Best Way To Discharge LiPO Batteries--Your Recommendation

[Case29247]

FWIW, I started this thread, but I don't spend much time on internet forums, so didn't monitor replies after just a few days of the original.

I simply bought a 50W halogen automotive bulb and a socket and fashioned a plug for the battery. I monitored the voltage and timed the discharges until I had a good feel for the time to discharge from various levels. I've been using the technique since then....Plus I have been more careful to charge only the number of batteries that I anticipate that I'll need for any particular outing, so I don't have fully charged batteries that I didn't need lying around afterwards.

I think I'll give Protocrash's Phantom Angel a try, assuming he's not charging an arm and a leg. I'll let you know.

 

[Case29247]

FWIW, I started this thread, but I don't spend much time on internet forums, so didn't monitor replies after just a few days of the original.

I simply bought a 50W halogen automotive bulb and a socket and fashioned a plug for the battery. I monitored the voltage and timed the discharges until I had a good feel for the time to discharge from various levels. I've been using the technique since then....Plus I have been more careful to charge only the number of batteries that I anticipate that I'll need for any particular outing, so I don't have fully charged batteries that I didn't need lying around afterwards.

I think I'll give Protocrash's Phantom Angel a try, assuming he's not charging an arm and a leg. I'll let you know.

Okay, I ordered one $24.00. I'll report on the delivery and whether I think it works.

 

[Protocrash]

Hey Case, good stuff! Thanks for trying it out. I'll get it packed and posted asap.

Really keen to hear your feedback!

 

[Case29247]

Hey Case, good stuff! Thanks for trying it out. I'll get it packed and posted asap.

Really keen to hear your feedback!

And I can't wait to get it. Wish it were on the front porch right now. I have one battery charged, with the others mostly discharged. It's a light overcast here today (relatively flat lighting without deep contrasting shadows), and I would really like to get out and get the video that failed due to some GoPro/Phantom glitch the other day. I'd love to charge up a second or third battery.....but most likely I'd come back with one or two fully charged batteries....and we know what that means. So I'll go out with just the batteries I think I'll need today and until I get your discharger.

 

[Case29247]

Well, I got the Battery Angel discharger today from Protocrash. It's very simple. One IC chip inside that I suspect is some sort of voltage referenced switch (zener diode or something like that). A few resistors and a relay. That's it.

Just install the 50W bulb that acts as the load on the battery. Then connect it to a flight battery, start the battery up (observing the polarity--the only instruction included), and press the button on the discharger. The bulb lights up for about thirty seconds, and then turns off.

I'm really not sure what's going on here. The battery I plugged in was nearly full--flashing 3-1/2 LED's, so it was at about 80-90 capacity. I expected the discharger to operate longer, until maybe about 2 or 1-1/2 LED's flashing, indicating below 50% charge remaining.

Unfortunately, I'm in crunch time tonight with a video piece deadline tomorrow; plus, that hurricane may be coming up the coast, which means that tomorrow I might end up going to the beach to haul out the boat that's still in the water, and tie down deck furniture, etc. Fun, fun, fun!

When I get past these, I'll put some time in trying to figure out the operation of he discharger. One thing for sure, the discharger comes with no instructions (not that it's too mysterious to figure it out). But I would have liked some instructions anyway.

$24 delivered (from Greece, no less) isn't going to break the bank, but I must admit I'm a little suspicious right now that it's not working correctly. Perhaps Protocrash can post here and provide additional insights.

I'll get back.

 

[Protocrash]

Hey Case,

Thanks for writing and sharing your questions as I’m sure others will have them as well! Let us take them one at a time:

How it works
As you rightly say its not magical or super complex. Actually most of my R&D went into finding the simplest, most reliable and most cost effective way of storage discharging. It would have been much easier to make something that looks twice as cool, costs twice as much and does the same thing!

The two main components in there are indeed a voltage reference and a relay. The 12k resistance controls the shut off point (so if you want to tweak it you know where to poke). Interestingly the most challenging aspect was handling the heat – hence the retro looking temp resistant box and the specific design of the lamp holder.

Instructions
Hmmm, I do post all Angels with instructions (very short but still). Perhaps you didn’t notice when you unpacked? Either way I’ve uploaded them here for you http://phantomangel.rocks/images/PA-instructions.png

Weird behaviour?
You may be onto something here. The way the Angel works is to shut off at 11.6v. I’d be quite surprised if yours is working differently as all Angels are tested twice, the first time by the board manufacturers and a second time when I assemble the whole thing.

So, two potentials here:
a) Despite the testing your Angel is shutting off earlier – in which case I’ll absolutely replace it at no cost!
b) Your LiPo may be worn so that with a small load (the 50W bulb) the voltage drops to 11.6 faster than it should. In this case I’d charge and discharge it a few times and monitor its behaviour carefully

Hope all this helps, and hope hurricane business isn’t too bad! Do write when things have calmed down.

All the best,
Proto

 

[Case29247]

Protocrash,

Thanks for getting back to me so quickly. Haven't had time to really test the Angel beyond my initial trial. The hurricane seems most likely now to stay off shore in the Atlantic, and I'm waiting for my video customer to show up.

I'm surprised that the programmed cutoff voltage is 11.6 volts, since the nameplate voltage of the DJI flight batteries is 11.1 volts, and when I fly, the voltage reading on my OSD quickly settles into the 11.2 volt range and stays fairly flat in that range for quite a while.

I would think the Angel's cutoff voltage would be around 11 volts or even lower, insuring that the battery is discharged to a lower level (i.e., 30% or 40%).

Attached is a discharge curve I found on the internet, which basically mirrors my flight experience with voltage.

Comment?

