Lithium battery PCM question - Page 1
Forum section: electronics / projects

author: chimera_786
author_rank: Regular Contributor
timestamp: April 21, 2012, 08:47:31 pm
content: Hello. I have a question regarding the PCM of a single lithium cell. PCM stands for 'protection control module'.It basically sets the charge and discharge, over-voltage and under-voltage limits for the cell. Basically, its tiny piece of pcb that is connected the to the tabs of the cell.Now, by default, I can CONTINUOUSLY discharge the battery @ 0.2C. This means that if my battery is 2400mAh, then i can only continuously discharge it at ? 490mA. However, i have seen on the data sheets of some of these PCMs that there is MAXIMUM CONTINUOUS discharge current of like 1C or something like that.How do I configure my present PCM to discharge at 1C. The issue around this problem is that the IC name on the PCM board have been erased so I cant look up the IC and then work around that. However, a good bit of researching brought me a typical IC that's does the whole protection: S-8261 by Seiko. That's right!!! SEIKO!!! did not see that coming.I cant go for a bigger size of the battery caz I have pcb real estate limitation. The only alternative I have is to use a bigger discharge rate.So does anyone think they can help me at this?? All help is much appreciated.

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2012, 09:03:49 pm
content: The "C" rating when talking about battery charge/discharge depends on the capacity of the battery in question.A protection PCB attached to a battery does not know (cannot know) the capacity of the battery, so it its behaviour is not defined in terms of "C".In fact the protection circuit will be designed for a maximum continuous discharge current specified in amps or milliamps. You need to know what this is for the PCB you have to hand.I rather doubt you can reconfigure an existing protection PCB. Youmightbe able to change the continuous current discharge limit by substituting a current sense resistor on the board, if you can locate it and you are really good with miniature SMT devices. But if you attempt this you should make sure your actual battery is safely capable of that discharge rate.Your best option is to purchase a combined battery/protection circuit module that meets the specifications you require, or purchase PCB and battery separately and combine them yourself--but you must still respect the discharge limits on the battery given in the manufacturer data sheet.

author: chimera_786
author_rank: Regular Contributor
timestamp: April 21, 2012, 09:12:54 pm
content: Quote from: IanB on April 21, 2012, 09:03:49 pmI purchased my battery from ebay. There is basically no info about the PCM circuitry involved. All of the info I have gathered is becaz of unrelenting searching on the net.I might just have to consider buying a new PCM circuitry or if my budget permits, a battery+pcm system that can be small enough but powerful enough to cater my needs.

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2012, 09:30:05 pm
content: What kind of lithium ion cell did you buy? What is its physical size, brand name, and rating?Lithium ion cells can be dangerous, and buying from eBay can be a bit of a lottery. Unless you know a lot about the vendor, you may not get what you think you are getting.The general way of designing with batteries is you consider the power requirements of your circuit or device, things like expected supply voltage, expected current draw, expected run time, that kind of thing. Then you do a parametric search for batteries that might meet those requirements. You look at the data sheets for shortlisted batteries to see if they have discharge graphs that are capable of your application. Then, lastly, you pick a protection device that matches the battery and the use case.You can bypass some of this if you don't want to get so fancy and just go by general design guidelines and general knowledge. For instance you don't generally want to discharge an 18650 size lithium ion cell at greater than 1 or 2 amps, though you might go up to 4 amps if you have a known high quality cell. Protection circuits attached to 18650 cells sold for hobby flashlight applications usually set the current limit somewhere in this ballpark.As with all questions on this forum, you will most likely get better advice if you give precise and specific details of what you are trying to achieve and avoid vague generalizations.(Also, the protection circuit is not normally supposed to cut in. The current limit is to guard against accidental short circuits that otherwise might cause damage or a fire. If there is a short circuit the protection device will isolate the battery and cut off the power.)

