
ASTREA1's Avatar
I'm not sure why "What should my battery voltage be" is not its own sticky?
Oct 14, 2013, 06:28 PM
Registered User
Quote:
Originally Posted by rampman
IMO, new batteries better hold much higher under load than 3.2, even 3.4. If not, time to upgrade your packs or reduce your load.
I guess you're not following the discussion. If the voltage does not reduce as you empty the pack then you're not measuring it correctly.

No battery, however good or new it is will hold a constant voltage for the whole discharge. If you leave the load on at some point it will get down to 3.4V or 3.2V or even lower. At that point you've damaged it. What we're discussing is how to avoid allowing it get that low.

Since most people can't see the voltage in flight they have to estimate it from the off-load recovered voltage after they've landed. If you disagree with the values I suggested what would values would you suggest people look for ?

Steve
Mar 13, 2015, 07:46 PM
Registered User
i got zippy 6cell lipo i kept it at the 3.86 a cell during the winter today i took it out and charged it and balanced it but i seen during flight that one of the cell lose charge faster then others like the other 5 cell are at 3.90 the 6th one goes to 3.8 any way to fix that or that cell is not good any more
thanks
Mar 18, 2015, 06:45 PM
Registered User
Quote:
Originally Posted by paktazh
i got zippy 6cell lipo i kept it at the 3.86 a cell during the winter today i took it out and charged it and balanced it but i seen during flight that one of the cell lose charge faster then others like the other 5 cell are at 3.90 the 6th one goes to 3.8 any way to fix that or that cell is not good any more
thanks
That's not a real problem. Just use the battery a couple more times and the cells will probably get closer to each other. ..
Mar 18, 2015, 07:01 PM
Space Coast USA
hoppy's Avatar
Quote:
Originally Posted by paktazh
i got zippy 6cell lipo i kept it at the 3.86 a cell during the winter today i took it out and charged it and balanced it but i seen during flight that one of the cell lose charge faster then others like the other 5 cell are at 3.90 the 6th one goes to 3.8 any way to fix that or that cell is not good any more
thanks

Do they all measure the same after 24 hours off the charger? Check it out.

Is it 3.80V? or 3.8?V? That much difference (0.1V) is not good as you probably have guessed already.
A 0.1V difference is about 20% of the packs capacity. If things don't change in the next few charges, it will be necessary to limit your flying time to where that low cell is no more than 80% discharged (3.7V) to keep from harming it more.
Mar 18, 2015, 08:32 PM
circle flyer
slow and low's Avatar
I use a battery alarm while flying, when the voltage gets to 3.5 on any cell the alarm sounds and I have about 5 minutes to land. The alarm voltage can be set by you from 3.3 to 3.9. I also use it to check battery voltage before flying, the display cycles through all cell voltages as well as the pack voltage. They are less than $2 shipped from overseas (EBay) or less than $10 at many hobby shops. The best $2 I ever spent.


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