Lipo Battery Standards, open discussion
 

This thread is not for debate of any particular brand, only to propose guidelines that may help standardize the ratings of lipo batteries. If nothing else, it will allow comparison between batteries on a level playing field. If you have hard discharge data on a particular cell, fell free to post it up with your interpretation on how it may fit in with said proposed guidelines.


I will start. This is what I think could be a good basis for battery ratings.



Continuous discharge rate with average voltage no lower than 3.3v/cell
Continuous discharge rate with no lower than 90% of 1C discharge capacity
Starting and ending temperature noted, along with any cooling and setup details. Temperature could be overlayed on the discharge graph.
Cutoff voltage should be 3.0v.

Burst ratings are much more difficult to pin down, but duty cycle may be a useful parameter to use with this.

10% duty cycle for burst rating (maybe 5%?), average pack voltage of 3.7v for instance.
Sustained duty cycle at 50%, average voltage at 3.7.
Pack should never drop below 3.05v or 3.1v under load.


To compare with enerland, they rate capacity aggressively. Their 2100LP cells generally hold closer to 2050 at 1C discharge, but at the continuous rated discharge at 18C yields right at 3.4v/cell and more than 90% of rated capacity. They do not show temperature or pack cooling during testing. They are up front that there is 70% capacity retention at 16C discharge after 50 cycles.


Please add your thoughts, objections, additions to this.

No thoughts or suggestions? I figure that one or two of you might have some insight. These can't be accepted standards if there is no feedback! I suppose I could just force them on you though :lol:

This sounds good to me! It would definitely help substantiate some manufacturers claims and weed out the packs with inflated ratings.

For continuous C rating:

Discharge graphs at 1C, 5C, 10C, 15C, 20C, etc should be done while measuring voltages and pack temperatures taken at 30 second intervals for each C rate.

The voltage drop at a particular current would allow you to graph internal resistance since it's non-linear and is a function of C rate.

The measured temps should then dictate the max discharge C rating the pack is good for. And anything over 120*F or 30*F above ambient (whichever comes first) should be considered non-useable.

Not sure what the minimum allowed voltage should be during the test for the associated C rate to be considered valid. 3.5v would be nice, but is that too optimistic? I think 3.4v would be acceptable.

For burst C rating:

This is a little trickier since I doubt anyone is going to agree what exactly constitutes a burst. Heli burst is probably different than land vehicle burst. And racing has a different burst profile than bashing. And what is the period (frequency) and the "resting" C value between bursts? We know current does not go to 0A in between these bursts, so that has to be taken into account. So, I would suggest using a worst-case test.

Discharge graphs of 15C, 20C, 25C, 30C should be take while measuring voltage and temps. A burst should consist of 2.5 second 10C "rest" period and 0.5 second bursts (~17% duty cycle). Anything over 130*F (or 40*F above ambient) should be considered unusable. That should cover basically anything.

Again. not sure what the minimum allowed voltage should be during the test for the associated C rate to be considered valid. I'm thinking 3.2v would be acceptable.

Results

Using the results from these tests/graphs, manufacturers could then rate batteries using something like:

This pack is rated for 20C continous with no less than 3.4v (for 90% of the cycle) @ no higher than 120*F temperature.
This pack is rated for 30C burst with no less than 3.2v (for 90% of the cycle) @ no higher than 130*F temperature.

Sounds similar to how amplifiers are rated "rated 100 watts RMS X 2 channels into 4ohms from 20Hz-20kHz with no more than 0.05% THD"...

I would be a bit more liberal with temperatures, around 140*f is my max temp for lipo. Of course as temp goes up R goes down. That is really what is missing in most discharge recommendations, and it may be why many cells do not seem to live up to specs. Testing a cell at 80*f and 120*f makes a HUGE difference in voltage under load. It really would be as easy as stating a few temps and the correlating bursts that the pack can handle without voltage dropout.