author: chimera_786
author_rank: Regular Contributor
timestamp: April 21, 2012, 10:31:39 pm
content: Quote from: IanB on April 21, 2012, 09:30:05 pmI wasn't trying to be vague on purpose. I figured that too much info sometimes hinders the easiest of solutions. The battery pack that I have is 3.7V 2400maH Li Cell. Its one of those flat ones. I have already designed, pcb'ed, and tested a stand alone Li charger. However, I wanted to see if I am able to extract a higher discharge current. Unfortunately, I got this battery last year (2011 June) and had not gotten around to use till march 2012 and buying it from ebay did not exactly help me with technical details of the battery and PCM.Now, based on the fact that I have tested my charger (works like a charm), I am going to go ahead with the AA Li secondary (rechargeable). They are small and pack a big punch. The power requirementsFROMthe battery are 550mA (4.2V to 3.0V--which means from fully charged to nominal). I am not going to be drawing current greater that that.I have a circuit in hand that will disconnect the load from the battery once it detects that the supply voltage (battery) has fallen below a predetermined value and it will automatically initiate the charger to start charging the battery and wait for the battery to reach 4.2V again where it will wait for the voltage to drop to the predetermined value and the cycles will repeat.See I told you..too much info hinders from simple solutions1- I am going to use a single 18650 cell. Its small and it packs a big punch.2- I am going to go for this battery holder and it also a PCM built it.http://www.batteryspace.com/battery-holder-li-ion-18650-battery-holder-1s1p-with-2.6-long-20awg.aspx3- I am going to be using my designed charger to charge it.Let me know what you think about the battery holder.

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2012, 10:41:41 pm
content: Quote from: chimera_786 on April 21, 2012, 10:31:39 pmQuoteQuoteOK, that's more information.So far you have described three different battery configurations:1. A flat one (rectangular, prismatic cell?)2. An AA size one (14500 cylindrical)3. An 18650 size oneIt is not clear which one you intend to use, but your last comments suggest the third 18650 option. In that case the battery holder with built in protection circuit looks OK. The current limit is 3.0 A, so you should be able to draw 550 mA successfully. Note that you need a compatible charger that will reset the circuit if ever it trips and isolates the battery. The charger will have to provide a short current pulse whenever it sees no voltage from the battery and wait for the battery to "wake up" before attempting to charge it.

author: chimera_786
author_rank: Regular Contributor
timestamp: April 21, 2012, 11:08:10 pm
content: Quote from: IanB on April 21, 2012, 10:41:41 pm1- Yes, it a prismatic battery. I did not know it was called prismatic. I Googled it quickly and its pioneered by Duracell (according to wiki).2- The 'AA' alkaline is not the one I am referring too. I had considered that earlier on and subconsciously I typed it out.3- 18650 is the one I am going for!For the charger, I am using MCP-73831 4.2V. Its a stand alone IC. Like I mentioned, it works and my prismatic (thank you for telling me that) 3.7V battery charges successfully.The MCP-73831 only gives up to 500mA of charge current (yes, the number just happens to be the same output current that I want to draw from the fully charged battery---coincidence). I am okay with a longer charge time. This is because the MCP is cheaper, i dont have the requirement of rapid charging and lastly, it is in a small SOT23-5 package (it has thermal issues but I over came that by using a nice copper pour around it when i designed the PCB).As for the actual charging circuit, I am using the configuration used in the application notes of MCP datasheet with an additional supervisory circuitry. I read up extensively on the data sheet and I am quite confident on the charger due to fact it works

author: metalphreak
author_rank: Frequent Contributor
timestamp: April 23, 2012, 06:35:48 am
content: Posted this in another thread before:http://www.all-battery.com/datasheet/32088_datasheet.pdfThat's the typical schematic. The current limiting is determined by the part number (it's a small voltage on the VM pin, measured as the voltage across the two mosfets basically). The over voltage/over discharge is also determined by the part number of the Seiko IC.S-8261ABJMD-G3JT2 = 4.28v and 3.00v limits. 0.08V maximum on the current detect pin before shutdown.http://datasheet.sii-ic.com/en/battery_protection/S8261_E.pdfAlso, the MCP-73831 is very goodBare minimum of components required and it works as intended. I designed my board very similar to the datasheet layout recommendation, with a few vias linking the top copper to the back layer as well.