High-rate Li-ion batteries - Page 1
Forum section: electronics / general

author: muvideo
author_rank: Frequent Contributor
timestamp: September 30, 2012, 03:52:47 pm
content: This isn't a teardown (yet)while performing the first charge on a pair of cheap lithium batterypacks I tought to perform a pair of tests.These are for fun and because I love graphsThe victims are lithium batteries for model airplanes with afunny name: “Loong-MaiLoong-Max Tipple 25C”nominal capacity 1,8Ahnominal voltage 11,1V 3 cells in seriesweight 153 gramsdimensions about 100mmx 33mm x 20mmHere can be seen the main power wiring and balance wiringwere all the cells come out, it's useful for connecting sensewire for the 2 psu I used to do the tests.For the ones (myself included) not accustomed with recent modelling batteries,these are intended to be discharged and recharged as fast as possible.Their life will be probably made of discharges at huge currents for 300-400sfollowed by wild charges of less than an hour, repeated for few tensof cycles and eventually disposal or, for the more unlucky, inflation, fire and explosion.The manufacturer claims that these packs are able to withstand dischargesof 25C (that for the cells in question translates to about 45A) but more noticeablythey can be charged to 5C i.e. 9A.Many know that fast charging of lithiums is not a good thing because1- their life is shortened2- you dont really gain much on charge times: doubling charge current usuallyreduces only marginally total charge time.The lithium-ion charging.This chemistry need to be charged in a way similar to Pb batteries:CC-CV, that means limited current in the initial phase, the voltage willraise and when it will arrive to the limit (usually 4.2V, but dependson the manufacturer and internal cell chemistry) maintain thevoltage while waiting for the current to decrease to a preset point.Since the cells dont have any mean to cope with overcharge,individual cells need to be balanced by the charger, usually atthe end of charge, and usually the balancer simply shunts theindividual cells to maximum voltage, waiting that all the cellsin the series stop taking current. There are many more thingsto do to have a long life for the cells, and battery managementis much more complex, but for these high rate batteries lifeis simple (and short)What I have done is simply to charge and discharge the batteriesand log the values, I was interested to see the real capacity andif there was any charge time gain using higher currents, as themanufacture implies.For the discharge the results are what you would expect froma very low internal resistance cell, given the rates I used thecells performed very well, they should be able to do muchmore when stressed.As for charging the surprise is that these cells really are ableto fast-charge. It can be seen that current falls rapidly oncepast the CC phase and the stored capacity at the end ofCC phase is more than 90% of total capacity, that is very good.Some numbers and graphs.Once manually balanced charging individual cells,batteries were charged and discharged at2C 3.6A and 4C 7.2A, end of charge was 12.6V (4.2V per cell)and 200mA, end of discharge was 9V (3.0V per cell).Between each charge and discharge the celle wereleft at rest for 2 minutes.Initial capacity is a little more than nominal,1850mAh and 21.1Wh at 2C, 1830mAh and 20.7Wh at 4C,Energy density should be about 140Wh/kg, not bad considering theheavy wiring.At 2C total charge time was 2200s that is slightly more than 36minutes. CC phase was 1700s, about 28min in this time thebattery stored 95% of total capacity.At 4C total charge time was 1420s or 23min 40s.CC phase duration was 813s or 13min 33s storing 90%of total charge. So doubling charge current doesnt reduceto half total charge time, but the battery can be rechargedto 90% in only 13 and half minutes.During 2C charge the battery temperature remains almostambient, while at 4C starts wo warm up few °C over ambient.All this performance should come at a cost, I expect that lifeexpectancy will not be very high, and I'm doing a few cycleson one of the batteries to see if these is an appreciable capacityloss, and I will also check if the cells will maintain balance.Voltage and Current versus Time, 2C and 4CVoltage and percent of nominal capacity:Voltage and Capacity vs Time:Comparison of 2C and 4C charge and discharge curves:To be followed.Fabio.

author: T4P
author_rank: Super Contributor
timestamp: September 30, 2012, 05:09:23 pm
content: It's not a Li-ion. It's a Li-Po .. Li-ion no matter how you manufacture them they cannot take such abuseAlthough Li-Po do blow up easier, but it takes quite some abuse (like 230V or overcharging) to blow them up unless you aren't using a balance charger, the cells need to be balanced externally and the lower W/kg is due to the fact that this is a VERY high drain and can be charged at high C rates without it giving up the ghost fast like normal Li-Po's do.Maybe you know what i said already, maybe you don't, but i'll just say itI delved into RC high-drain world because recently i bought a 300W-400W RC Brushless motor that was on clearance. What is your reason?

