connecting bms to battery - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: December 18, 2017, 01:37:27 pm
content: hello everyone.this post is to clarify my doubt regarding the terminals in a BMS which is connected to a 4S1P battery pack. I'm little confused with The terminals B1, B2, B3 on the BMS pcb. I have attached a pic of the BMS that I'm using. So the terminals B1, B2, B3 should be connected to points a, b, c other in addition to the balancing wires right?? what do these terminals do ??

author: Audioguru
author_rank: Super Contributor
timestamp: December 18, 2017, 05:34:09 pm
content: You are just guessing that "a" is not "d" so your cells might be connected backwards. Usually the charger is made by the company who made the battery then they talk the same language.Do you know what balancing is? It adjusts the voltage and current for each battery cell so that the weakest one does not overcharge while the stronger cells get hardly any charge. These extra wires and the balanced charger circuit doo dat.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 18, 2017, 06:07:52 pm
content: yes, terminals B3, B2 and B1 should be connected to points c, b and a respectively.these terminals are used to balance the cells, which is the reason for using a BMS in the first place.lets consider your illustration, which shows four cells connected in series. assume that each of these cells should be charged to 3V for a total of 12V. when you're using the battery pack, the cells might discharge at different rates due to slight variations in temperature, internal resistance, etc.lets say, after using the battery pack for some period of time, that the four cells have discharged to [2V, 2V, 2V, 2.5V] (total of 8.5V). you now plug the battery pack into a charger, configured to charge the pack to 12V. the charger will now pass current through the battery until it measures 12V between the end terminals. since the same current is passed through all four cells, we will assume that they are charging at the same rate. in this case, the charger will measure approximately 12V (~12.1V) when the cells are charged to [2.9V, 2.9V, 2.9V, 3.4V]. at this point, the fourth cell is already severely overcharged. this can be especially problematic when using for example lithium ion cells, which are often very sensitive to being overcharged. best case, it could damage the cell, worst case, it could explode.the BMS monitors the voltage level of each cell individually using the B1, B2, and B3 connections. it can therefore cut power/disconnect the charger when any of the individual cells exceed a certain voltage (in this case, 3V), providing an added protection to prevent overcharging. its use is not limited to protecting against overcharging, but it is one of the main features of a BMS.(a lot of the above assumptions are exaggerated and/or simplified to aid in explaining the concept)

author: Adhith
author_rank: Frequent Contributor
timestamp: December 19, 2017, 09:47:53 am
content: Thank you very much for the help but I still have one more thing to make clear. Sorry if I'm asking a stupid question. So I understood that B1, B2, B3 are for balancing then the 5 wires from that white connector should be for charging right?? but out of that 5 wires 3 are common for B1, B2, B3 also. So what is the necessity for again taping wires from same voltage points, could it be shorted within the board itself during designing??

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 11:41:47 am
content: i am not familiar with the exact unit from your illustration, and therefore also don't know the architecture of the board or the pinout of the connector.i do not however believe the connector is used to charge the unit. i would assume that the connector taps these voltages to allow you to monitor them externally.the charging of batteries often involve fairly high currents, which require a reliable and robust electrical connection. i would much rather solder the balancing wires to the large B1, B2 and B3 pads than connect the cells through the connector. you would charge the battery through wires connected to the P+ and P- terminals. you can them monitor these voltages either by measuring the pads directly or through the connector. i would suggest that the voltages at the connector pins might be buffered in some way, but connection diagram from your illustration suggests that this is not the case.no modification/shorting should be necessary. just ignore the connector if you do not need to monitor the voltages externally. but as i said, i'm not familiar with that unit, so I might be wrong

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 11:57:00 am
content: on second thought, it probably wont matter if you connect the balancing wires B1, B2 and B3 to the cells via the connector, as long as the B+ and B- pads are connected to the positive and negative terminals of the battery pack using a reliable, robust, soldered connection. the cells should also be connected to one another using a robust connection.so i dont think it would matter if you use the balancing connections from the pads or the connector. but once again, i would not have complete confidence that a low cost, no name connector would maintain a reliable connection for long

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 11:58:23 am
content: If that picture came with the BMS I would assume it has already been wired to use a standard LiPo balance JST plug. You could trace the PCB traces and probably find they connect to B1-3. Otherwise it would make no sense at all to have that plug there. It would also be highly confusing, since it looks awfully like a balance plug.Balancing (BTW) does not need to be done while charging. It can be done at rest using parasitic power of the pack itself, "passively" if you will.

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 12:03:24 pm
content: Quote from: nicalitz on December 19, 2017, 11:57:00 amI could be wrong, but it looks like a standard JST style balance plug. These are used everywhere for that purpose. The balance plug does not take very much current at all. I don't have figures, but differential cell to cell current is probably only 100mA at most. This is why balance leads on lipo packs are usually only about 26 gauge wire.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 12:12:38 pm
content: Quote from: paulca on December 19, 2017, 12:03:24 pmagreed, the balancing should not take much current at all, which i corrected in my follow up post. i am not familiar with using battery management systems with standard JST plugs, my experience is more with custom designs for high reliability products. but following up i assume that you are correct

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 12:38:11 pm
content: Quote from: nicalitz on December 19, 2017, 12:12:38 pmIt's the first one I have seen with a balance port on it too. It's not entirely clear it is a JST either. I was curious as all my LiPos have these JSTs and I am building a 18650 pack and intend to put a JST on it. So I would like a BMS to have such a port. Makes swapping the battery easier. I know this is a dying art in todays world, the ability to change the battery is taboo these days.They also attach very well to standard pitched PCB headers.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 01:04:57 pm
content: such a port is convenient, no doubt about it. would likely also consider one for personal projects. my line of work has however predisposed me to favor reliability above all else. i'd prefer a solder joint over a connector 99% of the time, and if i do end up using a connector, it should have some 'locking' mechanism, be it a clip, screw, etc. i guess it comes down to application an personal preference, both have their advantages

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 01:40:19 pm
content: Take your pick:https://en.wikipedia.org/wiki/JST_connector

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 02:09:04 pm
content: more of a general comment from my side, not aimed at JST connectors. as i said, i've never used them before. would definitely consider them for future projects!i would however be cautious of know offs. from the wikipedia article:The term "JST" is incorrectly used as a vernacular term meaning a small white electrical connector mounted on PCBs.. some ebay sellers might incorrectly advertise the connector as JST. but if bought from reliable distributors, they seem like a great option.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: December 28, 2017, 07:11:31 pm
content: here mate:I've done several videos on these units.