Connecting six protected 18650 cells 3S2p for close to 12V? - Page 1
Forum section: electronics / beginners

author: DaveC1964
author_rank: Newbie
timestamp: April 01, 2021, 01:17:29 am
content: I want to make a 12V range battery array with 2P 3S 18650 cells. The cells will be charged in an external charger NOT in the pack. They are removable in holders So I can pluck them out and charge externally. I have the basic plan below.Some criteria: The amp draw is relatively low at around 1A. I used the protected cells so one or more cells can't discharge below recommended volt limit. The protection also prevents a good cell from trying to charge and dump too much current into a bum cell that may be in parallel with it. That would be rare unless there is a mistake or a bad cell, it is a precaution. That is the theory.The question is do I need BMS and if so how and where would I put it? Would I use a regular 3S BMS and treat each two battery cluster as one cell? Would a BMS conflict with the protection chips in the batteries?Any suggestions?Thanks.

author: beanflying
author_rank: Super Contributor
timestamp: April 01, 2021, 01:55:28 am
content: Couple of things to be aware of first.In particular if they are salvaged or mixed age or batch cells. Capacity or near capacity matched when it comes to series parallel batteries is safer and will yield better results so if you have a method of testing this thenIf not take as much care to try and match them same brand age etc, some of the so called capacities out there just are not true.With your current draw of only one amp any individual cell will handle that current draw unless it is totally trashed or near EOL.SoIFyou put a BMS board on each of your cells like thiseBay auction: #124396847497gives you most flexibility to use the cells safely here or on other jobs and will look after a mismatched set of cells in the best way.ORShould you use a multi Cell BMS board and top and tail the wires to your holders you will likely have a spaghetti monster and a far less flexible set of batteriesNOW on BMS and do you need it/themYes and also No.I run 30-40 packs of LiPo's (I think) for R/C planes helis and drones none of them apart from the DJI drone packs have BMS boards or protection fitted. The Speed Controllers or Receivers in some cases monitor the overall pack voltage and shut down the motor at circa 3V/cell as we are pushing high to extreme currents the cells unloaded generally bounce straight back to 3.2-3.3V so there is simply no need for BMS and at 20-100A+ we would blow the stuffing out of the boards and we don't want additional losses.For all my 18650 based cells and toys/projects they go in I generally have chosen to at this stage use unprotected cells but only buy this sort of solution for powering circuits as it handles the charging and protection18650 UPS 3.3 and 5V holdersOther than that I have a couple of those single cell boards on some for set and forget use.

author: DaveC1964
author_rank: Newbie
timestamp: April 01, 2021, 02:15:46 am
content: I am using new cells such as the Konion VT6. Each holder has one of those protection circuits on them which is the same one used in the end caps of cells. It seems to use the same chips as the ones you pointed out but round instead of strips. I plan to charge them in an external charger to the same voltage for all. My concern is if one cell is a bit defective/weak or improperly charged would the other cell in parallel try to quickly charge it and overheat/explode? Or would the protection circuit prevent that? Or do I need a BMS to manage the charge? Those protection circuits really aren't "BMS" as in the ones mostly used for charging, they are just safety devices. I just wonder if they are sufficient to keep the magic smoke inside all of the cells.

author: beanflying
author_rank: Super Contributor
timestamp: April 01, 2021, 02:28:38 am
content: BMS in theory has NO part to play in the charging of cells. Think of the BMS as a simple watchdog device and no more than that, it simply keeps the cell/s within circa 3 and 4.2V. Time and again people try and push them as a charge solution they are not.Any sort of a sensible charger even the cheapest sensible alternativeeBay auction: #253066138178runs a multi stage charge profile. So when really depleted SLOW, Mid charge constant current of X (set by an external resistor) then SLOW again to 4.2V and then stop. Version with built in BMS hereeBay auction: #233887077427TP4056 Datasheet is worth a read toohttps://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdfSo this is where the issues can start. Simply throwing a constant current and relying on the BMS boards to control the charge of a multi cell pack will see the weak one charge first and it will end the charge for the rest. When that pack goes into use ALL the cells generally will have only been charged to the level of that weak one. This is JUST WRONG and is just here as a warning to others. This sort of DUMB charging of Lithium cells does damage at the low voltage end and also at the top end.Using individual BMS boards be they cap or strip versions will look after each cell and the pack overall from letting smoke and fire out as that cell simply goes out of circuit on low or high voltage

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2021, 03:25:46 pm
content: Quote from: DaveC1964 on April 01, 2021, 01:17:29 amI do not think the built in battery protection will work against that sort of fault with a series connection. In series if one cell is weak, then the other cells will drive it in reverse.

author: DaveC1964
author_rank: Newbie
timestamp: April 01, 2021, 06:23:11 pm
content: Quote from: David Hess on April 01, 2021, 03:25:46 pmHow is it done in hard wired packs? You see those welded packs used in e-bikes etc where the cells are just stuck together with no protection or no BMS at all. Wouldn't the same be true with those as they are always series and parallel.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2021, 06:27:02 pm
content: Quote from: DaveC1964 on April 01, 2021, 06:23:11 pmThe good ones that I have studied measure the voltage across every individual cell and shutdown when any one cell reaches the minimum voltage.

author: mikerj
author_rank: Super Contributor
timestamp: April 01, 2021, 10:52:49 pm
content: Quote from: DaveC1964 on April 01, 2021, 06:23:11 pmThe e-bike packs I have taken apart have all had some form of BMS.

author: camila
author_rank: Contributor
timestamp: April 03, 2021, 05:35:54 pm
content: BMS is always ideal, go ahead!