I like your idea for burst rating. I have come across 10 sec burst, 60 sec rest from a few factories, and that seems to be the same numbers I get when I talk to importers. The important point is that the method of determining burst is rarely disclosed by the retailer. I know of none that do right now.


Pack cooling will also play a larger roll. If airflow is used to cool a pack then the C ratings may be inflated since the temperature may not hit the cutoff.

yeah, the temps are a little on the conservative side, but you also don't want to give people the idea that it is OK to run them at 140*F because you KNOW people will push it (just like everything else).

As far as the rating, maybe change to:

This pack is rated for 20C continous with no less than 3.4v (for 90% of the cycle) @ 120*F (+/- 5*F).
This pack is rated for 30C burst with no less than 3.2v (for 90% of the cycle) @ 130*F (+/- 5*F).

And then a little note stating the absolute max temperature and the fact that performance will suffer with cooler temps. Also, maybe a little blurb to summarize the burst test: Burst = repetitive 30C rate for 0.5s with 10C rate for 2.5 seconds.

If the cont. ratings are 3.4v average it will exclude many many brands from their ratings. Even some of the enerland cells don't hold that.

This is where the 90% rule can come into play. If you can get 90% capacity at 3.3v under cont. load you are good. If you can get 90% capacity at 3.4v under cont. load you are better.

Almost too much to consider.

Ok, then, how about this:

This pack is rated for 20C continous discharge with no less than 3.3v (for 90% of the cycle) @ 120*F (+/- 5*F).
This pack is rated for 30C burst discharge with no less than 3.1v (for 90% of the cycle) @ 130*F (+/- 5*F).

The maximum temperature allowed for safe lipo use is 140*F. Performance will be reduced when running cell temperatures other than the ones listed. Burst rating is defined as the repetitive 30C rate for 0.5s with 10C rate for 2.5 seconds.

yeah, thats reasonable. The burst voltage@load will certainly be very relative to pack temp too. Man I really want to get some test equipment for burst now. I need to make some phone calls.

Get them... I love looking at graphs...

No offense, but I doubt you would be considered an "impartial third party" for some people. :wink:

Ideally, it should be someone with the right test equipment that the industry would recognize. That way, resellers would be willing to send sample cells to rate - heck they might even WANT to do this! Then, various resellers can post the graphs/specs of the cells on their site (maybe even a datasheet?) and put little rating stickers on the packs containing these cells.

Even though the testing isn't perfect due to the various variables at play, at least it would be a VERY big step in the right direction.

The only problem is that the people willing to spend the cash and time will generally never be "impartial third party". I really don't care that I won't be considered impartial. I just like having complete data when I make a decision on something and to me that is worth it. If anybody could be considered impartial with the knowledge to test this stuff, it would be you. Do you have the time and resources though?

I could make the time, but don't have the resources.

If I was to do it, I'd probably get a bunch of dummy loads and put them in combinations of parallel configs to get the approximate load needed. Then, hook the cell up to an Eagletree (or something similar) and measure voltages and temps in 10 second intervals for the continuous test, and 10-50 milli-second intervals for the burst test (to capture the 0.5s pulse readings). This is assuming the ET device can be modded to handle higher currents without insertion losses (only being used for voltage). Also, I assume the raw data can be saved? Need those for custom calculations and graphs.

That said, what reseller is going to go on the word of just some dude doing these tests in their basement? I was thinking more of an already established third party testing company.

A number of testers on rcgroups have really slacked off producing the graphs. There were at least two that were very good and had capacity to test at high current and power.

Why have the battery graph vault slowed down? Maybe it can't keep up with the changes and various vendors. I definately checked the graphs and like the format of the CBA tester. At least there's one constant (the current) a pull for each current is done and overlayed nicely.

I also like the way kunlang.. is sorting his currents all on one graph too. It is a lot of info on one graph.

Would you like to see single cells or pre-made packs?

It comes as no surprise that the two of you would be able to work out some reasonable baselines for testing of this sort. It would surely benefit the consumers the most and help the technology gain acceptance more readily.

