BMS 3S 60 Amps: unequal voltage outputs - Page 1
Forum section: electronics / general

author: panoss
author_rank: Frequent Contributor
timestamp: February 17, 2024, 08:41:42 pm
content: I havea BMS 3S 60 Amps.('0V'= bottom left, '4.2V'=bottom right, '8.4V'=top left, '12.6V'=top right (of the PCB))Between points '0V' and '4.2V' I measure 4.26V.Between points '4.2V' and '8.4V' I measure 4.06V.Between '8.4V' and '12.6V' I measure 4.29V.(measurements taken with batterries connected on the BMS)My problem is the second one, 4.06V (20mV less than the others!).Maybe I can change this to 4.26V? What should I check?(maybe this is not really a BMS? I see no MCU)

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2024, 08:47:58 pm
content: It's a BMS, not a cell balancer. If you want your cells to have equal voltage, you need to add a balancer.

author: panoss
author_rank: Frequent Contributor
timestamp: February 17, 2024, 08:49:34 pm
content: What? I thought every BMS is also a balancer!

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2024, 08:53:34 pm
content: Quote from: panoss on February 17, 2024, 08:49:34 pmNot necessarily. They are different devices. Sometimes they come in a single unit, sometimes they don't.The job of a BMS is to provide several types of protection: overcurrent (including short circuit), overcharge, overdischarge, some will also do overheating protection. Cell balancing is strictly speaking a separate task, and it's not always considered to be required.Yours however does have a bunch of resistors and SOT23 parts, and that might be a balancer, I can't readily tell this (maybe someone else can). Even if so, it will be a low-power one, and if it's there, it doesn't do a good job. What do the specs of your BMS say, if there are any?

author: panoss
author_rank: Frequent Contributor
timestamp: February 17, 2024, 08:58:46 pm
content: I found no specs (the link of the seller is posted on my 1st post).I 'll search on other sellers, maybe I find something.

author: johansen
author_rank: Super Contributor
timestamp: February 17, 2024, 08:59:05 pm
content: Its a 2$ bms, what do you expect?They typically ballance the cells once there is a 20mv difference. And they may only do it once a cell reaches 4.2 volts, that is, only during charging

author: edpalmer42
author_rank: Super Contributor
timestamp: February 17, 2024, 09:01:07 pm
content: What size batteries were connected and for how long?It's possible that the batteries in the middle of the string aren't quite finished charging. Combine that with normal component tolerances and you might not have a problem at all.By the way, the voltage difference is 200 mV, not 20 mV. But is that difference even significant?Ed

author: panoss
author_rank: Frequent Contributor
timestamp: February 17, 2024, 09:03:06 pm
content: The batteriies are of 18650 type.I also took measurements without any batteries connected, the results are the same.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2024, 09:06:37 pm
content: So the item description page says that it does have a balancer. I stand corrected.The question, however, remains, how accurate it can be and whether what you observe is its expected behavior. Try to charge it to something like 11 volts, instead of full charge, and check cell voltages again -- will they have the same difference as when fully charged?

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2024, 09:08:48 pm
content: Quote from: edpalmer42 on February 17, 2024, 09:01:07 pmIt may very well be. Consider 12.6 V end of charge voltage, then we can have 12.6 V = 4.0 V + 4.2 V + 4.4 V. Not good!...but overcharge protectionmaykick in earlier and not let that cell ever reach 4.4 V. This has to be verified, especially with cheap boards like this.

author: panoss
author_rank: Frequent Contributor
timestamp: February 17, 2024, 09:09:17 pm
content: Quote from: johansen on February 17, 2024, 08:59:05 pmI don't think it's a matter of price, I also have two other batterieswith these 2$ BMSsbut they 're working fine.Quote from: johansen on February 17, 2024, 08:59:05 pmI will discharge and recharge it to see what will happen.

author: panoss
author_rank: Frequent Contributor
timestamp: February 18, 2024, 10:36:16 am
content: I discharged the battery, I recharged it (not fully) and I got:Between points '0V' and '4.2V' I measure 3.99V.Between points '4.2V' and '8.4V' I measure 3.87V.Between '8.4V' and '12.6V' I measure 4.00.This is 130mV (@edpalmer42 you 're right) difference from lowest to highest.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 18, 2024, 12:57:56 pm
content: Quote from: panoss on February 18, 2024, 10:36:16 amCharge the middle one separately until it settles to about the same voltage as the other two.In the long term, you'll probably want to get an active balancer.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 18, 2024, 01:03:30 pm
content: Spec says 4.20V, yet you measure 4.29V. Unless your multimeter is off by that much, put the crap into e-waste where it belongs.Any BMS (except colossally oversized one, which makes no sense) requires initial manual balancing of cells. Balancing current is usually small to only maintain the balance.

author: panoss
author_rank: Frequent Contributor
timestamp: February 18, 2024, 03:17:40 pm
content: Quote from: Siwastaja on February 18, 2024, 01:03:30 pmI just have to lower the charging voltage, I gave it too much.Quote from: Siwastaja on February 18, 2024, 01:03:30 pmDidn't know this. So if I ballance it manually it might (later on) work ok?

author: johansen
author_rank: Super Contributor
timestamp: February 18, 2024, 03:57:16 pm
content: Quote from: panoss on February 18, 2024, 10:36:16 amCharge it at 20mA until some cells get to 4.2 and stay there and see i the balancer can hold them down while the others come up.

author: tunk
author_rank: Super Contributor
timestamp: February 18, 2024, 06:35:30 pm
content: No expert on this, but there looks like to be three 100ohmresistors which could be the balancing "bypass". If so, youcan only charge with up to 42mA (4.2V/100ohm) for balancingto work. Also, most 18650s should not be charged above 4.20V.

author: panoss
author_rank: Frequent Contributor
timestamp: February 18, 2024, 06:40:09 pm
content: Quote from: johansen on February 18, 2024, 03:57:16 pmYou mean I should limit the charging current at 20mA?

author: johansen
author_rank: Super Contributor
timestamp: February 18, 2024, 06:41:49 pm
content: Quote from: panoss on February 18, 2024, 06:40:09 pmYep, because who knows what the balancing current actually is until you measure it