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| Gregor99 | Elite Veteran - Location: Western Wa - |
OK, so here's topic for some debate. The old rule is discharge to only 80%. What is behind this rule? What happens when you go below 80%. There seems to be very little detailed technical information about this guidance.
Although dated, the FMA tech docs provide some of the better details about this.
http://www.fmadirect.com/tech_data/techdocs/
Here's an interesting discharge graph from the Handbook Section 2.
What is interesting is that at the lower discharge rates, the voltage stays much higher, far deeper into the unsafe 20% zone. At the highest discharge rates, the voltage drops to 3v at roughly 80% of the pack's capacity. Unless you know exactly how much current you are drawing, and as well as the discharge curve, then once you are beyond 80% there's no way of knowing how quicky you'll hit 3v. This is likey the reason for the general guidance that is suitable for most users and most applicaitons. However if you are running a low current setup, and you have access to your pack's discharge curve (which is now quite rare), then you may be able to run past 80% without issues. The problem with that approach is that after a battery ages, and degrades, the discharge curve changes. Unless you know about it and shorten the discharge cycle, the battery could be damaged.
The other danger is that past 80% all cells in a pack don't always discharge equally. Its possible that one of the 3 cells reached the unsafe 3v level much faster than the others. During charging, monitoring the individal cell voltage at the start of the charge can give an early indication of a cell starting to deteriorate. That pack shouldn't be discharged past 80%.
The other question that has bothered me is what "damage" occurs past 3v? In fact some users report that they've discharged past 3v and were able to recover the pack. My guess is that discharge past 3v is not the curse of death to lipos that we've been told about for so many years.
I suspect that the real danger to lipos is not over discharging, its overheating. During the discharge cycle, the heat generated as the voltage drops relative to the internal resistance of the pack ramps up dramaically toward the end of the discharge cycle. I've noticed the a huge difference in tempature between 80% and 90%. Past 90% and medium to high rates of discharge is likely to push pack over the safe limit of 140 degrees.
Conversely a pack that is under 3v per cell could draw a lot of current from the charger, unless the charger detects this and ramps down the charge voltage so the current through the pack doesn't exceed safe levels. Alot of chargers now have a recovery mode where they will charge the pack at a very low current until its up to about 3.7 volts per cell. I suspect this is to prevent over heating during the charge cycle.
Newer packs with thier very high discharge ratings often don't even get that warm at the end of a run. This is similar to the older packs discharged at low current. Or put more accurately at much lower than the batteries rated maximum. This is one reason I buy packs rated at far more than my heli will ever draw. It makes it safer to dip into the 20% range if needed. Plus it helps prolong the life of the pack since the cells never really get all that hot.
The other mystery yet to be fully explained is what is Air Thunder and Kong power doing to allow full discharge? I wonder if its as simple as a PCB board that ramps down the voltage when the pack temp approachs unsafe levels.
All of this makes me question the age old 80/20 rule. It seems that we should be basing flight times on pack temps as taken at the end of a flight not strictly 80%. I have older packs that still balance and work fine. But they are MUCH hotter at the of the run than when they were new. As the battery ages the internal resistance goes up. This increased the run temp of the pack and more quickly brings the pack to unsafe levels even when only discharged to 80%. If you aren't monitor temp of the pack after the run, the pack could easly become damaged if the run time is not shortened. 80/20 doesn't account for age. It seems that standard Lipo practices are missing one of the most important data points for safe discharge levels and monitoring lipo health.
I'm guilty of this too. Can anyone recommend a good IR tempature gauge?
Canton MO backyard flying club
West Coast Chapter |
| 06-14-2008 08:18 PM | | | |
Another quote from fenderstrat.
