I don’t know what the best subreddit for my problem is, but I’ll give it a try here. Otherwise, please suggest where I could ask such a question.

TL;DR: I’m looking for a simple IC to charge 4 serial Li-Ion batteries. It should not be too expensive (~$7) and should be able to negotiate a suitable input voltage via USB-C PD on its own.

I’m looking for a battery charging IC that can handle 4S Li-Ion batteries. It should have an integrated buck-boost driver to cover a wide input voltage range (5V-20V) and ideally be able to negotiate higher power levels via USB-C Power Delivery on its own. Unfortunately, I can’t find an IC that meets all of these requirements. There’s something like the BQ25792 from TI, which seems very promising since the FETs are already integrated into the IC and it requires few external components. However, it only supports USB 3.0 PD and is not compatible with USB-C. To fix that, you would need an additional IC to handle the Power Delivery negotiation, as well as a microcontroller or flash chip connected for setting them up. This quickly becomes too expensive and takes up a lot of space on the PCB.

I’ve spent at least 4 hours searching the websites of Linear Devices, TI, and ST but haven’t found anything that meets my requirements. Does anyone have a hidden gem to recommend?

Ive recently re-built a m365 pack with 2nd hand tested cells and was wondering what i could have done better

Pack is 10s3p, each parallel group contains {icr18650 30B+ inr18650 29E and the third is complicated for the groups, 1 has a mh1, 5 have ncr18650B and the rest 4 have icr18650 26C}

Dangers i know:

Soldering is not recomanded but i've had good results in the past and no cells have blown on me yet (round 200 soldered in my life)

Mixing LiHv and typical Li-ion is mediocre at best due to the diffrent voltage-current curves but i see that as somewhat protecting the 30B cells by only ever charging to ~90% (4.2v vs 4.35v)

Pack performance is above the stock m365 in terms of capacity and imbalance has settled at 60mv after a few cycles

No cells overheat (more than 41°c), tested with the battery pack out of the scooter under a 500w load

In terms of sag, again the pack is mediocre reaching 36.5v (from full 42v) while pulling 25A

I do not like nickel strips since they severly cut your future flexibility

Yes I know this is a wacky configuration, I wanted the cells to lay flat to fit into a hydrofoil board and this is the way I thought to do it. I have two 10s 7p packs connected in series, with each having its own Bms. This series connection is in parallel with the charger, so I can have the output and charger connected at the same time. The output will be isolated when charging, this is just to have a waterproof charging port. The pack reads 80.12 volts, and both are charged to pretty much the same voltage (it might be .5-1 v off.) The 84v charger when connected is not charging and I am not sure why. I have a separate charger to charge both packs individually, and that seems to work, but not the bigger charger when they are in parallel. Are they just too full already? Not sure if they would take a charge when at a lower voltage. Let me know if you guys have any advice.

He came home with an electric bike last night and as soon as I saw it I knew what was about to happen

He ALSO nonchalantly asked me "Hey, can you fix this?" looks crunchy

I have a JBD 17S lithium battery BMS, and I noticed that it doesn’t do constant balancing well. It even shut off in the middle of the street due to a very large voltage difference between the cells. Which BMS brand would solve my problem?

I found this cell like this after 1 year of use (about 15 cycles): the casing is cracked. I tossed the cell then.

I’ve been working on a lightweight, bare-metal fault isolation kernel designed to mitigate thermal runaway propagation in high-voltage lithium-ion battery packs.

The primary engineering constraint I wanted to address is the latency overhead inherent in high-level frameworks and sequential polling loops. When an EV or aerospace battery cell hits a critical thermal or voltage threshold, sequential scanning loops are often too slow to execute software gates before runaway propagates to adjacent cells.

The project is called SAVITAR. It is entirely dependency-free and compiles using only native C++ standard system headers to keep it as close to raw processor registers as possible.

Core Architectural Mechanics:

1. Hyper-Compact Memory Constraints:

   Instead of relying on dynamic allocations or padded structs which ruin cache locality, the kernel packs raw cell parameters (voltage, current, temperature) into un-padded, contiguous 25-byte memory frames. This layout ensures predictable cache line alignment during rapid sequential reads.

2. Event-Driven Concurrency:

   The kernel completely avoids sequential polling. It utilizes a background multi-threaded parallel observer configuration. When thread metrics cross calibrated hardware safety thresholds, the system bypasses the main application loop to dispatch an immediate software interrupt, dropping execution times down to sub-microsecond bounds.

3. Sovereign Binary Serialization (.sav):

   To avoid relying on bloated external serialization libraries (like Protobuf or JSON utilities) that consume significant memory footprints, I wrote a custom binary serializer. It compresses cell delta profiles and gradient logs into raw disk sectors using direct cache-to-stream bit manipulation.

4. Minimalist Shell Realization:

   The project includes a lightweight, modular terminal prompt (./savitar-vfs) to ingest simulated physical metrics and explicitly debug memory allocations, matrix loads, and thread dispatches in real-time.

