"Modifying" a Li-Ion cutoff circuit to work with multiple cells. - Page 1
Forum section: electronics / projects

author: corrado33
author_rank: Frequent Contributor
timestamp: May 17, 2015, 05:01:12 am
content: Hi all. I'm attempting to make myself a battery pack for my bike lights. Currently I'm using a 12V lead acid battery which isn't ideal as.... well it's 12V, and it's big and heavy.2 Li-Ion cells will come in around 7.4V nominal 8.4V fully charged, which is closer to the voltage my lights require (and will consequently have fewer losses in my linear regulators.)Anyway, the cells I ordered were name brand (samsung) but cheap because they don't have protection circuitry (I think.) I ordered a few of the Samsung INR 18650 cells. I THINK the reason they're so cheap is because they don't have the protection circuitry, as the datasheet doesn't mention any circuitry.Anyway, after googling around for a while, I found a simple comparitor bases circuit that uses very little power. I found it at this website. Here's the schematic.http://electronicdesign.com/power/li-ion-battery-protection-circuit-draws-only-45-muaNow, I was doing some of the math, and I think the only modification NEEDED if I wanted to use this circuit for 2 cells in series instead of one is to increase the zener from 1.25 to 2.5V, provided I don't mind the bit of extra current drawn by the circuit from the increased overall voltage. I possibly might need to play around with R5 to increase the hysteresis, as I can't seem to find the ESR of those batteries, but that seems about it. Am I looking at that wrong?Would this work?

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 17, 2015, 06:03:46 am
content: Quote from: corrado33 on May 17, 2015, 05:01:12 amWether it works or not, you should monitor each cell seperately so one of the cells does not discharge lower then the other due to aging, damage or slightly other specs.You get proper multi cell monitors with a much smaller footprint from different chip vendors.

author: jt
author_rank: Regular Contributor
timestamp: May 17, 2015, 06:32:43 am
content: Agreed with PeterFW on both points.A few other misc. points to consider:1. cell protection circuits can become very feature rich and very complicated/intimidating (ex. string balancing, I2C or SPI interfaces for telemetry collection, fuel gauging ect.). Unless you are interested in these features, I'd suggest looking at Seiko cell protection ICs; they provide a more bare-bones solution to protecting the cells. I'm sure there are other brands that fit within this category as well.2. the circuit you present does not have over voltage, or over current protection - which should be considered mandatory features.3. If you are newish to electronics I might suggest you splurge on a pre-built pack and avoid safety-critical design work.4. Putting cells in series adds a lot of complexity compared to a single cell 3.7V pack with regard to the protection circuitry, wire harnessing and the need to balance the cells before assembling the battery. Also the battery life-time will suffer from the cells becoming unbalanced over time unless you add balancing circuitry. A simpler solution may be to use a 3.7V pack and a boost converter to achieve the voltage you need. I think you can find boost converter circuits prebuilt in places like sparkfun if you don't want to build your own.

author: Neilm
author_rank: Super Contributor
timestamp: May 17, 2015, 06:46:33 pm
content: Genuine 18650 cells do have a certain amount of protection built in - if they get too hot a one shot thermal fuse will blow and the cell will go open circuit. They are not usually sold to the general public so I would be a little bit skeptical about whether this protection is there. Fake cells may be nice and cheap as they don't have this protection.Remember that this is still not guaranteed to prevent a fire when charging. If you are not sure about what you are doing - I would look to buy one with an appropriate charger and not risk the house fire you could have otherwise.

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 17, 2015, 06:52:54 pm
content: Quote from: Neilm on May 17, 2015, 06:46:33 pmNone of my 18650 cells have such a fuse, at least it is nowere mentioned.I do not know what you call genuine but mine are from panasonic, sony and samsung.They are new and definately not recycled china crap.QuoteThey are sold to the general public, often and in great numbers. You may not have come across that but they are used a great deal in electronic cigarettes.

author: HKJ
author_rank: Super Contributor
timestamp: May 17, 2015, 07:17:42 pm
content: Quote from: PeterFW on May 17, 2015, 06:52:54 pmCell do have some internal protection:There is the vent that will blow if the pressure gets to high, it is often designed to break the connection, but not always.Many cells do also have a PTC resistor in to reduce the current if it gets hot.I have opened some cells:http://lygte-info.dk/info/batteryDisassemblyUltraFire%20UK.htmlhttp://lygte-info.dk/info/batteryDisassembly5000mAh%20UK.htmlAnd a simple explanation:http://www.lygte-info.dk/info/battery%20protection%20UK.htmlWith the OP's circuit it would be simpler to change some resistors, instead of the zener, but as has already been said: It is better to get a dedicated chip for protection.I have seen "professional" equipment where 7 LiIon was in series, but without any cell protection. In the pack I checked there was 6 cells with over voltage and one cell with 0 volt (It was supposed to be a brand new pack).

author: Siwastaja
author_rank: Super Contributor
timestamp: May 18, 2015, 08:58:04 am
content: Quote from: PeterFW on May 17, 2015, 06:52:54 pmAll 18650 cells from the major producers (Samsung SDI, Sony, Panasonic, Sanyo, LG Chem, etc.) do have a PTC polyfuse which self-resets. If you look at the datasheets, even the PTC DC resistance is often specified. I would like it to be a non-resettable type (because I want it to stop functioning after a critical safety failure), but the merit for the PTC is that it provides two functions in one - overcurrent protection and overtemperature protection.What Tesla basically did with their pack was to special order the Panasonic cellswithoutthe PTC, and use a small, robot-welded fuse wire to connect the cells in parallel instead. This is because the construction of the cell cap with the PTC and the pressure valve is relatively complex and expensive.Re multiple cells, it IS possible to build a 2s pack without cell level voltage sensing - this is routinely done in mass products. But you really need to know what you are doing. (But preferably no more than two cells, although I have seen a 4s nontapped pack from BOSCH, for example.)

author: G7PSK
author_rank: Super Contributor
timestamp: May 18, 2015, 10:23:13 am
content: You can buy the protection boards so cheaply it is not worth trying to make your own.http://www.ebay.co.uk/itm/2S-Li-ion-Lithium-18650-Battery-Input-Protection-circuit-Board-PCB-7-4V-2A-/301421249144?pt=LH_DefaultDomain_3&hash=item462e1b4278If you just want to learn make one but buy a ready made unit for actual use.

author: dexters_lab
author_rank: Unknown
timestamp: May 18, 2015, 02:45:16 pm
content: there are also 18650 battery boxes that have the protection built in like:http://www.ebay.co.uk/sch/m.html?_odkw=18650&_ssn=directtouch&item=390845006978&_osacat=0&ssPageName=STRK%3AMEBIDX%3AIT&_from=R40&_trksid=p2046732.m570.l1313.TR4.TRC1.A0.H0.X18650+protection.TRS0&_nkw=18650+protection&_sacat=0