Charging 18650 LiPo cells via BMS, using a non-constatant current PS - Page 1
Forum section: electronics / projects

author: bill6560
author_rank: Newbie
timestamp: June 11, 2024, 06:41:36 pm
content: I'm in the process of building a 4S/14P power pack (for ham radio use**) using 18650 LiPo 2600mAh (2.6Ah) cells, for 16.8V (4.2Vx4) at 36Ah total. With the battery pack, I'm using a smart JK-BMS 4S BMS for charge/discharge control. I don't have a constant current power supply for charging, but what I intend to use is a regulated 18 Volt (±10%), 180W, 10 Amp switching PS to charge the pack, with the PS's voltage output set to 16.8 Volts. The PS (https://www.amazon.com/dp/B07STRPMFY) specs state that load current should limited to 80% of rated current, or about 8 Amps for safe use.My question is, using this non-constant current PS, the maximum charge current will be limited to PS's 8-10 Amps. If my math is correct, for 14P cells, at 10A that's about 714mA (0.28C), or at 8A that's about 570mA (0.22C) charge current per cell. The PS does have Over Current, Over Voltage, and Short Circuit protection.Another unknown is whether the BMS will control the charge current to limit it to 8A, however the JK-BMS app does have a setting for "Max Charge Curr (A)".Will this work and be safe for the cells?Thanks,Bill** P.S. To limit the initial voltage of the pack's 16.8V, I'm inserting a 60A full bridge rectifier in series to limit the voltage to a safe 15.4V for my ham gear. Once the pack settles down to it's nominal 3.6V/cell voltage (~14.4V at 4S), I'll switch out the bridge rectifier.

author: tooki
author_rank: Super Contributor
timestamp: June 11, 2024, 07:22:39 pm
content: Could you share the precise, full model number of the BMS, and link to the documentation if you have it handy?If the BMS lets you set a current limit, it stands to reason it can control it. Bear in mind that controlling charge current is a fundamental function of any lithium charger that even pretends to do it properly.Only a willfully reckless lithium charger would rely on the power supply’s own current limiting — or conking out — to limit charge current.As for whether this is suitable for the cells: find and read the cell datasheet.Where in this setup is the cell protection?

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 07:29:21 pm
content: QuoteThere is no evidence that PSwill limit currentfor you. I'd rather say thatyoushould provide this current limitring for PS. Without external current limit it just turn off load on overload protection.PS. IMHO PSU is very like to be remarked MeanWell PSU (it has the same part number as MW one)

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 11, 2024, 07:40:18 pm
content: Are you sure your PS will limit current? Most commercial SMPS will happily provide huge amount of current when connected into low impedance load, only to then completely shut down when its OCP trips instead of limiting current.

author: bill6560
author_rank: Newbie
timestamp: June 11, 2024, 08:33:55 pm
content: No, I'm not sure. I don't know what kind of initial charge current would be needed for a 4S/14P pack, assuming that charging would start at around the cell's 3.0V level, or whether the BMS will limit the charge current.

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 08:38:04 pm
content: QuoteIn general BMS will not regulate charge current. Maximum it can turn off charging if current will be too large (or do not turn off - it depends on BMS).

author: bill6560
author_rank: Newbie
timestamp: June 11, 2024, 08:43:08 pm
content: If push comes to shove, I guess I could construct an off-board current limit circuit.

author: bill6560
author_rank: Newbie
timestamp: June 11, 2024, 09:04:36 pm
content: Quote from: tooki on June 11, 2024, 07:22:39 pmIt's model JK-BD4A8S-4P (https://device.report/JK/BD4A8S). On Page 27 (i)"Maximum charging current", "Charging overcurrent delay", "Charging overcurrent release"), it talks about the BMS shutting down when the max overcurrent value is exceeded after XX seconds of delay.As for the 18650 cells themselves, they came out of one of my e-bikes 48V battery pack that had a bad BMS. It only had about 30 hours on it, so the cells are essentially new. However, as is common in these cases, I've been unable to identify the cells manufacturer (see attached image), so specs, other than what's printed on the cell itself, is unknown.For protection, I plan to use the "Tesla type" fusing method for each individual cell, using a 1/2" length of 30AWG copper plated wire as the "fuse". Additionally, the entire pack will be fused, with several cutoff switches.

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 09:10:24 pm
content: QuoteLinear current limiter for 8A is a quite bulky thing - it should be able to dissipate about 25-50W. It requires a huge heatsink.DC/DC current limiter mostly as complex as original PSU.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 12, 2024, 02:39:44 am
content: Quote from: bill6560 on June 11, 2024, 06:41:36 pmJust lower the charging voltage to 16V or so, you won't lose much capacity but it will help the battery age slower.

author: Freesurfer
author_rank: Regular Contributor
timestamp: June 12, 2024, 09:25:15 am
content: I think it is a common misconception that if you have a PSU rated for a certain current, that it will also limit to this current.That is not the case, and very often you will end up with an uncontrolled and undefined current that can overload the PSU. The current rating is just what the PSU can safely deliver.Even if it may not be 100% academically correct, I usually tell people that the power supply is responsible for controlling the voltage. The load is responsible for controlling the current.I know this is for your HAM project, but it should be remembered that when you have device with a missing power supply, and you're looking for a compatible one, the rule of thumb is that you need a power supply with a matching voltage/polarity, and one that is at least capable of supplying the current demanded by the device. The rule never says that it needs to match the current, and you don't know if somebody is going to use a 1.5A or a 10A supply, which further backs up the claim that the device must control the current on its own.

