Tablet/cell phone battery - Page 1
Forum section: electronics / beginners

author: Jane
author_rank: Frequent Contributor
timestamp: April 08, 2020, 07:48:25 pm
content: When I open a tablet battery or cell phone battery I can see some electronic inside. Is there available the description how this electronics ( inside the battery) works?Is it BMS circuit?

author: Rick Law
author_rank: Super Contributor
timestamp: April 09, 2020, 03:50:29 am
content: First, I am just a hobbyist, so this is my hobbyist experience only -- not a professional opinion.I suspect it is most likely some form of a battery management or battery protection circuit. Depending on how old it is, its capability will vary. Then there is the problem of determining what the connectors do and the boards' limits.I did removed a couple from an ancient StarTAC phone (dated back in the G2-phone era, and when NiMH was still the most common phone battery). Attached is a photo of it. I managed to map the "pin out" for them. The two outer (silver color) tabs connects to the cell's +-. The two center gold tabs are +- output to the world. The two gold ones next to the silver ones are "reset" (after cut-off), and (I think) a temperature-sensor. The cut-off voltage I can DMM, but with load, I just guess it should do around 1 to 2C of the battery. I never put more than 1Amp (+-10% ish).When I first started (before I have other methods), I used these two boards quite a bit - particularly when I was doing battery capacity test. It serves as a good guard against over-discharging an unprotected cell. Now I use something I purchased (with specs).Edited: corrected a couple of typos/errors

author: Jane
author_rank: Frequent Contributor
timestamp: April 09, 2020, 02:37:42 pm
content: Thank you for your reply.I have Lenovo Yoga X50F tablet with a battery that starts charging( USB tester used) about 1A but in about 3 seconds it starts dropping until it reaches about 0.26A and remains there.Is it because of battery cells( the battery consists 2pcs of 18650B cells and one 18500A cell) poor quality (high resistance) or that electronics inside the battery?( Another battery remains at about 1A during charging)@Rick LawQuoteCan you please explain a little moreQuoteAnd what device you use now?Thank you

author: Rick Law
author_rank: Super Contributor
timestamp: April 10, 2020, 03:26:49 am
content: Quote from: Jane on April 09, 2020, 02:37:42 pmre: "The cut-off voltage I can DMM..."The very basic function a battery protection circuit should do is to prevent over-discharge. Cheaper protection circuits will do over-discharge protection only. The next up (in my opinion) is to prevent charge voltage from exceeding the battery's designed max voltage. There are of course other more "advanced functions" that fancy BMS may do.That board that I removed from the StarTac will do over-discharge protection - it will cut-off discharging when the voltage drop beyond a certain value, but without specs for the board or the chip(s) on that old ancient board, I had no idea what that board's cut-off voltage was. So, I use a DMM to log the voltage, and determined that the cut-off voltage was 2.4Volt.re"...but with load, I just guess it should do around 1 to 2C of the battery. I never put more than 1Amp (+-10% ish)"Without specs, I don't know the board's maximum current capability. Like most batteries, the capacity is printed on the battery. I noted that before I took it apart. If I recalled right (I lost my note book), that was a 800mAH or a 900mAH battery. I will pick 900mAH since I don't recall, so one C is 900mAH, so I expect (typically) the board would be designed to handle at least 1C (900mA current), probably a bit more. So I "unofficially" set 1A as the limit for that little board.I have purchased battery protection circuits capable of handle 4A, so 1A is a very low limit.Quote from: Jane on April 09, 2020, 02:37:42 pmI have a collection of different types I played with, but what "I use" would be the ones I installed on used 18650 cells recovered from old laptop battery packs. Typical DW01 based ones. Those boards are the tiny round PCB you fit onto the end of batteries. Boat loads of vendors sell them. They have specs on line, and I randomly select one from the shipment (typically I buy 2 to 6 at a time) and test it against their claims. Many of them claim to do charge over-voltage protection, but my tests show they did nothing of the sort.Quote from: Jane on April 09, 2020, 02:37:42 pmThat would be anyone's guess. It could be your batteries, or perhaps other reasons.With my limited experience, I cannot narrow down much further. Taking the batteries out and testing them individually is what I would do. Then again, Darwin works on me too... Dealing with those lithium batteries carelessly could be deadly...No joke, I was playing around with those old StarTac batteries for a while and having a lot of fun - and then I must have made a mistake somewhere along the line. The darn battery was getting so hot I quickly grab my pliers and use that to grab the battery, walk out to the backyard and dump that on the lawn to let it cool. (or let it catch fire there instead of inside the house.) In later days, I always tape a temperature sensor on the cell when I test any recovered cells. Only after a few cycle of "normal results" for charge-discharge test would I then re-wrap them and put it into circulation for my use.EDIT: for wording (better choice of word) and typo correction.

author: amyk
author_rank: Super Contributor
timestamp: April 10, 2020, 04:44:51 am
content: Quote from: Jane on April 09, 2020, 02:37:42 pmThat's an odd arrangement to have mismatched cells, but if they're all in parallel it should be fine; the current drop is likely because the voltage has risen to the limit and the cells are charging in the constant voltage phase. Cells of lower capacity will charge more quickly.

author: Jane
author_rank: Frequent Contributor
timestamp: April 10, 2020, 07:50:51 pm
content: Thank you for replies@Rick LawFrom your experience , what parametrs should 18650 cell have so that it is good to use such a cell?@amykThe cells are connected in series

author: Rick Law
author_rank: Super Contributor
timestamp: April 12, 2020, 07:18:54 am
content: Quote from: Jane on April 10, 2020, 07:50:51 pmIt depends on what you want to use it for. I keep basically most cells I have recovered - until proven dangerous. Dangerous are those that refuse to charge up and just converts charge current into heat.First, my definition of "too hot". My daily charger cuts out at ~90c. So I define "too hot" as "going above 70c at any point during charge (1A charge current limit)". So, the "not too hot" ones can be charged by my daily charger with some margin.These are my rule of thumb for 18650 cells only:For my use high-end good cells (>85% of nominal capacity, discharges >1A, and never get too hot while charging at 2A), I use it on my demanding flash-lights.My "run-of-the-mill" are those that fails high-end but above low-end.Low-ends are those that can recharge to <60% but >40% nominal at 1A without getting too hot, The low end ones, I use on my transistor radio, my night light, and store them for emergency power outages.Really low ends are those needing charge current reduction to 500mA but otherwise can charge fully without getting too hot, and have >40% nominal capacity. They have special labels (such as charge at 500mA only) Those I keep as last-resort, like keeping an employee you know you cannot rely on, but have some skill(s) that you know you likely need with future projects already in the pipeline. They are watched carefully.Those that fails to charge to nominal voltage (regardless of capacity) are immediately disposed. Those <40% nominal capacity are garbage waiting to be disposed (wont recharge those, but may finish discharging).I do keep a few I consider dangerous (charges but fails to charge to nominal and gets hot) - with highly visible labels. When I play with recharge circuitry, I like to have something that fails so as I can test my "fail-detection" setup. Testing is a use, so they are useful.Now, I am a hobbyist with limited experience. So, do not take what I use as rules everyone can use. Darwin works on anything alive until they stop being alive...ps: if you think I am testing Darwin with my "rule of thumb", please do let me know. I don't intend to test Darwin every time I recharge a battery. I know I will kick the bucket some day, but I am in no hurry.