Ma Ti Posted January 11, 2020 Share Posted January 11, 2020 I'm new to this kind of project. I created a battery pack with 3x3.7v lithium cell and i connected it to a dc-dc step down buck converter and a while ago its working fine i can charge my phone with it but after a few hours it started acting as what you can see in the video. The USB indicator light is not running or it not stable. So Please if anyone can help me . I would really appreciate it. Thank you Very much. Quote Link to comment Share on other sites More sharing options...
Popular Post Chriull Posted January 11, 2020 Popular Post Share Posted January 11, 2020 (edited) Welcome @Ma Ti You have choosen a wrong forum - as the forum's name and the forum's and subforum's descriptions tell this is for electric unicycles (and some other pev's) However, what you made seems to be a ticking time bomb. LiIon accumulators offer great possibilities, but can be very dangerous (and deadly) if mistreated! At first you should improve your soldering skills - bad solder joints not only (can) lead to bad connections but also the wire (or the whole solder joint) could come loose and cause a short curcuit. If the wire shorts a cell it's damaged or dead (in the best case) or could cause a fire/explosion in the worst case. Googling for "how to solder" gives one tons of nice tutorials. For learning details about treating LiIon cells right i'd recommend www.batteryuniversity.com Now regarding your project: You mentioned your cells have 3x3.7V. This 3.7V are their nominal (average) voltage. Fully charged the have normally 4.2V and can be discharged downto 2.5V. Better and safer would be discharging only downto 3V. Depending on the used brand/chemistry this values can differ. So most likely your project worked the first couple of hours, as long as the cells provided enough voltage. At one point, once the voltage got too low it was not enough anymore to charge your phone over the DC/DC converter. So you'd need some low voltage protection circuitry between the liion cells and the dc/dc converter to cut off once the cells reach ~3x3V=9V! Every cell discharged below 2.5V gets damaged, as they are slowly "damaged/aged" the more the are at their extremes. Damaged cells "just" loose capacity until, in the best case they cannot be recharged anymore. In the worst case they could with every charge/discharge vent(the internal safety vent opens and lets the high pressure gas out from the evaporated chemicals)/burn or explode. Also you should use a cell balancer with cell overvoltage protection for charging liion cells connected in series - as they age slightly different over time! Such a balancer can also provide your needed functionality to shut off the connection to the dc/dc converter as soon as any cell reaches a minimal threshold voltage. This devices (balancer, cell over and low voltage protection) are called BMS (battery managment system). Have fun with your project and handle your liion cells with care! Edited January 11, 2020 by Chriull 6 Quote Link to comment Share on other sites More sharing options...
Rehab1 Posted January 11, 2020 Share Posted January 11, 2020 @Chriull I love your sincere teaching skills and clarity. 2 Quote Link to comment Share on other sites More sharing options...
RockyTop Posted January 11, 2020 Share Posted January 11, 2020 @Chriull You tell him he is on the wrong forum and then destroy that statement by giving him the perfect answer to his question. You are a generous teacher. 3 Quote Link to comment Share on other sites More sharing options...
Chriull Posted January 11, 2020 Share Posted January 11, 2020 Ps.: @Ma Ti soldering durectly on the liion cell caps is not recommended. The heat from the sildering iron could easily destroy the cells internals! As you will have noticed, soldering on the caps is a "pain in the ass"... But one can buy liion cells with prewelded easy solderable tabs. If you intend to make this often you can buy/build your own welder. Or you use a battery holder - just be sure he is specified for the necessary current. Quote Link to comment Share on other sites More sharing options...
Ma Ti Posted January 12, 2020 Author Share Posted January 12, 2020 Thanks Mr. Chriull. Thank for taking the time to comment and i really appreciated it. And Sorry for posting in a wrong forum its just that im really desperate for help. Btw im using 3s 12v 25amps 3.7v Battery BMS with Balancer, is this good enough for a bms? And the Dc-Dc Converter why is it that the reading is stuck at 6.1-6.2 ? And also why is it that the dc-dc converter's USB light indicator beside the port is blinking before its steady ? Quote Link to comment Share on other sites More sharing options...
..... Posted January 16, 2020 Share Posted January 16, 2020 Feed a cat once..... Quote Link to comment Share on other sites More sharing options...
Chriull Posted January 16, 2020 Share Posted January 16, 2020 On 1/12/2020 at 7:47 AM, Ma Ti said: desperate for help You need to read the manuals/specifications of your used components, search for adfitional info in the internet, and do some voltage measuring to understand once some things do notwork out... Without the basic understanding i would recommend you to not mess around with liion cells. Quote Link to comment Share on other sites More sharing options...
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