Noddy question re. charging - Page 1
Forum section: electronics / beginners

author: Mattyuk
author_rank: Newbie
timestamp: October 05, 2023, 08:12:10 pm
content: OK I'm new to this and am fairly sure this is a very basic question, so please go easy on me! I can't find a simple answer on the web.I've made a 3s2p battery pack using 6 LG18650M26 18650 cells and this BMS -https://www.aliexpress.com/item/1005002117390409.html?spm=a2g0o.order_list.order_list_main.53.1c3d1802D2xB1A.I was planning to set it up so I could charge it using a phone USB-C charger via one of these little step up modules adjusted to say 12.5v output -https://www.aliexpress.com/item/1005005133852642.html?spm=a2g0o.order_list.order_list_main.54.1dd418021Jff5j.But I've just had a thought. What is it that will determine how much current the 3s2p pack I have built will draw when it is hooked up to a fixed voltage charger? I have a feeling ohms law has the answer but like I say I'm new to this. I've been charging it so far just using a 1500ma 12v charger and it seems to go ok. However is it the case that:- The pack will draw 12v (ie. whatever the charger/step up module is fixed at) and as much charge as it can pull out of the charger/step up module? So a very low current supply at 12v will still work but will just take a long time to charge. Or- The pack will draw 12v and pull whatever current it wants to pull, and if that is more than the charger/step up module can provide, then the charger/step up module will just overheat and die.If it's the latter, how do I go about working out how much current the power supply needs to be able to provide?Thanks all in advance. I'm just getting into electronics so this might be the beginning of a fun journey (if I don't burn my house down).

author: p.larner
author_rank: Frequent Contributor
timestamp: October 07, 2023, 07:21:42 am
content: can you not use tp 4056 moduals in series maybe?

author: Dan123456
author_rank: Regular Contributor
timestamp: October 07, 2023, 12:48:53 pm
content: 18650’s like a constant current then constant voltage charge.EEVblog did an excellent video on this that helped me resurrect a whole bunch of cells from some old power packs.From memory and in short, a single cell will first charge using a constant current of X amps until they hit the correct voltage (often 4.2V). They then want constant voltage from there until they are drawing a certain amount of mA (I think 30-50mA is common).In short, charging Lion battery’s is a little more complex than just feeding them the right voltage but I hope that helps somewhat

author: Eraldo
author_rank: Regular Contributor
timestamp: October 07, 2023, 03:16:26 pm
content: The modules you linked from aliexpress are battery charging modules which have built-in current limiting (as well as the voge selection). If you use that module it will charge with constant current of the set current (the current is set by a resistor, read the 9 photo pages on aliexpress in the link) and then when the battery reaches the set voltage, it will charge with constant voltage.The modules aren't that powerful so it's going to be a slow charge (1 Amp max for 4.2 V. So if you set the voltage to 12 v its going to be more like 500ma max).

author: Mattyuk
author_rank: Newbie
timestamp: October 07, 2023, 04:24:10 pm
content: Thanks All.Eraldo I think you're closest to where I'm wondering. Do you know what would happen if I DIDN'T use a charging module like the one I linked to? ie if a power supply was just hooked straight up to the batteries without any charging module or BMS? Assuming the 18650s were the unprotected type, would they draw an amount of current that reflects their resistance (I assume total internal resistance?) based on ohms law?Dan apologies if that is the answer you have given but I'm just testing my understanding.Anyway sounds like the good news is I can just use one of the piddly little usbc charging modules if I want to, it might just take a looong time unless I find a module capable of higher current output.

author: tunk
author_rank: Super Contributor
timestamp: October 07, 2023, 04:56:12 pm
content: Not sure if that board does CC/CV/cut-off charging.Could look like a BMS that has these protections:- over-charge voltage- over-discharge voltage- over-discharge currentEdit: And cell balancing.

author: sleemanj
author_rank: Super Contributor
timestamp: October 08, 2023, 12:24:24 am
content: Quote from: p.larner on October 07, 2023, 07:21:42 amNo you can't use TP4056 in series, they are not isolated, you would short out cells through the common grounds.