author: muvideo
author_rank: Frequent Contributor
timestamp: September 30, 2012, 06:15:56 pm
content: Quote from: T4P on September 30, 2012, 05:09:23 pmTwo days ago I repaired my brushless controller, crappy soldering work.I had to rework all the mosfets.This will be the intended uselast time I played with these things there were still NiCd around...

author: muvideo
author_rank: Frequent Contributor
timestamp: September 30, 2012, 06:57:06 pm
content: Quote from: T4P on September 30, 2012, 05:09:23 pmForgot this part.As for the definition, I know that modellers call these Li-Po, but as far as I know they are notsolid polymer electrolyte, probably they are more similar to li-ion, infact dont like over or undervoltage,and take fire if abused.As for High rate li-ion cylindrical cells, there are also IMR (the ones used in power tools) that should beLithium Manganese instead of Lithium Cobalt, and are capable of high rates and should be moredurable that these RC batteries, but are also heavier and more costly.Fabio.

author: T4P
author_rank: Super Contributor
timestamp: September 30, 2012, 07:10:54 pm
content: So far they have been very durable for me

author: muvideo
author_rank: Frequent Contributor
timestamp: October 01, 2012, 11:03:40 am
content: And now the results after 20 cycles, 2C chargingand discharging.Here are the cycles, there was a power outage thatscrewed one of the discharging cycles:The battery holds decently, but the aging is noticeable.The stepped graph is the discharge capacity, in the last cyclesthe capacity recovers because of power outage, that triggeredtwo charge cycles without discharging the battery, the straightline is a linear regression with an aging of 1.7mAh per cycle:So the cell age should be around 1.5-2mAh per cycle.If they keep this rate, the battery will be at 80% initial capacityafter about 200 cycles.As for balance, at the endo of charge the cell voltage difference wasbelow 2mV, so no noticeable imbalance in 20 cycles.I have somewhere a cyclindrical Lithium-Spinel cell, it's anotherhigh rate chemistry, and the manufacturer sayt that it has a verygood aging. I will put under test that and check it against this one,I think will be interesting.

author: HKJ
author_rank: Super Contributor
timestamp: October 01, 2012, 03:18:30 pm
content: Quote from: T4P on September 30, 2012, 05:09:23 pmTry checking A123, they can take a lot of abuse. I tested a cell up to 30A discharge:http://lygte-info.dk/review/batteries2012/A123%2018650%201100mAh%20%28Yellow%29%20UK.html

author: T4P
author_rank: Super Contributor
timestamp: October 01, 2012, 03:30:17 pm
content: Well i withdraw my statement, not as well as Li-PoJust don't overcharge or do anything silly

author: muvideo
author_rank: Frequent Contributor
timestamp: October 01, 2012, 04:13:21 pm
content: Quote from: HKJ on October 01, 2012, 03:18:30 pmYes, that are Lithium Iron Phosphate chemistry, lower density and voltage, butvery tolerant to abuse, and if well built should have very log life, A123 is a manufacturer(for the specs, THE manufacturer) and their Rate and life specs are impressive.Lithium chemistries are becoming confusing, manufactures list 4 or 5 different formulaitons,each with slightly different characteristics.See for example samsung's page of li-ion cells:http://samsungsdi.com/battery/cylindrical-rechargeable-battery.jspNow I'm testing the INR18650-15L:I found in a small battery powered tool:

author: muvideo
author_rank: Frequent Contributor
timestamp: October 02, 2012, 09:45:28 am
content: Like I mentioned before, I opened a power tool battery to test theINR18650 cells it contains.These are high rate Li-ion cells used in recent power tools, themanufacturers usually tells that they are capable of high currentcharge and discharge with good life expectancy.Into the Metabo cordless driver I found 3 samsung sdi INR18650-15Lcells, and a thermistor, nothing more, no safety apart probablya ptc fuse glued in the center.The only document I found is this:http://www.batteryspace.com/prod-specs/6615.pdfthat is of -15Q version of the cells.The battery weight without plastic shell is around 170g,maximum discharge current around 10A and max charge 4A,with a 2.5V minimum voltage.I tested with 2.75V per cell discharge cutoff and 100mA 4.2Vper cell charge cutoff.The rates tested are 2C 3A and 4C 6A for discharge, whileI used 2C 3A and 2.67C 4A for charge, now the batteryis cycling at 2C to see if there is any evident aging.Mnufacturer says that internal resistance should be around25mOhm, that makes a total 75mOhm.If we ignore the eventual fuse (I should try to find if thereis any excess resistance in the series) we should seea voltage drop of around 0.2V at 2C and 0.4V at 4C.There are the Voltage-Current curves for 2C and 4C,the cell capacity is around 1.45Ah, see VIT curves.And there are Voltage charge percent, different fromthe RC battery, the cells arrive earlier to CV phase andthe charge process slows down, the steps in currentcurves are due to the fact that I used two paralleledPSU for the charge/discharge process and one of thetwo kicked in earlyer than the other, the 4C test wasdone at 4C discharge and 2.67C charge because theof the maximum 4A charge rate.

