d0uglas

Hey now everybody. Because continuous safe discharge rate/C rating is, unlike capacity for which anyone with a $12 wattmeter can test, a bit of an arcane, nuanced thing that's hard for average consumers to test themselves (if they know or care what it means), there is pervasive, wholesale lying by battery vendors, exaggeration of this particular number, and correspondingly they jack up the price of their "100C" batteries. It's apparently profitable to do this and there's very little risk in doing it, so it seems.

I want to publish the true safe discharge rates of batteries of various brands based on my own testing using the Wayne Giles ESR/IR meter. I slapped together a crude website and a table of data I've collected on a small handful of batteries so far and outlined the procedure I follow when testing each battery. Obviously, it's absolutely critical that I have a procedure in place that yields accurate data and makes that data stand up to any hostile reactions from battery vendors irked by what I am trying to do.

The site address is https://lipo.lol

I don't know much about physics and electricity, whereas many of you do. Whenever I want to learn something both complex and RC-related, for example the rules of thumbs for motors and watts, I'll append site:wattflyer.com to my google search come up with this gem of a thread. So if you could kindly eyeball the site, namely the procedure outline toward the bottom of the front page, and let me know if I'm doing it right or not, I'd be most grateful.

Cheers, Doug

Wildflyer

Very interesting.
I have considered buying one of those Wayne Giles meters, just don't have the extra money for it right now.

I would also be interested in how the actual C-rating changes over time and use.

Seems like we should have the WG meter and test each battery before we put a high demand airplane in the air. That is is not the place to find that you have a weak battery. (found that the hard way, landed an 8' plane in a tree)

d0uglas

Thanks Wildflyer. To offer a "use case" of the site, $27 plus shipping is a bit much for an 1800 3S (unless of course it's truly 75C) and you want the elbow room as you intend to fly it with your 40A-consuming VisionAire, you don't want it to die on you in a short period so it's worth the extra twelve bucks you figure. While it's possible I got a bum pack from a "bad batch," I want that man to know that this 75C battery after only four gentle 1C cycles I ran is testing at 22C which is already less than 40A before he clicks the paypal button.

As for the degradation over time, interesting suggestion -- a C test of a fresh battery that I offer may be less relevant to people than the battery's test results after twenty flights. I'd like to give both, or chart out every flight. But I only have so much time, and then the data becomes cluttered and the procedure less uniform for each battery. But it would make the site more useful. My purpose isn't just to shame vendors, I want to offer helpful data.

Having tested my older packs, since I bought this meter and moving forward I will be much more disciplined in leaving them at storage levels after flights. Once in the winter, when lipos struggle more (Giles advises somehow heating your packs), I was flying with what I thought was over eleven volts in the tank and then wham, 9V out of nowhere. The plane crashed into a cemetery, no need to hire a herse. Brought the pack to the basement, accidentally dropped a wrench on the battery from only three feet, WOOOSH, I barely manage to grab the ec3 tip to hurl the flaming thing out of the basement hatch without frying my flesh enough for yet another ER visit.

dereckbc

Couple of comments.

Been a PE for over 40 years working with batteries and all major manufactures. What you are really talking about is the Battery Internal Resistance (Ri), and most of the good Hobby Chargers will read Ri. Once you have Ri, it is just a simple math formula to calculate the C-Rate. No need to buy a special uni-function meter when all you need is a multi-tasker hobby charger that does everything for less money. Fine if you can afford it, but it is a dust collector.

Secondly you are reinventing the wheel. Those list already exist.

In the end what you will find is very few batteries actually have the specified C-Rate. What you will never find is any battery with a true C-Rate of greater the 30C, and not many will test above 20C.

d0uglas

None of my cheap things that detect IR gave consistent values and after reading about this meter, for example how it takes one measurement, draws a significant current and takes another measurement during that pulse, I was eventually sold that this is the most accurate way to get C ratings within my reach.

So for the purpose of the site and to share with the men at my club it was worth it, as well as to use for my own regular flying purposes.

I see your points, and I'm coming to grips with the the realities of C ratings you noted. I'd still like to put some of this data in one place, in part as a buyer-beware message along the lines of what you just wrote.

