Li-Ion cell.  When to declare full discharge? - Page 1
Forum section: electronics / projects

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 18, 2012, 07:56:40 pm
content: Hi,I have a design that draws short 20-50msec pulses of relatively high 1A current from a small 50mAh single cell 3.7V Li-Ion battery. The duty ratio of pulse load is less then 5%.What would be the best strategy for discharge cut-off? Is Li-Ion battery declared dead when it reaches 3.0V under actual load or under no load? I can monitor both voltages. Or maybe there are some standard load conditions for measuring cell voltage?The battery voltage seems to recover after I have stopped discharging it at 3.0V. Is it safe to draw additional power until voltgae drops to 3.0V again or does it need to be fully charged after hitting 3.0V the first time?ThanksLeo

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2012, 08:20:40 pm
content: depends on the battery model and the manufacturer ..... read the datasheet of your specific battery model.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 18, 2012, 08:35:39 pm
content: Quote from: free_electron on April 18, 2012, 08:20:40 pmThanks for nothing!LeoP.S.http://www.fentbattery.com/Upload/LeSeno.Com_2012228101223613.pdf

author: muvideo
author_rank: Frequent Contributor
timestamp: April 18, 2012, 08:52:54 pm
content: The problem is that a discharged lithium is a dead one.If it recovers it's probably damaged.A damaged lithium is dangerous.So you should stop when you are reasonably sure that the cell is still safe.Does the circuit draw any standby current?Can the cell remain connected to this circuit after discharge?For how much?The answer to your question is in the cell datasheet,how the manufacturer tested the cell, how much isthe internal resistance of the cell.The rest voltage can overstimate the residual charge,but the loaded voltage depends also on age of the cell,an old cell has higher internal resistance, same withtemperature.In this example:http://samsungsdi.com/battery/cylindrical-ICR18650-26F.jspthe values are valid at 0.2CIn less words:what said free_electronFabio.edit: trying to read the datasheet you attached...read it, that document lacks decent info, like free_electron said theymention 3V at 1.5A as discharged point.

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2012, 09:00:58 pm
content: Quote from: Leo Bodnar on April 18, 2012, 08:35:39 pmYou are welcome. Success with your project...Now, if you are willing to listen , the answer is in the datasheet.The manufacturer has characterized the battery.In the two cycle life plots they mention a 3 volts cut-off. There is a reason they don't characterize beyond that !In the charge time plot you actually see the voltage plot as well... the battery is considered 'empty' at 3.4 volts.. ( See the red capacity meter in mAh . it gives 0 at 3.4 volts )You will not be able to pull a 1 ampere spike out of a battery in that low a state of charge. The cell voltage will collapse and permanent damage will be done to the cell.You cannot rely on the battery to supply you with the spike current. Put a capacitor in parallel and use that one as the buffer.To protect the cell you can charge this capacitor with a resistor. Time the Rc constant so that the capacitor can be fully charged during your dead time.You can work out a ratio so that the battery remains operable down to its 3.4 volts.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 18, 2012, 09:20:22 pm
content: Quote from: free_electron on April 18, 2012, 09:00:58 pmIf you are willing to read you may find the wordconstantin front of thedischarge. 20ms pulse with 2% duty ratio is not constant by any means.Thanks for RC network introduction. You have opened the whole new world to me! If I only knew your name to thank you.Leo

author: bfritz
author_rank: Regular Contributor
timestamp: April 19, 2012, 02:30:35 am
content: I've designed battery safety and charging systems for over 15 years...But with the amount of attitude by the OP, why would I wish to lend my opinion. I'm just going to get bashed if I tell him something he "knows".Good Luck!

author: muvideo
author_rank: Frequent Contributor
timestamp: April 19, 2012, 07:03:36 am
content: Quote from: bfritz on April 19, 2012, 02:30:35 amHello, I don't want to "flame", just my opinion:I hope that OP's attitude doesn't stop you,your experience will be useful to many other people,that simply read this thread, or have written in it.I'm interested in your opinion.Fabio.

author: sorin
author_rank: Frequent Contributor
timestamp: April 19, 2012, 07:09:50 am
content: from the datasheetwhen the battery is charged with 2A the 0% capacity correspond at 3.4v, I think that is the same with the dischargeso with 1A discharge you have 3.4 - (1A*0.1ohm)=3.3vbut, I don't thing that is a good idea to discharge the battery down to 0%Where did you buy this battery?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 19, 2012, 07:31:50 am
content: Quote from: sorin on April 19, 2012, 07:09:50 amHi sorin,I have bought this battery from an RC online shop but I am more interested in general science of Lithium Ion chemistry.You have highlighted something that is obviously not clear from the datasheet. There should also be a difference between short bursts of discharge and continuous drain. If same approach had been applied to LEDs we'd never had any PWM controlled brightness or scan matrix display. LEDs peak current is pretty much defined by the die thermal dissipation. I don't believe chemical processes are the same at constant current of 1A and standalone 1A pulses of 20msec.Leo

author: muvideo
author_rank: Frequent Contributor
timestamp: April 19, 2012, 07:41:16 am
content: Quote from: Leo Bodnar on April 19, 2012, 07:31:50 amThe difference is in part addressed by the "internal resistance" figure:if the cell has 100mOhm internal resistance the voltage will sag approximately100mV per Ampere when loaded.Fabio.

author: sorin
author_rank: Frequent Contributor
timestamp: April 19, 2012, 08:32:28 am
content: I thing that here is a big of confusion!because batteries that are capable to deliver more than 3C usually are LiFePO4, which are 3.3v (I never see Li-poly or Li-Ion drive so much current )

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 19, 2012, 09:11:26 am
content: Quote from: sorin on April 19, 2012, 08:32:28 amHi sorin,Have e.g. a look here:http://www.fentbattery.com/en/Cylindrical.asp?id=472&pid=108These 3.7V Li-Pos allow up to 30C continuous discharge and up to 40C what they call burst discharge (less then 5 seconds.)I would like to learn whether there is a major chemistry process difference between 5 second burst discharge and 20 msec burst discharge.Leo

author: sorin
author_rank: Frequent Contributor
timestamp: April 19, 2012, 09:19:12 am
content: one of the the problem is the heat produced by the internal resistance.http://batteryuniversity.com/What is the website when you bought the batteries?