author: muvideo
author_rank: Frequent Contributor
timestamp: October 02, 2012, 09:56:54 am
content: And here a little comparison between RC battery and power tool one,the internal resistance of the RC cells is much lower, and this is reflectedalso by the rapid charge times. Also the Samsung INR cells showed astrange behaviour, 4C dicharge was actually better than 2C, I dont knowwhy, sure the battery was warmer during 4C discharge or maybe thebattery was charged better before 4C discharge.These are Voltage vs Capacity, the different internal resistance andprobably also chemistry is evident:These are Voltage vs Charge percent, normalized to 2C dischargecapacity for each battery:Fabio.

author: muvideo
author_rank: Frequent Contributor
timestamp: October 03, 2012, 09:34:40 am
content: And now the aging test for samsung INR18650, thetest was similar to Tipple RC battery: 20Cycles at 2C chargeand discharge.The INR cells dont show any significand aging at this timescale,the aging is there, but is smaller than RC battery and shadowedby other capacity variation like ambient temperature, the test totaltime was about 26hours, there is variation in the exact end of chargeand discharge point, due to the fact that I was sampling every 2s,and that alone is responsible for 1.5mAh "granularity".Also I should rectify an error: the battery weight is 140g,so considering that is capable of 15.8Wh storage, it's energydensity is about 113Wh/kg.Here the graph, horizontal divisions are 1mAh each,Blue discharge capacity and orange charge capacity:Fabio.

author: saturation
author_rank: Super Contributor
timestamp: October 03, 2012, 11:39:47 am
content: Thanks muvideo. Great data and posts. More later!

author: muvideo
author_rank: Frequent Contributor
timestamp: October 04, 2012, 10:39:21 pm
content: Last post with high rate data, let's see if theTipple battery is able to discharge at full current.During the discharge the battery warms up substantially,but it is to be expected while delivering aroundhalf a kilowatt of power, the Tipple discharges almostto full nominal capacity, only slightly lowering terminalvoltageThese are discharge times, the current wasnt exactlythe stated value, so the times are slightly differentof what you would expect, for example the 10C dischargewas performed at 18.24A instead of 18.0A and thetotal time was less than 360s :To put things in perspective this is the Voltage andcurrent logs, the discharge isfastand thecurrenthighQuote from: saturation on October 03, 2012, 11:39:47 amThank you, I hope it will be useful or at least interesting.Fabio.

author: nebs
author_rank: Newbie
timestamp: November 11, 2012, 10:07:59 pm
content: Hi muvideo, just out of curiosity, what type of battery cycler are you using for this testing? I'm slowly trying to put together my own cycler since they're a bit expensive, especially for a hobby. I figure I could get constant current without too much problem but I'm not yet sure about pulsed currents.

author: muvideo
author_rank: Frequent Contributor
timestamp: November 11, 2012, 11:11:38 pm
content: Hello, I cannot do pulsed currents, but for this kind of testing it's not necessary(also if 25C is sort of a slightly longer pulse).I made the tests using a (pair of) HP6632B, it's a programmable "two quadrant" psuthat means that can be used also as programmable load, it's rated to 20V 5A.For high currents I built a dirty load "amplifier", a cc load follower that multipliesx10 the 6632B capability.The automation was made using a python script running into a pc, connected to the psutrough a serial cable, used for both sequencing and datalogging.I used what I had around, a better setup would be a programmable loaddedicated to the task, and a switching programmable psu.The 6632 is limited as load, noisy, inefficient (it's linear after all) that means thatlong sessions cost many kWh of energy from the mains, and around here it's notthat cheap.The good thing about 6632 is that you can easily measure very slight variations inparameters thanks to it's stability and good performance as a meter.I used a 6-1/2 digit meter to check the battery under test andthe display was like a counter, digit after digit up during charge and down duringdischarge, no glitches, no double counts, no noise down to last digit.In my setup the resolution limit you see on capacity readings is due to coarsetemporal sampling, because I tried to keep file size reasonable.Fabio.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 12, 2012, 06:30:55 am
content: I have done similar research on specific batteries in these types of packs and discovered that on packs with high rated discharge currents the capacity on the battery much higher than rated if used at a normal 1C rate, so they can achieve their rated capacity at the higher discharge rate.Eg, a 20C 5000mAh cell can provide all 5000mAh at 20C, at 1-2C it is closer to 5700mAhAdditionally, with care taken, (high LVCO (3.55v), low charge rates (0.5c) and discharge rates (1-2C), low CV voltage (4.1v vs 4.2v for a cell rated at 4.2v)) the cell aging is significantly reduced (less than 1mAh/cycle).With the incredibly low internal cell resistance, using the cells in parallel can significantly increase lifespan.