 

[Protocrash]

Hey Case, some interesting points and I'm sure others are having the same questions. So, here goes:
- 3S LiPos may have 11.1 written on the box but at full charge are actually at 4.2v/cell = 12.6v. Theiir voltage then changes depending on their charge state with a curve similar to the one you attached.
- However... voltage also changes drastically in relation to the load. A heavily loaded LiPo will register significantly less voltage than when at rest
- So, your OSD readings are highly affected by whether your motors are spinning and how much
- The graph you attach is interesting. Seeing as it starts 12v and is, overall around 0.6v lower than the normal LiPo chart I can only conclude that it is showing the voltage under hover load (which, at around 5-10C I'd expect to be 0.6v lower than at rest)
- FYI I attach a chart for 3S LiPos under various loads. You can actually google LiPo curve and view images and you'll find loads of similar ones
- So, it actually all adds up with the Angel cutting off at 11.6v under minimal load
- You also mention that perhaps the cutoff should have been at 30-40%. Actually, all LiPo manufacturers (including DJI) recommend anywhere between 50 and 65% as safe storage level. I've taken a point towards the higher end as it is both safe for the LiPo and means you need to spend a little bit less time re-charging when its time to fly

Finally, as this is a (strangely) interesting topic I'll add one more comment to spice things up. Estimating LiPo charge % from its voltage accurately is... impossible! All of these curves and calculations are estimates and depend on loads of things. Temperature, the age and state of the battery, load and other parameters all affect how you translate voltage to capacity. The only accurate way to truly understand what's going on in your LiPo is to discharge it as close to fully as feel comfortable, recharge to 100% and measure how many mahs you're putting back in. Then, as you use it you measure the mahs consumed and calculate your percentage. This is what DJI does with the Inspire where they ask you to discharge down to 8% every 20 cycles (note - this is really bad for your LiPos!).

The good news is that when it comes to storing batts this level of accuracy isn't necessary. Which is why, when you throw it all in together, I believe 11.6v is an appropriate cutoff.

Wow - that was a long reply, got carried away!

Cheers

 

[Case29247]

Protocrash,

I read your reply with interest, and I like it. I think everything you say is very credible, and I really don't think I need to spend a lot of time doing my own research and bench-top testing. Thanks.

I'm not an expert by any means, but my engineering and video production careers did sometimes involve batteries of several different types and capacities. So, I do know, for instance, that capacity ratings are invariably variable due to various factors, two of the more important often being discharge rate and temperature. Thus, actual usable capacity is normally less at higher discharge rates due to some extent to chemistry, but I think mainly due to internal resistance losses (the good old P= I^2 x R ). Also, capacity is very much diminished normally by lower temperatures.

So, it doesn't surprise me when you talk about how difficult it is to control the discharge cut-off point with great precision, using such a simple voltage-only scheme. But I also suspect that your scheme should be good enough to hit the "correct range."

So, I tested my Angel using one, nearly new DJI battery with a voltmeter connected to the leads. I charged and discharged it using the Angel three times. Summarizing and averaging:

Fully charged at rest: 4 LED's, 12.5V.
Start Discharge: 0 minutes, 4 LED's, 12.3V
11 minutes, 3-1/2 LED's, 11.9V
21 minutes, 3 LED's, 11.6V
Auto Shutoff: 25 minutes, 3 LED's, 11.5V

Right after shutoff, rebounded at rest to 11.6V, and after about ten minutes 11.7V.
After shutoff, the LED indicators show 3 LED's remain lighted (no load).

If you look really closely at the DJI chart, you will see the red curve actually starts at about 12.3V. This corresponds favorably to curves for one cell at 1.0C discharge rate, times 3. The DJI flight battery I'm using is rated at 5200 mAh (57.7 W-h). The point is, give or take and allowing a little latitude in the numbers, the Angle with a 50W bulb discharges at about 1.0C rate. And if you go back and look at your chart and the 1.0C discharge curve, you see that my measurements show that the Angel cuts out at about 57%. (GIMME SOME LATITUDE HERE---my measurements are pretty good, but I'm rounding off some of the data and eye-balling the charts, etc.

So my conclusion is that you're hitting your intended 50% - 65% cutoff range pretty much in the middle. That's good. If it does anybody any good to know, that's with 3 full LED's shining. The 3-LED indication is pretty rough because 3-LED's shine across a pretty broad range, but it's better than not knowing anything.

So Protocrash, you get a good mark. I think your simple and very economical device is a great value and a everybody should consider getting one.

BTW, I LOVE Greece. We were there for a couple of weeks in 2009 as part of my daughter's college graduation family celebration. We rented cars and did our own touring and "our own thing." We spent five days on Santorini, and the rest of the time in and around (day-trip range) Athens. Love the Greek people. Y'all know how to live. Love the food. I just hope you can get your fiscal house in order.

Thanks for everything.

 

[Protocrash]

Hey Case. Thanks for taking the time to write such a detailed reply and of course for the kind words (especially the "everybody should consider getting one." )

Seriously, clearly you're not new to LiPos and your reply is a great confirmation of the approach I've taken when designing the Angel.

And yes, Greece is absolutely lovely and Santorini especially so! Life on the ground is not quite that glamorous though these days as you can imagine.

Thanks again!

 

[Protocrash]

Hi guys,

Since launching the Phantom Angel a couple of months ago I've had loads of discussions with Phantom pilots about their batteries.
What I saw was that the majority of new pilots were not aware of standard LiPo practices and were surprised and grateful when I shared some of what I've learnt over the years.

So, with that in mind, I've created a concise, newbie-friendly guide to all things LiPo. If you don't know how to store your battiers, how to keep them from exploding or how to tell your watts from your amps, this guide's for you

Top tips for happy Phantom batteries

Hope you find it useful and please do send over any and all thoughts and comments.

Proto