100 cycle lipo test on different brands

[GiantShark]

This article was written by Bob Smith from QEFI

Testing to find the cyclic Life of Lithium Polymer Batteries.

This is the second part of a two-part article dealing with the cyclic life of lithium polymer batteries.
Part 1 - dealt with the general specification of LiPos and their performance, and with the testing equipment I have available to
carry out cyclic testing of them.
Part 2 - will cover the test procedure and the results of testing a series of packs.

Test procedure.

The equipment I detailed in Part 1 operates to the following pattern. I try to start all of the tests at the same
point in the charge/discharge cycle and although this could be anywhere I try to begin with an empty pack since this is
easy to eastablish by checking the pack/cell voltage under load (nominally 3 cells at 3 volts each). The first stage in the
cycle is then a normal balanced charge using the recommended pattern of a 1C constant current up to 4.2 volts per cell
followed by constant voltage as the charge current reduces down to 10% of the 1C value. This normally takes around
70 minutes and is followed by a 10 minute pause or dwell to allow the pack to stabilise. The discharge follows at a rate
which is normally the manufacturers’ maxmum recommended value e.g. 20C, and the discharge is stopped when the
poorest cell in the pack reaches 3 volts. This is again followed by a 10 minute dwell before the whole cycle begins
again. You can see a pictorial representation of this single cycle in graph1. The equipment is designed so that this cycle
is automatically controlled and repeats as long as needed to reach the required number of cycles. The data produced by
each cycle in terms of the pack voltage, elapsed time, discharge capacity, and pack temperature is recorded by data
logger and periodically downloaded to a computer.The set-up I am using with the logger gives me around 27 hours of
recorded data with each cycle taking about 1 hour 40 mins so I get 16 or so cycles in each batch which means six days
for the 100 cycle test.
One of the problems associated with this type of long term testing is the vast amount of numerical data you
produce and I am only able to include examples of the data in the form of a typical HiBox plot and a set of
temperature/time figures. Even here you can see that the HiBox plot of only 20 cycles is crammed to the point where it
is difficult to interpret the data visually. Remember that the temperature data is a dual purpose record since it also
provides a back-up discharge capacity. Since the discharge is at constant current the product of that value in amps and
the time period of discharge in hours is the discharge capacity in Ah. In the event, the main recording system via the
HiBox proved to be sufficiently reliable that no back-up was needed.

Data Interpretation.

The basic intention of the whole programme was to determine the loss of capacity suffered by the test packs
over the 100 charge/discharge cycles. Of the two traces on the plot, the blue voltage values are basically a check that
the process is proceeding as intended and that the maximum charge voltage does not exceed 4.2 volts per cell, and that
the minimum discharge voltage did not drop below 3 volts per cell. The values on the chart are for a 3 cell pack so are
12.6 and 9 volts respectively. The balancer built into the equipment did control the discharge cut-off on the basis of the
worst cell, i.e discharge was ended when the lowest voltage cell dropped to 3 volts.
The second green trace is for the discharge capacity and is accumulative, i.e. each step in the trace is the
discharge magnitude for that particular cycle in Ah but the scale reading is the total of all the discharges since the test
commenced. Over multiple cycles the vertical scale would be difficult to interpolate accurately but the HiBox software
has a floating cursor which gives the reading of any point on the trace to two decimal places (10 mAh).
Once a hundred cycle test is complete, each of the discharge capacities is measured and a graph plotted of this
capacity against cycle number. There is, of course, a fair amount of variation in these values which arises partially from
experimental deviations and partially from timing and other minor errors. In spite of this there is clearly a downward
trend in the plot which indicates a loss in capacity. If a mean line is plotted through the fluctuations (which are only
some tens of mAh in value) then, in all of my results to date, this produces a straight line with a negative (reducing)
slope. This implies that the capacity of a LiPo battery degrades with use, that the rate of degradation is constant and
cumulative, and that the quality of battery can be assessed by the magnitude of the degradation.

Test Results.