I have a couple of thoughts on this that I'll toss out for your consideration.

Since there is no current standards per say why not set the minimum per cell voltage to the nominal cell voltage under its continuous discharge rating? I understand that this would likely shift the relative C rating we are used to seeing current packs rated at. But since the current ratings are vague and somewhat random why not?

I think that testing and rating should be done at 2 levels really. The initial tests should be of the individual cells from the mfgr's and then packs from the various builders should be done also since materials and methods of pack building vary widely and will contribute to actual performance and thermal characteristics in actual use. I know that personally I could care less about individual cell performance if there is any change once it is assembled as a 2s4p pack for example.

One of the things I'm unclear on is the degree that variations in operating temperature will effect performance of a cell. I have heard that some cells work best at 110 - 120 df for example. I can only assume that this may be somewhat brand specific as there are various chemistries being used in different cells. This may be something that needs to be considered if possible during testing.

One of the packs I have lists only the following concerning thermal conditions.
During discharge do not exceed 60 degrees C.
Do not charge in freezing temperatures (assumed < 0 degrees C)
Optimal charging temperature 20-25 deg. C

I would like to see some sort of testing that would reflect use in 1/8 offroad vehicles as it seems to me that they present somewhat unique operating conditions compared to boats/planes/helis and the like. This may not be needed but I have never seen comparative graphs of packs under high constant load and repeated extremely high peaks like you see in offroad use.

I may have more ideas after some sleep but thats all for now. Hopefully some of this makes sense or is possibly useful to you.

Seems to me that the most relative tests would mimic actual real world RC use. You guys have mentioned how spikes for heli-boat-plane-car are all different. Also the spikes are relative to vehicle weight and performance.

I would think that a massive collection of eagle tree data logs would be the best indicator of how to test the cells. An average of the data logs for cars would give the average need for the lipo. Burst duty cycle, amps between bursts, continuous, braking, ect. Devise a standard car cycle to run the lipo through, note the voltages and temps, calculate a "P" rating from there (performance). Devise a heli cycle, plane cycle, ect.

The trick of course is the test equipment, the environment, and the guy with the time and cash who has nothing to do with anything :)

Long term test would be good too. It might take long, but would help to see at what cycle the cell(s) starts to loose some capacity.

I personally like the CBA graphs at various "C" ratings. Various current ratings are even better. To keep it simple a single cell should be graphed at the various currents and a temp rise and max put in description at each current.

I like to look at th graph and make my own interpretation as to what I would "rate" the cell at.

Doing packs opens up another can of worms, but is far more realistic. If someone wants to discharge a certain pack at a certain "C" factory rating it is nice to see the results.

Question - Is it worth plotting charts under varying load - sign wave? - In real world you will hardly every draw constant ampage. I wonder how well different packs take this.

Personally, a graph using the testing procedures found in my previous post is best. It's not perfectly ideal, but should encompass the worst-case use a lipo will endure. At the very least, it will rate all cells using the same conditions and procedures for a better apples-to-apples comparison.

Yeah, that would be nice... Worst case scenario would be best.. I like it when batteries are underrated.. I usually underrate them myself by about 20% when figuring out speed, power, voltage & capacity.... It's always nice to have a battery pack that can perform whatever is marked on the sticker.. without any fine prints.. but I guess that would be hard to do..

LOL, I bet you'd love to be the person to perfom all these test... I know I wouldn't mind.. house full of LiPos, imagine that...

I think that both constant and burst testing will prove useful. The Constant rates will allow easy comparison between cells. The Burst rates will allow the true ability of a pack to be tested and empirically logged. Even 1/10th second bursts could be used to get an idea of how high a pack will spike before dropping out.

I can definitely tell which one is good :lol:. If they can handle our stuff, they can handle anything. A video of the test should also provide some form of where the data came from. A burst test should be done to confirm the pack can withstand the burst rating. Afterwards one should be able to establish a safe continuous and burst C rating on their own and not just go by what a factory puts on the cell. It all comes down to a little commom sense on the rating of cells and to avoid situations like the ones mentioned. All the batteries I've tested and owned does come with a factory C rating, but I like to beat them up a bit to see if they can live through it.