Thanks Dereck.

d0uglas

Been flipping through your threads Dereck, learning along the way. Gotta say parenthetically that this site is uniquely humbling, the wealth of knowledge here that I don't yet have but would like to acquire. Like at my club, I'll be the rookie for a long time, but lucky that the veterans are eager to help and teach. Thanks again.

Turbojoe

I've had Wayne's ESR meter for almost 3 years now. Almost wish I'd never bought it. Makes me cry every time I test one of my so called High "C" packs. For me Glacier and Gens Ace consistently test closest to their claimed 20 or 25 "C" ratings. The worst packs I have are Thunder Power. Regardless the "C" rating not a single one tested above 23 "C" and IR between cells was all over the place! These were 4 brand new packs carefully cycled three times before testing. They've never been and never will be in an airplane I care about. I still have another 2 T.P. packs brand new in the original packaging sitting in the fridge. I'm not going to waste my time testing them. T.P. used to be the best on the market. They sure went swirling down the toilet.......

Joe

Stay Quiet

T.P. swirling down the toilet......

Turbojoe

And I wondered if anyone would catch that......

Joe

dereckbc

Doug no big deal. All you need is a volt meter and a couple of resistors. iCharger line of chargers uses Delta Voltage to measure Resistance. Super easy to do with any charger.

What you are describing is Delta Voltage method and here is how it works. We will make two voltage can current measurements any hobby charger can do.

1. First current measurement is going to be our High Current Measurement. So we hit the battery with say a 10 amp load and measure the voltage. We record both the voltage and current. Example let's say we measure 9.9 amps we label A1. Voltage is say 3.6 volts we call V1

2. We repeat the same test at a much lower current say 1 amp and we get say .99 amps we call A2 with a voltage of say 3.7 volts we call V2.

3. OK just some simple math is all that is left to be done.

Ri = [V2 - V1] / [A1 - A2]
Ri = [3.7 - 3.6] / [9.99 - .99]
Ri = .1 / 9 = .011 Ohms

Method 2 is even easier and only takes a resistor, volt and current meter. Any hobby charger can do this.

1. Measure the Open Circuit Voltage (OCV) of the battery say 4 volts. Now apply a load of say 10 amps which will require a .4 Ohm 50 watt resistor and measure both voltage and current. Say you measure 3.9 volts @ 9.0 amps.

Again simple math.

Ri = [OCV - Loaded Voltage] / Current
Ri = [4.0 - 3.9] / 9
Ri = .1 volts / 9.99 amps = .011 Ohms

Don't get me wrong, nothing wrong with the ESR meter other than it is a UNI-TASKER and expensive. It works fine, but if you have a hobby charger you can get the same information if you know how the measurement is made, and use the money saved on the ESR meter for something else other than a dust collector.

OK here is what I have learned. Buy quality batteries. I have tested quite a few brands and the 3 best brands I have tested are Thunder Power, Gens Ace, and Glazier. Of those 3 Gens Ace and Glazier are the best value, and if money is no object a slight performance boost using Thunder Power.

One more note. Of all batteries I have tested and yes even a few Thunder Power Elite 55C Series have never tested greater than 30C. Moro of that story is do not buy any cell over 30C rating because it is a flat out BS. Where I have ended up is I use Gen's Ace 25C lineup. Save my coins, and about 2 or 3 times a year Gens Ace puts batteries on sale and I gobble them up. Example 3S 2200 mah for $14. A Thunder Power cost $30 fro the same battery and equal performance.

Give you one more fabulous tip, one that will double your battery cycle life. Real damn simple lower charge voltage from 4.2 to 4.1 volts. That simple. All you give up is 3 to 5% capacity, but double you battery life. Last comment is when you finish a flight, immediately feel the battery with the back of your hand or any other body part sensitive to heat and cold like a great big pair (.)(.) A good battery will only feel warm. If hot, time to retire battery because C-Rate is to low or Resistance is now to high.

d0uglas

That's gold. Especially the tip on 4.1V.

