Homemade flashlight with 2x 26650 batteries. How to charge? - Page 1
Forum section: electronics / beginners

author: PaulSimon
author_rank: Newbie
timestamp: May 29, 2023, 12:28:30 pm
content: Hi,First time posting on these forums.I do various electronics projects from time to time just for fun, but I don't have any education within it so I sometimes get stuck on very simple things - and this might be one of those simple things.So for this project I want to use two 3.7v lithium batteries, and I want to be able to charge them both internally.To reduce the amperage in the final DC-DC converter, it makes sense to me to run the batteries in series configuration.But this gets me a bit confused about how to wire things.Here's a diagram of how it would look with parts I currently have:How can I charge the two batteries internally while still maintaining the series configuration?I assume that the current configuration just wouldn't work. Or would it?Would be nice with some feedback here, this is a bit confusing to me.

author: Ian.M
author_rank: Super Contributor
timestamp: May 29, 2023, 12:41:09 pm
content: That will let the magic smoke out, because the single cell LiPO charger modules are non-isolated and almost invariably have a direct (or fractional ohm) internal connection from In- to Batt-, so the 26650 cell on the left will be short-circuited.You need a two cell series (2S) LiPO balance charger, which is quite likely to need >9V input

author: PaulSimon
author_rank: Newbie
timestamp: May 29, 2023, 01:03:44 pm
content: Thanks for the response. Looking around, I'm finding small convenient 2S battery chargers that accept USB 5v input, which is perfect for my purpose.Some have 2 outputs, and some have 3 outputs. I assume the 3-output ones are the ones that will actually balance the batteries, like this one:https://www.aliexpress.com/item/1005005589570229This would actually simplify my circuit, which is nice.Assuming I buy the chip I linked, would this be the correct way of wiring it?Edit.Actually, it might be like this:

author: Ian.M
author_rank: Super Contributor
timestamp: May 29, 2023, 01:16:20 pm
content: Top schematic lets the magic smoke out again, and as you've hard-shorted both batteries, expect to trip their protection, or even get a battery fire!Bottom schematic looks OK, however until you get the module, you cant be certain how much the 2s charger will drain the batteries when there is no USB power.

author: PaulSimon
author_rank: Newbie
timestamp: May 29, 2023, 01:23:24 pm
content: Thanks again. Very nice to get help this quick.I'll watch out for phantom draw, and if necessary have a secondary switch to disconnect the board when not charging.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: May 29, 2023, 03:14:55 pm
content: You can simply toggle the ground between the charger and boost modules. Be sure your switch can handle charge and discharge currents. I assume this is not going to magic smokeWhat you really need is a BMS with over-discharge protection.

author: PaulSimon
author_rank: Newbie
timestamp: May 29, 2023, 03:56:56 pm
content: These batteries are supposed to have discharge protection built-in. Doesn't this make a BMS redundant for my use?https://www.orbtronic.com/26650-battery-protected-li-ion

author: thm_w
author_rank: Super Contributor
timestamp: May 29, 2023, 11:54:16 pm
content: From your link: "YES - Advanced Safety protection (overcharge, over-discharge, current-overload [cutoff @ 15 Amp], short circuit)."Mostly, yes, a BMS might offer balancing though to keep the two cell voltages within range of each other.But if you are sure to fully charge or discharge both cells before installing them in the circuit, then they should be balanced, and should be fine for a long time.I would just have them in parallel to keep things simple. 10W is ~2.8A at 3.6V, not that crazy.

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2023, 02:47:21 am
content: The 2S charger module PaulSimon intends to use does balancing.https://www.aliexpress.com/item/1005005589570229.htmlThe only remaining reason to consider some sort of BMS would be the impact of discharge cutoff voltage on cell lifespan, as the cells own protection board discharge cutoff voltage is likely to be fairly low, and a higher cutoff voltage may be desirable, especially if there is a risk that it will be stored fully discharged.

author: Peabody
author_rank: Super Contributor
timestamp: May 30, 2023, 02:54:22 pm
content: Quote from: thm_w on May 29, 2023, 11:54:16 pmI've always assumed that parallel batteries should not have built-in protection. The reasoning is that the protection on one of them may trigger at a different voltage than the other, and then when charging current is applied again, and everything reconnects, there will be a voltage difference, which is not what you ever want with parallel batteries. So unprotected batteries should be used when they are in parallel. Does that make any sense?Edit: Of course you would need protection somewhere, but perhaps it should be incorporated in the charging system, like a DW01 circuit.

author: thm_w
author_rank: Super Contributor
timestamp: May 30, 2023, 11:35:02 pm
content: Quote from: Peabody on May 30, 2023, 02:54:22 pmIt makes sense, but can't see it being a major issue as the voltage difference would be some tens of mV.And yes as you say, get a $2 USB-C charger module and it will have a DW01 built in.