author: sleemanj
author_rank: Super Contributor
timestamp: October 08, 2023, 12:36:03 am
content: Since you have a protection board ("BMS" as the chinese rather generously call them), all you really need is a CC/CV boost or buck converter as appropriate for your source voltage.Set the Current Limit to your maximum desired charge current.Set the Voltage Limit to your maximum desired voltage.There are plenty of such CC/CV converters in the usual cheap stuff places (random example). Some will also have a "charged" indicator (sometimes adjustable) which just lights an LED when the current drops below a set-point (typically 10% of the max set current).Simply take the module (not connected to your pack!), and your multimeter, set the output voltage to your max pack voltage (or better, a bit less), put multimeter into 10A range and short the module output (again, not connected to your pack!) and adjust to get the maximum current you want to charge with. Now connect it to your pack (through the protection board you have).

author: Mattyuk
author_rank: Newbie
timestamp: October 09, 2023, 08:27:33 pm
content: Thanks Sleemanj/tunk that's useful to know how I can achieve the end goal.What I'm trying to understand however is the electronics/physics behind it. ie what happens if I do it "wrong" eg. without any BMS? That's what my question below is trying to ask. Any ideas?Do you know what would happen if I DIDN'T use a charging module like the one I linked to? ie if a power supply was just hooked straight up to the batteries without any charging module or BMS? Assuming the 18650s were the unprotected type, would they draw an amount of current that reflects their resistance (I assume total internal resistance?) based on ohms law?

author: sleemanj
author_rank: Super Contributor
timestamp: October 10, 2023, 12:14:37 pm
content: Yes. By ohm's law. I = V/R the R is the internal resistance, and the V is the voltage across that resistance being the supply minus the cell's current voltage.Example, internal resistance 0.01 Ohms, cell at 3.2v, apply a 4.2v constant voltage capable of infinite current, and you end up pushing circa I = (4.2-3.2)/0.01 = 100 Amps, which would be probably not fun.Which is why you use a Cc/Cv supply limited to an appropriate current and voltage.

author: ledtester
author_rank: Super Contributor
timestamp: October 10, 2023, 02:51:56 pm
content: Note that the current limiter protects the power supply as much as the battery. Over loading a power supply will generally result in it not working as intended and the result could be as benign as it shutting down or blowing a fuse. But it also could result in damage to the supply (through components overheating) or even starting a fire.

author: tunk
author_rank: Super Contributor
timestamp: October 10, 2023, 04:41:16 pm
content: Also note that the BMS has a 100A over-current protection.This is more than your cells can deliver, so you don't reallyhave any over-current protection.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2023, 04:48:33 pm
content: Quote from: Mattyuk on October 07, 2023, 04:24:10 pmDO NOTconnect a lithium ion cell or battery directly to a power supply unless you know what you are doing, andespecially do not connect one to a constant voltage supply.Lithium ion cells like 18650s can fail spectacularly with flames and fireballs if overcharged. There are documented cases of houses burning down due to mishaps when charging lithium ion batteries.It cannot be overstated:Only use a proper chargerInstall a protection circuit on the battery

author: Mattyuk
author_rank: Newbie
timestamp: November 10, 2023, 06:05:13 pm
content: Thanks everyone for the very interesting responses and advice and apologies for my slow reply.Don't worry I'm not going to hook anything without appropriate protections up, just interested in the theory.The lithium battery fire question is an interesting one and I'd guess one that will spark significant debate here and elsewhere. Looking at the limited stats available online eg.https://www.theguardian.com/news/2023/may/02/e-bike-e-scooter-battery-fires-uk-datacould it be argued that the risk is being somewhat exaggerated? The above article forecasts 338 fires from scooters and ebikes this year based on responses from 38 fire services (I believe there are 50 total). Based on info herehttps://www.gov.uk/government/statistics/fire-and-rescue-incident-statistics-year-ending-june-2023/fire-and-rescue-incident-statistics-england-year-ending-june-2023the numbers of fires in England alone per year number in the hundreds of thousands.I'd be interested in other's (civil) thoughts on this.

author: IanB
author_rank: Super Contributor
timestamp: November 10, 2023, 07:07:22 pm
content: Lithium battery fires are unusual if properly designed batteries and chargers are being used.Trouble may happen if there is a fault, or if people are using bare lithium polymer cells or batteries (e.g. for RC hobby applications). Lithium polymer is much more dangerous because it just uses a thin plastic pouch that is easily punctured, rather than having a protective metal shell.