The data I have gathered could be used in several different ways to measure the performance of the packs but
in this article I am only referring to the loss in capacity over 100 full charge/discharge cycles. You could start with the
manufacturer’s specified capacity but this is usually measured at a very low rate of discharge and I have tested the
packs at a much higher rate. All batteries produce a lower capacity if they are discharged under high loads so it would
be unfair to base the assessment on the capacity on the lable. The figures in the table are therefore calculated by taking
the the capacity of the mean line at 100 cycles and subracting this from the capacity of the mean line at 1 cycle and
expressing this as a percentage of latter value. In graph 2 you will see that the indicated values are 2.08 Ah at 1 cycle
and 1.92 Ah at 100 cycles giving a loss of 0.16/2.08 x 100 = 7.7%. In terms of the use of these packs it is obvious that
we want this value to be as low as possible.
The table of test results presented here has been compiled from two sets of batteries. I had already completed
the tests on several packs before I was invited to write this article, and in addition the editor contacted a number of
manufacturers/retailers to submit packs for testing specifically for the article. The combination of makes is now fairly
broad but remember that new makes of LiPo batteries seem to be hitting the UK market on what seems like a monthly
pattern, so there are still some gaps. There are also some slight anomalies with the discharge rates. Although the
majority of the packs are 2100 to 2400 mAh packs rated at 20C discharge, some are of different capacities and ratings. I
attempted to be as consistent as possible with the 20C test and used this value even if the labelled rate was higher, but it
would have been unfair to use it where a pack was rated at less than 20C and in this case the specified maximum was
used. In all cases the packs survived the test regime although some were producing very low readings by completion.

Analysis.

There are several points to make about the results.

1) In terms of the cyclic life the results are probably better than I expected. All of the packs lost capacity but
some lost very little and some a significant amount. Although the losses appeared to be linear over this 100
cycle span, I suspect that they would become progressive (at an accelerating rate) if the testing had been
extended, especially with the poorer performing packs.

2) There are also large differences in the comparison between the rated capacity (the value on the label), and the
initial test capacity (cycle 1) at maximum loading. Remember that the percentage loss over 100 cycles is based
on the latter value rather than the former, but it is equally important to know how much you will get from a
pack under load when it is new. It is clearly not always possible to rely on the value on the label.

3) There is absolutely no indication that LiPo packs should be bedded in by slow charge/discharge cycles when
first used (as we were advised to do with NiCd and NiMH packs).

4) I hope that the data provided in this article is of use to the modellers reading it, but I have to add a word of
caution. I believe my equipment and procedures are sound and reliable, but they are not up to the standards of
a commercial testing laboratory in areas such as calibration. In addition to this, I am reporting on the results of
testing a single sample of each pack type and statistically this is obviously not ideal. I am repeating the results
I obtained from my tests and I hope you have found it interesting.

Contacts.

OverTec, Jesmond Dene Trading Estate, Forton, Nr Lancaster, Lancs PR3 0AT – Tel 01524 793328 website
http://www.overlander.co.uk

West London Models, 214 High St, Harlington, Middlesex, UB3 5DS – Tel 020 8897 2326
Websitehttp://www.westlondonmodels.com

Puffin Models, Unit D3 Backfield Farm, Wotton Rd, Iron Acton, Bristol. BS37 9HD – Tel 01454 228184
Websitehttp://www.puffinmodels.com

Ripmax Ltd., Ripmax Corner, 241 Green Street, Enfield, EN3 7SJ
Tel: 020 8282 7500 Fax: 020 8282 7501 Website http://www.ripmax.com

BRC Hobbies, P O Box 226, Whickham, Newcastle upon Tyne, NE16 4WU
Tel 0191 4887879, website -www.brchobbies.com

Electrolite RC Ltd., 6 Westmoreland St. Harrogate, N.Yorks, HG1 5AT email info@electrolite.eu
Website http://www.electrolite.eu

ModelPower, 3 Church Walk, Mancetter, Atherstone, Warwickshire, CV9 1PZ
Telephone: 01827 711501 email james@modelpower.co.uk website http://www.modelpower.co.uk

J Perkins Distribution Ltd., Northdown Business Park, Ashford Road, Lenham, Kent ME17 2DL
Telephone: 01622 854 300 email sales@jpmodels.co.uk website http://www.jperkinsdistribution.co.uk

RCM Direct, 9 Kenilworth Close, Hemel Hempstead, Hertfordshire, HP2 4EY
Telephone: 0845 205 5050 email websitehttp://www.rcmdirect.co.uk

Graupner GmbH & Co. KG, Postfach 12 42, D-73220 Kirchheim/Teck, GERMANY.
Website http://www.graupner.com

Photographs etc.

RCMW2.pdf
Page 1 - An assortment of the packs tested.
Page 2 - Graph 1
Page 3 - Graph 2
Page 4 - Typical Temperature/Time data record
Page 5 - Typical HiBox plot of cyclic voltage and capacity.

[pauljack]

ALL MY FLIGHTPOWER PACKS HAVE LOST SO MUCH CAPACITY!VOLTAGE RETENTION IS VERY POOR AFTER A FEW CYCLES.
ID LOVE TO SEE FLIGHTPOWER EXTRACT THE CLAIMED 168 AMPS FROM THEIR 3300MAH 3S PACK!

[whitemice]

tghj

[pauljack]

What lipos are you using now?