OK, so who's gonna spring for the equipment and begin testing? :smile:

It just sucks that we have to do a live test to see. It would be nice to know the true capabilities before they puff. ...Kinda like testing to see if food is poisoned by tasting it. :lol:

If I had a CBA, that's what I would do. Then people wouldn't have anything else to say. Who knows where those graphs came from. I can get you great graphs from Enerland, change a few things here and there to make it seem like it was done by me. Make a video of the test doing the 1C, 10C, 20C and burst ratings. That will definitely be the most effective way for me.

How about a graph where the cell is loaded to what ever amps it takes to drop it's voltage to 3.2v. Then as the test progresses the amps will fall off to keep the cell at 3.2v untill the amps hit 0. Note the temp rise from an established 80*f. Cutoff voltage may need to be higher or lower for the cell to never pass the "maximum safe" temp ~140*f?


It would of course take multiple tests to place the proper cutoff voltage with each cell. Example cell#X test 1, LVC=3.8v (test begins) tester loads the cell to what ever is necessary to drop the voltage to 3.8v (say 60amps then decreasing, dictated by LVC) cell temp only rises to 115*f during the test. Recharge cell to 4.20v cool it to 80*f. Cell#x test #2, LVC=3.6v (test begins) tester loads cell (say 85amps then decreasing dictated by LVC) cell temp rises to 135*f during the test. Again at 3.55, 3.50, 3.45---

So on and so forth untill either the max safe temp is hit or the LVC drops below 3.0v. At that point a graph is made, showing the cells maximum constant performance.

The variables for air flow would need to be set, and a tester would have to be fabricated. We could use a data logger to show the volts, amps, time, and temp in a graph.

Wouldn't an ESC with a soft cutoff (proportional to lvc) be able to do this for us? We would just need a load that will be able to stay constantly above the required amp draw.

Exactly. Do the test for several C points like how I originally stated. Eventually, you'll find the C rating that maintains the minimum voltage you are looking for. Some people may find 3.0v an acceptable cutoff, while others may accept only 3.3v as the cutoff. The graphs would show this and allow people to make informed decisions.

Then, compile all the graphs and make a little datasheet just like how electronic components are rated.

I don't know about other people, but my goal would be to provide the information for buyers and let them make the decision about what is adequate for their needs.

When finding the constant load on a battery with voltages as low as 3.2 or 3.0, we should find that the capacity does not hold up to the 90% standard. Drawing a battery at a rate that depresses the voltage to 3.1v/ cell average should make it fall out at a terrible capacity.

Well, the graphs would show that as well. The end-rating would take that into account, but the graphs would detail it out.

Yep yep. It will show the constant rate that drops capacity below 90%, or that falls below 3.X volts/cell average.

My question is whether there will be much variation from brand to brand. Since all lipos are made with similar construction there should be a fairly close trend on now far down the average V can be pushed before the capacity drops significantly. Of course inferior brands or brands with new manufacturing techniques may be able step outside of the norm. This has been a question of mine for a long time actually. Are enerland cells showing higher average V on continuous capacity only because they rate them conservatively?

I think that would be the case, but just a suspicion...

I think that the C rate is still useful info, but I also thing that varying the C rate to a constant LVC and maximum temp would show the ultimate durability and performance of the cell.

I also think that using pack that is assembled should be manditory. I've been pondering how the tabs have been failing on some brand name lipo's during use, but the manufacturer has obveously tested them at huge amp numbers. The problem I believe is external to the cell, it's related to the method of assembly. Specifically the manufacturer is spot welding the tabs and them folding them over. I'm confident that the pack no longer has the ability to perform to the specs that the individual cell had.

I think assembly methods could even cause localized heating at the tabs of the cell, causing heat damage while the rest of the cell is at an acceptable temp.