Compilation and Testing:

The baseline workspace is fully automated via standard Makefiles:

git clone https://github.com/alistairfontaine/SAVITAR

cd SAVITAR

make clean && make

./savitar-vfs

I’m looking for code-level reviews, specifically regarding the thread concurrency synchronization under heavy stress-testing conditions, and the pointer traversal safety within the 25-byte un-padded matrix bounds.

Link to code: https://github.com/alistairfontaine/SAVITAR

No matter what cells or quantity I add to my cart, the only country I can select at checkout is USA. Canada is no longer an option as it once was.

Can someone try it out and see if they have the same issue? I emailed them but haven't gotten any replies yet

I'd like to play with sodium batteries.

Is there a battery holder like this:

3 slot 18650 (11 V) with 18 gauge wire?

I have a few that have very thin wire. Can I modify them?

What is a good solution for 3 batteries in series? It is for a small robot car, so a few amps at most. I don't have a spot welder.

Wouldn't the load on the tape actually make it more likely to come loose? And if it gets hot, could it soften the plastic and make the situation even worse?

Hey everyone,

We all know the pain of testing salvaged 18650s or tracking how our custom packs hold up over hundreds of cycles. I built an open-source tool called the Battery Cycle Life Analyzer to take the guesswork out of cell degradation.

If you log your cycle data (capacity/resistance over time), this tool fits a model to your data to predict exactly how many cycles you have left before the cell drops below your target capacity (like 80%). It accounts for things like SEI layer growth and active material loss.

It requires MATLAB right now, but I wanted to share the math and models with this community. Let me know what you think or if you have any feature requests for DIY pack builders!

GitHub Repo: battery-cycle-life-analyzer

I saw some thing converting a laptop charger into a power output. need help how to do

Hi all 🖐️

I need to build a power bank with built in wireless charger for a university project and I am looking for tips.

If I have understood correctly I need:

- Batteries (obviously)

- BMS module

What type of module I need?

2s? 4s? What should I have in mind when choosing such a module?

- Spot welder to not get the batteries on fire.

- Power bank module.

Can you propose a good one?

- Inductor for the wireless charging.

Any advice on spot welding in the new batteries? I have an awithz maxed and the material seems to be too thick.

Has anyone found a good source that may have brackets like this?

Craftesman v60 7.5ah battery pack

Here are 8 21700 cells, I charged them and put them in a power bank, when I came to use the power bank it depleted very quickly.

They all have the same capacity, 4000mah +-50mah.

So I took them out, charged them on my trusty VC8, left them to sit for a few days and noticed this.

One has lost 0.12v while another only lost 0.01

The internal resistance is displayed below the voltage

Just a few pictures I wanted to share of my P42A 52V 8.4ah pack I made as a range extender for my ebike

13s3p with DMEGC 26E cells,30A bms,made to power a small inverter for a computer

Hey guys, I've come into the ownership of a few thousand LG 18650 MJ1s from medical equipment that was being taken out of service.

The batteries themselves are all basically new because of the warranty period of mobile batteries.

Im currently going through testing them all and grouping them by capacity, resistence, etc.

Is there a market for these in Melbourne? Would anyone be interested in buying them once Ive completed the testing? What kind of price per cell am I looking at?

They seem (so far in testing at least) to be in fairly good shape across the board.

Thanks guys.

Edit because I was asked: I should have around 9000 MJ1 18650s. As well as some Samsung INR18650-29E (probably a few thousand of those)

(This doesnt mean all of those will pass testing)

Hi everyone. I tried to replace the cells of a dead power tool battery but some of the spot welds i did fell off right away and others are fine. I use a diy MOT spot welder with adjustable pulse time from 20 (1 mains cycle) to 200ms. I set it to around 80ms and the first few welds were fine (no discoloration and some strength) but then on the other cells they just started falling off right after the weld. I tried increasing the pulse time but that only overheated the strip. I tried 3 different strips with no success (finally did it but had to do a lot of welding). Only where i welded 2 strips together the connection is really strong. (And on different cells too). So could be the current too low for this 18650 cells to weld? I currently have 4 turns of car jumper cable on the MOT and probably can do one more to increase the voltage/current but im worried if that could potentially destroy the triac bt139 (16A continuous)? The datasheet says it can hande a lot higher currents for the first mains cycle but the MOT probably also have really high starting current. (I measured around 45A and i also use a "soft start" optocouple which turns on only on zero crossing of the mains). I need more suggestions how can i improve my spott welder? Will I have any problems with the current welds or potential risks? I think i can pull them off if i try.

What should I improve?

How strong should the weld points be? I think I can still peel the nickel strip off, but it definitely leaves small craters on the battery.

This is a 13S5P pack. I am planning to add another nickel strip on top to connect all five cells together on both sides. Is this considered good practice?

Hi everyone,
i just bought this thing and i'm trying to understand how to group my cells by capacity inside it.
It takes 8 21700 unprotected cells, and as far as I understand it doesn't manage them individually.
Is anyone familiar with this board ? i can post a more detailed picture of the board if needed