author: BravoV
author_rank: Super Contributor
timestamp: November 12, 2012, 09:17:28 am
content: Just want to say thank you for sharing this, really appreciate your effort.

author: muvideo
author_rank: Frequent Contributor
timestamp: November 12, 2012, 10:29:07 am
content: Quote from: peter.mitchell on November 12, 2012, 06:30:55 amHello Peter,Li-ion battery aging is an interesting matter, I'm interested in your findings.As for the capacity,the two chemistries I tested perform almost equally atlow rates and high rates, the RC battery probably has 1/2 to 1/3 internal resistance of INR18650.The total discharge energy is reduced mainly by voltage depression during discharge.I've never tested other manufacturer's of RC cells.As for the aging: the first consideration is that the Tipple was new, probably it needs more cycles to stabilize.As far as I understand li-ion suffers aging that is independent of total cycles more than cycle aging per se.My understanding (but I confess I didnt much research on the subject) is that the electrodes aredamaged mainly by high voltage, the effect is worsened by high temperature.So if the cells remain charged resting at 4.2V, they start losing capacity also if not cycled, and the worseis when fast charging end of charge: the cell will be pushed to high voltage and high temperaturefor some minutes. One could also consider fast charging to 4.1V and then to 4.2 after a little rest to cooldown the cells.I second your advice, given the good energy density and low cost I use the cells charged at 4.1V and I use3 packs instead of 2, if I expect that the cells will not be used for a while I charge (or dicharge) them at less than 4V.I cannot use lower discharge rates because I'm limited by power to weight ratio, I use them on a model airplaneand the flight power is between 100 and 200W.The INR is interesting, because it seem the have a better life expectancy than the RC battery;for me the interesting finding was that the RC battery was capable of fast charge, probablythis is the main effect of low internal resistance, the cell charges almost fully at CC (90% at 4C).Fabio.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 12, 2012, 11:08:41 am
content: I presume the capacity difference between discharge rate in my scenario and yours my be less to do with C value discharge rate and more to do with absolute value, in my scenario 20C is 100A whereas in yours 20C is 36A~, producing more heat and losses in cables ect.I agree whole heartedly that Lithium chemistry batteries suffer capacity degradation simply for retaining a charge, and that to minimize the degradation over time, they should be discharged to approx 3.8v/cell for LiPo chemistries, with that said, whilst the battery is in use, the battery will also degrade, but how you treat it will determine by how much.Indeed using multiple cells in parallel is not always a viable solution, however, if using them in applications outside of their main purpose (possibly in place of SLA batteries or NiMh in stationary equipment) this can be useful.The packs I used for my testing were Turnigy 5000mAh 4S Hardcase with a 20C constant discharge and 30C 20second peak discharge, 5C maximum charge, tested over 10 cycles over 9 packs after being cycled initially 20 times to stabilize.

author: muvideo
author_rank: Frequent Contributor
timestamp: November 12, 2012, 12:30:06 pm
content: I extracted another diagram from the battery data, it's end of charge voltage vs stored capacity,it' useful for evaluating how much of the total cell capacity one will use stopping the charge at lower voltagesthan 4.2VAnother (unsuspected for me) advantage of the low internal resistance is evident here:for the tipple stopping at 4.1V stores still about 83% of capacity, while for the INRwe are around 70%; at 4.0V the tipple is slightly less than 70% while the INR is around 58% ,at 3.9V the difference is lower, capacity of the tipple has dropped to 50% while te INR isat 47%So the lower internal resistance permits to charge the Tipple to 4.1V storing 83%,that seem to me a good compromise.Diagram for RC Tipple battery:Diagram for Samsung INR battery:Quote from: peter.mitchell on November 12, 2012, 11:08:41 amAre you using them for their purpose (RC car I imagine) or for other uses?

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 12, 2012, 01:31:53 pm
content: I came accross them when looking for new batteries for my RC car, yes, but I do use them for other things.