That seems to have been the case for the trakpower bricks. It seems that the current 25c packs are the same as the previous 20c packs that were just conservatively rated.

I must admit that I would prefer to buy packs that have been rated conservatively over the alternatives...

I agree that their seems to be no "standard" for providing the individual cell and/or pack ratings on Lipos - one manufacturer's "20c" pack is another's "15c" pack, and without reliable test equipment and a controlled environment, the information is somewhat useless and speculative - purchase decisions must be made largely based on whether or not the customer "believes" the ratings posted by the pack manufacturer - based on other's opinions and experience I guess.
I test my batteries in actual use with an Eagle tree data logger - before anyone asks, I don't plan to post graphs, but rather I use the information to make recommendations to customers who ask - I do take the job seriously when someone has a particular application and is seeking advice on a "safe" setup, and a 12c pack in the right capacity and application is often times a better choice for the consumer than a 25c pack with less capacity - ultimately depends on the customer's end goal.

I test everything in the same manner - starting with a fully charged pack at ambient temperature and discharging it in harsh use until it trips the LVC. I then scroll through the data to see pack voltage at given amp draw throughout the run(10 frames/second). I also test multiple cells/packs in any given session - same day, same temps, same conditions - this way I am "apples to apples" as much as possible.
In RC Cars and trucks, burst rate capacity is more important than constant discharge rate IMO - the average draw of any given run is typically below 30 amps, but I see spikes beyond 180 amps throughout the run. 100 amps continuos draw would yield a 3 minute runtime at 100% through-put with a 5000mah pack - this just ain't gonna happen, and is therefore fairly useless IMO.
I make by judgements based on whether or not the pack can maintain 3volts/cell throughout the run. Constant discharge rate isn't important(or possible to record with my method), so long as the pack can maintain 3 volts/cell throughout the run - at least this is my opinion. If it can't, then the pack isn't suggested for the application, regardless of its posted specs.
I have tested packs from every brand listed on my site(and then some), though I have not tested every individual pack. PolyQuest, Flightpower and NeuEnergy packs all have handled beyond their stated burst rates(at least up to 175 amps, which the most I can accurately measure - special praise to the PQ2500 packs, which maintain 3volts/cells up to about 70c!!). I have some other cells "in testing" that have exceeded the above packs significantly as well, but long term testing is still under way.
Anyway, these are my thoughts - until a standardized and non-biased test system comes into play, the only info I trust is what I can prove in actual use. :)

I like real world testing as well. But for the companies that do sell cells/packs, they should have the testing others and I mentioned to see how well they do with different C ratings. If the cell/pack states a burst of 50C then do a 50C burst to see what the cell/pack can handle. Like Mike mentioned about the average AMP, now that would be something a company can do as well. Try and get results on a constant discharge of various AMP rating. Get a constant 10A, 20A, 30A and so forth. That way we can see how well the pack can hold at those ratings. I have seen spikes of over 180A, but my highest average so far has been the most 35A. I also like how my 2200mAh packs can handle spikes of 180A+ and still provide plenty of juice for a 15-20 minute run. This is not race conditions.

Have you guys seen this before?

It appears to be a step in the right direction.

It sure is a step in the right direction. All they need to do now is have repeat tests and find out how many cycles it take for the battery to start loosing power. The only problem is it'll take a long time and lipo cells are getting better.

Those may be Common Sense ratings and perhaps their batteries too.

A little bump for new discussion.

John, more than C ratings, I'd like to see some SIZE standards. As a pure basher I have no real need for insane discharge abilities, but it has been quite irritating to be limited on pack dimensions. Seems like many of the better performing packs were designed for RC aircraft, which are more adaptable to the available space. Even the cheapo Rhino 2S 4900's, which I would love to try-are too long for my application. I see Hyperion is the first to address this with their Swift 4000 packs, those are 20C rated (and I would trust Hyperion to be honest with that) which is fine for what I want to do. Hopefully the the others will follow suit.