[Wayne Giles]

Whitemice
I agree with your points about lipos in general with regard to overclaiming and overpricing. I only know of Loong, Kokam and one other where the packs will actually deliver claimed currents. Unless the ESR of the pack is low enough to keep heat dissipation within reasonable bounds, then the pack is damaged and the cyclic life will be very short. Loongs have not been subjected to the 100cycle test, but I would expect them to come through with flying colours as they display extremely low ESR and therefore low stress levels during discharge. (I stand ready to be proved a plonker if Bob Smith runs a 100 cycle test on them!! - they may degenerate over 100 cycles for some other reason) The low ESR is also why they will actually deliver claimed current levels.
Remember that the stress is a function of the SQUARE of the current you take from a pack; double the current generates four times the heat and stress. ESR also rises at lower temperatures, so low starting temperatures are bad news.

I would offer the following for reasonable pack life:-

(1) Unless you are using the above named packs, derate your max "C" rating to 60% of the makers claim.
(2) In present (winter) conditions, derate to 40%, certainly at the start of a flight. I know you want to use full throttle at take off, but that is when the temp. is low and the pack most vulnerable to damage. Even Loong and Kokam struggle at low temperatures.
(3) Don't assume because a XYZ 2200 pack is good, that a XYZ 3700 pack will be the same. I tested a 2600 pack which was very good, bought two 2200 packs from the same maker and they were both identically rubbish.
The problem with finding which are the best packs is that you have to buy one to find out. Whoever started this wild overclaiming caused the problem. If you have a 15C pack for sale and tell the truth, nobody will buy it so now most people overclaim to keep up.

You notice that Bob's 100 cycle test article which was written for RCMW, at their invitation - but never published!! (?)

Wayne Giles

[whitemice]

tghj

[Wayne Giles]

Whitemice,

Must be your lucky day! Saw your post and would you believe, it just happens that I am in the middle of testing a Loong 5S 4300 20C pack! I had already done a 15C (65A) discharge run and was wondering whether to go straight to a 20C (max rating) run or to do an intermediate one at 17.5C which is, of course, 75A. So just for you I have done a personal 75A full discharge on the pack this afternoon!! I will post the results in a day or so as a full set of results in 'Product Talk' as I did with the 3S 2250 30C pack last week. To allow you to sleep, I can say now that at 75A the pack starting from 21deg.cent showed no sign of voltage "sag and recovery" which is good, and showed 17.78V at 50% discharge with a capacity delivery of 4.14Ah and 73.3Watt.hr and a temperature rise of 30deg.cent. (51deg final)
This means that two packs will give you over 35V average (Note I measure at balance connector as your power connector is up to you) and the packs will deliver 146Watt.hrs. A good rule of thumb is that allowing for motor, connector and ESC losses, 10 Watt.hours will give you about 1 Horsepower.minute at the prop., so you would get about 14.6Hp.mins. eg about 2.5Hp for 6 mins etc (NOT 14.6HP for 1 minute!!)
If you have Medusa Powerplus software loaded I can send you a file to view the discharge curve, assuming I can drive my computer. If so send me an e-mail address.

Wayne Giles

[whitemice]

tghj

[gmb42]

btw, rcm&e are not going to rock the boat with advertisers who are willing to pay a few grand for a few pages, and i am aware of FP using various threats and tactics in many different areas and places.

Not wanting to pick on RCM&E in any way (who may or may not be guilty of this), this attitude is unfortunately understandable, but doesn't help the paying customers sort out the wheat from the chaff. Unfortunately this will always be the way of the world when magazines (and websites) rely on advertisers to enable them to produce their words.

The need for accurate info was part of the reason for my post in response to Wayne's thread thanking him for the test results for the Loong Max 2250 packs.

I do wonder if some of the "bad" reputation of Lipos is due to folks taking manufacturers at their word and subsequently overstressing packs due to the inaccurate ratings.

Graham

[Wayne Giles]

Graham,

I agree with your last statement , that the bad reputation of Lipos is largely due to the fact that most manufacturers do wildly overclaim "C" ratings on their products. The customer is hardly responsible for believing their claims and I am not suggesting you meant that, but people like me are seen as cynics. The fact is that most engineers live in the real world and know that PR will not change physics. Sales people have always been optomistic but here that has spilled over to the point where they must know they are seriously misleading customers.
I am looking at the possibility of setting up a site to publish independant tests. The two potential problems are cost of the packs (I cannot see many suppliers volunteering their products for test) and the hassle I am likely to get which I don't need as I am retired and only do it as a hobby.
BTW it was RCMW and not RCM&E which the original article was written for.

Wayne