18s ebike charger - Page 1
Forum section: electronics / general

author: m3vuv
author_rank: Super Contributor
timestamp: March 20, 2021, 02:18:51 am
content: hi all i have an 18s/8p ebike battery,its a standard 18650 l-ionpack not lipo4 so 4.2v per cell,i have a charger for a 13s pack,ie 54.6v the 18s pack needs 75.6v,can i set my lab psu too 20v and wire that in series with the 13s lipo charger and use that to charge the 18s pack?,thanks in advance,m3vuv

author: Siwastaja
author_rank: Super Contributor
timestamp: March 20, 2021, 08:45:33 am
content: No.

author: m3vuv
author_rank: Super Contributor
timestamp: March 20, 2021, 04:05:35 pm
content: any reason in perticular why not?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2021, 08:32:51 am
content: The information about the packs and their internal battery management system, if any, is completely missing.

author: beanflying
author_rank: Super Contributor
timestamp: March 21, 2021, 09:04:47 am
content: That for a start and if you opt for an unbalanced charge solution with that number of cells/parallel and you are more than a good chance of smoke and fire WHEN you overcharge one or more of them. Most if not all apart from the most trashy of chargers of multicell packs charges each cell to a voltage limit if for example one of those cell banks is or has failed even slightly will allow another to go over voltage with the system you propose.Example one bank has one parallel cell of reduced capacity will hit maximum voltage before the other banks. You are proposing not to monitor 'some' of those even if the charger will work under that system and they will continue to charge and go over the safe voltage limits.

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2021, 09:15:29 am
content: Quote from: m3vuv on March 20, 2021, 04:05:35 pmOr more specifically, go to a bit over 1 minute in:Louis insists that it was 100% NOTHING to do with his modifications/installation, etc etc.(EDIT: He doesn't necessarily say that as such, but that is the jist of what I thought he thinks, when viewing the video and/or other information about that fire)

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 21, 2021, 01:45:03 pm
content: Put it in the category of "it's fine if you really know what you're doing but if you have to ask, you probably don't". Definitely supervise it if you want to try anyways.Quote from: beanflying on March 21, 2021, 09:04:47 amAny BMS that's even half decent specifically protects against that case and the good ones even correct that situation.

author: beanflying
author_rank: Super Contributor
timestamp: March 21, 2021, 03:23:16 pm
content: Quote from: NiHaoMike on March 21, 2021, 01:45:03 pmThis is pure 'assumption' on your part of a BMS being fitted and how it is fitted on an unknown pack of unknown internals. Sweeping statements like this are potentially dangerous to others. In addition to this a typically BMS will not 'correct that situation' it will in a 'typical case' go open circuit and stop the charge due to an open circuit.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: March 21, 2021, 03:48:18 pm
content: I can understand what @m3vuv is trying to achieve, but my understanding is never connect two unmatched power supplies in series to create a combined voltage? It is the same reason why AA batteries should not be mixed in, for example, a remote control car. The stronger rated power source will always back-power into the weaker one. I hope the lab PSU will detect a 'short' and trip out. Else, the charger's output Mosfet will evaporate. Possibly?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 21, 2021, 11:13:05 pm
content: Quote from: beanflying on March 21, 2021, 03:23:16 pmBringing down the high cells is exactly what the balancing feature on a good BMS does. If the pack does not have a BMS at all (often the case for R/C packs), it is only safe to charge with a charger that is aware of individual cell voltages.Quote from: Syntax Error on March 21, 2021, 03:48:18 pmBoth the charger and the lab power supply are going to have current limiting so theoretically it should be fine.

author: beanflying
author_rank: Super Contributor
timestamp: March 22, 2021, 12:20:09 am
content: Quote from: NiHaoMike on March 21, 2021, 11:13:05 pmAGAIN YOU are making ASSUMPTIONS on this pack! BMS systems DO NOT further 'bring down' cells they switch them in and out they are not a magic 'fix'. For information ALL of my R/C packs of LiPos don't run BMS of any sort as I would blow the crap out of them.Your last statement about 'it is only safe .....' doesn't square with your earlier one of the BMS looking after the pack in spite of the charger. I suggest you do some reading on how multicell balancing chargers and BMS boards actually work. Chargers don't 'generally' give two stuffs about if there is or is not a BMS fitted anywhere the charger is what keeps the cells within the limits not the BMS.

author: m3vuv
author_rank: Super Contributor
timestamp: March 24, 2021, 05:54:10 am
content: the battery pack has a bms fitted from the factory,the reason i ask is{i lost the original charger and looking for a work around while im waiting for a new charger to arrive,

author: beanflying
author_rank: Super Contributor
timestamp: March 24, 2021, 06:21:40 am
content: I doubt it will have a BMS fitted for each cell btw far more likely they are grouped into banks. So the answers you got relating to BMS then also still make it a no they are not a charge control or limiting device they are only an over or under volt protection device. The first cell/group to trigger its BMS will stop the charge as it will/should go open circuit.Worst of all is even if you current limit the supply you proposed using it is way less than an ideal way to charge Lithium cells toward their last 20% and you risk doing lasting damage to the pack cells.

author: m3vuv
author_rank: Super Contributor
timestamp: March 24, 2021, 10:52:41 am
content: the battery is an 18s 8p 20ah pack,the 13s charger is for another pack i have,i just want to use my psu in series with it to make the total voltage up to 75.6v,the psu is cc/cv.,i gues the 13s charger will still sense when the battery bms cuts off at the end of charge?

author: beanflying
author_rank: Super Contributor
timestamp: March 24, 2021, 11:00:21 am
content: Lithium Batteries are not 'generally' charged at constant current for best life. Plenty of information around but they run a slow if really really flat, then a constant current to 80-85% then tapering toward full. Do a bit of online reading while you wait for the proper charger.Also if your 13S charger is designed to be used with balancing leads and you plan not to it will be worse than a power supply as it will have no idea of what you are doing to it externally. A voltage offset may cause it problems too.While this is 'generalist' advice it is given trying to avoid you letting the smoke out of electronics or worse fire out of your pack

author: Siwastaja
author_rank: Super Contributor
timestamp: March 24, 2021, 02:50:03 pm
content: BMS does not mean any standardized component doing a certain thing. BMS can do anything, or may not do anything at all. It very likely does something else than you expect. The three letters alone are meaningless.You need:1) the full documentation of the BMS,2) to understand itIt will also answer all of your questions.Before that, there is no way to make any assumptions about the system.Practically, there are three common classes of BMS:1) Those that monitor and inform you, andyouare responsible for implementing the required disconnect switches.2) Those that do include switches internally, so that the pack terminals are protected against overcharge and overdischarge; but the voltage/current/temperature limits are beyond normal operating limits, only as last-resort safety.Youneed to implement another layer of BMS for good safety and good cell life.3) Like 2, but with limits chosen within designed operating ranges of the cells; this is the only type of BMS that acts like an "ideal battery" which you can just use.Without the documentation, guessing is meaningless.Balancing is completely irrelevant question. You are ready to go when you understand why I say that. You need the basic understanding of the li-ion cell operating limits, and how the packs are managed to see this.

author: m3vuv
author_rank: Super Contributor
timestamp: March 25, 2021, 10:23:40 pm
content: well i have just ordered these,fingers crossed they are ok!https://www.aliexpress.com/item/1005001676531280.html?spm=a2g0s.9042311.0.0.2d784c4d5Wh70ahttps://www.aliexpress.com/item/2045636323.html?spm=a2g0s.9042311.0.0.2d784c4d5Wh70a