author: strawberry
author_rank: Super Contributor
timestamp: June 12, 2024, 05:25:50 pm
content: resistor to limit maximum charging currentvoltage will limit BMS(thats what they do in cheap BT speakers, screwdrivers,..)it will be better anyway when compared to cheap and unreliable DC/DC or CC SMPS

author: bill6560
author_rank: Newbie
timestamp: June 12, 2024, 06:07:44 pm
content: Quote from: Freesurfer on June 12, 2024, 09:25:15 amGoing in, I understand all that's being said. First, my ham gear wouldonlybe powered by the battery pack, with good useful batteries that were essentially free. I'm just looking for a frugal, but safe method for charging (through the BMS). I also know a lot about PSU design, but very new to LiPo 18650 battery use, maintenance and their very specific charging requirements. The blessingand cursewith YouTube (and the web) is you'll get a multitude of solutions, not all good, or even safe. I come to forums like this, who's base seems to be technically knowledgeable, to try to get solid and accurate answers.I also realize that over-current protection in most PSU designs are typically a crowbar that shuts down the PSU if its rated current is exceeded. I was just hoping against hope that someone would come back and say they did the same thing and it worked for them. Oh well, I'm an incurable optimist.Bottom line. It seems that my low cost PSU is just not suitable for LiPo charging without some sort of auxiliary constant current add-on. I do have an older BK bench PS with is CV/CC, but only with 30V/3A output. This will of course work, but at a much slower charge rate.Finally, one other option is to use one of those low cost buck/boost converters that say that they are CV/CC, but is seems that these (Chinese) options out there are of tenuous quality and reliability. If anyone can make a recommendation on this option, it would be appreciated. What's seen on Amazon and eBay are less than a dozen variants of the same thing. One nice thing about Amazon, is if you look at one that has many reviews, you'll get a good handle if these are junk or not. One review said it blew up and caught fire.Thanks to all for your input,Bill

author: xvr
author_rank: Frequent Contributor
timestamp: June 12, 2024, 06:13:25 pm
content: There is another option: you can try to add CC circuit to your existent PSU. You need schematic of it to do so. I think you can try to trace its PCB. May be you get a little luck and CC circuit can be created.

author: strawberry
author_rank: Super Contributor
timestamp: June 12, 2024, 07:20:41 pm
content: could look for something available on TindieI made adjustable sepic CV/CC 12->0..20V ~0..10A . basically for battery chargingcells in series dont balance them selves wellsome BMS are only over-chage over-discharge protected (no current protection other than poly fuse)

author: tooki
author_rank: Super Contributor
timestamp: June 12, 2024, 08:45:58 pm
content: Quote from: bill6560 on June 11, 2024, 09:04:36 pmOK, so the BMS doesn’t have a current limit, it has overcurrent protection, and the “max current” is you telling it where to trigger.Anyhow, if you look at that manual, you can see the diagrams showing that you need a charger in between the power supply and BMS. The charger is not optional! It’s the thing that manages the CC/CV lithium-ion charging scheme, and you cannot do without it.Don’t try to jerry-rig something, don’t pass GO and collect $100. Buy a charger and do it right. But before that, study up on lithium ion charging, and the hazards, because this is an area of power electronics where getting something wrong can mean your house burns down.Quote from: bill6560 on June 11, 2024, 09:04:36 pmThat is unfortunate. Tread lightly, in that case.Quote from: bill6560 on June 11, 2024, 09:04:36 pmIf you think a fuse is all that’s meant by “cell protection”, you do not know enough about this topic to do this safely. I’m not trying to dissuade or discourage you from ever doing it, but you need to put on the brakes and do a lot more homework first.

author: bill6560
author_rank: Newbie
timestamp: June 12, 2024, 09:20:19 pm
content: Ok. I tried to read up on all things LiPo batteries (apparently not enough), but not exactly sure what you're referring to as "cell protection"?I'm using multiple fuse protections, disconnect switch's, and most importantly, using a good BMS to protect the cells on charging and discharging. The JK-BMS also has dual thermistors to monitor cell temperatures.Please provide a link or two that describes your "cell protection" concerns.Thanks

author: tooki
author_rank: Super Contributor
timestamp: June 13, 2024, 06:48:55 am
content: Cell protection means protecting against overcurrent, overvoltage, and undervoltage. It’s typically either part of a BMS, or built into a cell itself. (18650 cells exist in both protected types with the protection IC on a tiny PCB built into the top of the cell, or as bare cells with no protection, in which case you have to provide it yourself as part of your charging circuit and BMS.

author: bill6560
author_rank: Newbie
timestamp: June 14, 2024, 05:46:22 pm
content: Just a clarification for BMS wiring. While JK-BMS seems to be one of the better BMS's, it seems apparent that they mostly serve the Asian market. As such, it's documentation, originally written in Chinese was translated, and as we all know, a lot gets lost in translation. Referring to the top part of the attached image, JK's wiring diagram shown in their documentation is a bit misleading. You see the "controller" and "Charger" tied together, which is incorrect. You can't have both attached to the BMS at the same time. Also, my guess is that "controller" was really meant to be "Load".Looking a the bottom section of the image, you'll see the correct wiring of a typical BMS system. The (JK) BMS is smart enough to know which direction current if flowing, so it knows whether it's batteries are being charged or discharged.