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protecting LiPo's from deep discharge...

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Patrick

Thu Apr 25 2013, 04:24AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im here to ask for some help and comments reguarding LiPos...

Question: what is the deep discharge region (at which cut-off should be done) for RC batteries?

let me explain further, my batteries are 4S, LiPo, 30C, 248 grams, 14.8V, 2200mAH, Chinese no-name type. (24 US$ each)

ive heard 2.9 all the way to 3.3 should be considered for the stop and charge point. I realize the deeper we discharge, and the more frequent, we shorten any batteries life. but For multirotors we need balance. the difference between 2.9 and 3.3 volts could be a lot of flight time. (I need to look at a discharge curve though) my cells charge to 4.2 volts reliable, though one is puffing after 100+ cycles, so its going to haz-mat disposal soon.

I was also thinking, since my safety features are cutting in too soon at 3.3 to 3.4 volts, I should disable them, and use the piezo out of a smoke alarm, for a LOUD in flight warning (hey get me on the ground human pilot before you kill me) kind of warning.

Please comment on voltage vs lifetime please, im thinking of cut-off under 15-20 amp load voltage.

TY, Patrick

Carbon_Rod

Thu Apr 25 2013, 06:10AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Mostly, when entering a roll-off discharge area the 3-phase driver Mosfets and Motors enter a "stress" zone where they start inefficiently radiating excessive heat rather than doing useful work.

Keep in mind, this is true only if they actually survive the above normal draw.
Some just die mid flight without warning if people don't notice the built in ESC stutter warning at a distance.

You may find it anecdotally interesting that the overall capacity doesn't degrade as quickly as the discharge C rating.

Cheers,
Rod

Dr. Slack

Thu Apr 25 2013, 07:07AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

I was also thinking, since my safety features are cutting in too soon at 3.3 to 3.4 volts, I should disable them, and use the piezo out of a smoke alarm, for a LOUD in flight warning (hey get me on the ground human pilot before you kill me) kind of warning.

Absolutely, in extremis it will always be better to shag your batteries than to shag the entire quad and whoever is standing under it, who will then sue the hide off you.

Better than that would be more of a continuous gas-gauge sort of function so that you don't get to the last ditch warning unexpectedly. One beep/yellow LED flash for 3.7v, two beeps for 3.6, etc etc

Patrick

Thu Apr 25 2013, 08:10AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carbon_Rod wrote ...

Some just die mid flight without warning if people don't notice the built in ESC stutter warning at a distance.

yes i realize this is a risk below a certain voltage.

Carbon_Rod wrote ...

You may find it anecdotally interesting that the overall capacity doesn't degrade as quickly as the discharge C rating.

please elaborate on this comment? i dont see your meaning.

Dr. Slack wrote ...

Absolutely, in extremis it will always be better to shag your batteries than to shag the entire quad and whoever is standing under it, who will then sue the hide off you.

Better than that would be more of a continuous gas-gauge sort of function so that you don't get to the last ditch warning unexpectedly. One beep/yellow LED flash for 3.7v, two beeps for 3.6, etc etc

yes, ive seen idiot pilots at the "X-factor london" and sporting events fly over peoples heads, which scares the hell out of me. becuase some idiot will screw the rest of us over, and cut some pretty girls' face off, and the media will never shut up about it... the FAA is already screwing us multirotor-ists over.

but i still wonder if i should set the first alarm at 3.2V per cell, or 3.1V...

edit: i am putting super bright LEDs together now, along with searching for the loudest piezo device i can get. is 102dB loud?

Steve Conner

Thu Apr 25 2013, 09:25AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The problem is that due to the cells' internal resistance, the terminal voltage sags as more current is drawn. So terminal voltage under heavy load can only be a very rough indication of the remaining capacity. This is related to what Carbon Rod said: the faster you discharge the battery, the less of its capacity you can access. When you set the UVLO threshold you need to take all of this into account.

The "gas gauge" in a laptop battery measures current and integrates it to clock up the actual amp-hours consumed. Maybe you can get a device for RC models that does this, but the variation of capacity with discharge rate might mess it up.

Dr. Slack

Thu Apr 25 2013, 11:59AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Steve Conner wrote ...

The problem is that due to the cells' internal resistance, the terminal voltage sags as more current is drawn. So terminal voltage under heavy load can only be a very rough indication of the remaining capacity.

I would hazard a guess that the battery's internal resistance is not constant enough to correct for by estimating the current drawn? The Law of Sheer Cussedness says that it will increase as the battery discharges. However, what's the time behaviour of the extra drop due to load?

How fast does the voltage drop recover with no load? Could you switch off the loads for 10uS, or 10mS, and get a better indication of remaining capacity. That would completely correct for the actual physical internal resistance. I would expect the cells' inductance and capacitance to be negligible for these sorts of times, but I could imagine that polarisation effects might take orders of magnitude longer to sort themselves. A motor drive need not notice a 10uS supply hiccup, and the navigation would probably sort out 10mS without power, but I'm not sure a 10s free-fall would be well-received, even if it was in the service of being more accurate about your battery.

Patrick wrote ...

is 102dB loud?

It's loud, but is it loud enough? What would get your attention is to stutter the lift. You're concentrating on the multirotor as it goes 'buzzzzzzzzzzzzzz ...' and then it goes 'buzzzzzzzzzu - uzzzzzzz'. That's an example of a warning message that isn't arbitrary (now what does that ping mean?), it contains its own hint about the action to take (OMG, the fans are going to stop). Another example of that sort of message was used by wind turbine managers on a remote Scottish Island. As the wind dropped, did they start Diesel? Not at once, they let the output voltage drop first. The islanders noticed the lights dim, and rushed around turning off less important loads, delaying the time when they'd be forced to start spending money on fuel.

Conundrum

Thu Apr 25 2013, 08:42PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

May be better to include a backup battery as well, so that the quad has just enough power to make a controlled landing even if the main battery drops out.

Smoke alarm piezo is a good idea, google "MicroScream" for a LVCO based on the TL431.

Carbon_Rod

Sat Apr 27 2013, 01:51AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Steve noted something rather important,

People typically use paralleled smaller packs with a combined rating higher than a larger pack.
Note this is typical even if the Ah rating is slightly less, as sustained output maximum often increases. Additionally, the packs are usually cheaper to buy, last longer, and provide performance class value for lower than a B-grade price.

For example, given a choice between:

I.)
The 3.3 Ah that will survive fewer charge cycles, and have less capacity at 20C

II.)
Or two 1.5 Ah units with combined output rating of 40C

Option II. will likely give longer flight times, and the airframe will lighten a bit too.
Hint: if you scroll half way down the page a wait a few moments for the JavaScript to trigger an event.
These discount packs go for $3.39/unit + shipping, and work well on quads.

Cheers,
Rod

Patrick

Sat Apr 27 2013, 01:59AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I have an older, heavily used Lipo, its moderately puffed, and getting worse, slowly. should I poke a tiny hole, let out the evolved gas, then cover the hole with tape that I know is chemical resistance?

Carbon_Rod

Sat Apr 27 2013, 07:35AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

I guess you want Oxygen gas for some reason.

The batteries puff up when they are internally damaged, or too old to survive the charge cycle properly.

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