In defense of my blowing money on the uni-tasking meter in spite of presumably being able to follow the steps of both of those methods, I am not a professional engineer (nor a mathematician) of forty years, and I am still a rookie to the hobby, especially relative to those hereabouts and to the men at my club.

It is important to me that the data I put out is not only accurate but is also received with confidence that I didn't screw something up. My having done my best to make sure the pack temperature is 22C and to press a well-regarded single-purpose device's single big red button a few times inspires more confidence in the data's accuracy than relying on me to do what that device does in manners that require both more steps and calculations (however simple). And that makes me not regret the purchase. Though now I want to try it your two ways if only to learn.

That said, if you decided to make this site too, I'd probably turn to yours for information over mine. I'd learn quicker -- instead of a big messy table, I could simply read "Hi. Don't buy over 30C, stick to these three brands, charge to 4.1V, reallocate your $95 uni-tasking meters toward some flowers for your wife for putting up with your hobby obsession EOM" and have myself a better, more cost-effective hobby.

By the way, among others, last night I read the whole Smoking Motor thread. Fascinating. For example I had no idea this sort of mathematics is at our disposal. Better late than never to start learning... Obliged everyone.

solentlife

I have a dedicated meter for Ri as well as my chargers that give readings ...

My dedicated meter has been back in its box for over a year now ... and to be honest I rarely use the chargers function unless I have reason to believe after flights that a pack is down.

I think like many - we just get used to thinking high C rate does not exist - you only have to look at the wire gauge to see how ridiculous claimed C rate can be ! We then just buy based on what we expect to see.

Nigel

dereckbc

You cannot defend yourself when you get caught with your hand in the cookie jar. Your busted.

Seriously no defense necessary, and I apologize if I made you feel that way.

My point, and the point others are making is the Tables you are trying to generate already exist. Might not be the exact model you have, but good enough to tell which manufactures are honest. They will also confirm by the absence of any 30+ C-Ratings. Now to be fair, you will occasionally se one test over 30C, but they are rare, far, and few between.

I am the odd man out here because I do LOG most of my Batteries capacity and Internal Resistance. However not as frequently as I did. I do not do that to check C-Rate, I do it as an accurate measure of battery health.

Example I have a lot of 3S 2200 mah 25C and 30C packs. When I receive new batteries, I cycle them 3 or 4 times, then record Capacity and Ri at storage voltage at room temps. That alone confirms a good battery from the start, and having that info makes it a snap for warranty replacement when I do occasionally receive a bad battery. The main reason is to know when to retire the battery from the flight line. When I see either the capacity decreases to 80% or rated capacity, or Ri doubles in any one cell, it is time to retire the battery. New a 2200 mah battery cell Ri is around 4 to 6 milliohms or 12 to 18 milliohms total on a 3S pack. Go plug those two numbers into a calculator.

What I can tell you from experience, and I am sure other experienced members have learned is when you land and have a battery warmer than normal battery, you have a PROBLEM. Its internal resistance is too high and C-Rate has crashed. They are directly related. As batteries age, the internal resistance goes up. As resistance rises the battery operates at higher temps, and C-Rate diminishes.

If you look at the formula, the resistance of the cell and AH Capacity are the two variables used to calculate to limit battery cell heating to 6 watts per AH of battery capacity.

First step is to determine the maximum current the battery can safely deliver without bursting into flames. To do that we have to use a heat value of 6 Watts per AH of battery capacity We limit that to 6 watts per AH of battery capacity. Example a 2200 mah battery has 2.2 AH x 6 watts = 13.2 watts.

Next step is to convert 13.2 watts to Current or Amps. That is where the Internal Resistance comes in and a simple Ohm's Law equation Current = Square Root of Power ÷ Resistance [A = √P/R] . So we measure say .006 Ohms. So we take 13.2 and divide that by .006 = 2200, and we now take the square root of 2200 equals 47 Amps maximum current that battery cell can dissipate without over heating or limited to 6 watts per AH.

Now we solve C-Rate = Max Amps / Battery AH.

So 47 Amps / 2.2 = 21.3C Good enough for a 45 amp ESC and motor. Take away here is if resistance goes up, C-Rate goes down.