Hobby Battery Overview

 I belong to some interesting technology groups, which, pre pandemic, would meet on a regular basis, conduct a bit of club business if needed, and then enjoy informal technical discussions and lots of "show & tells".

Then we switched to online Zoom meetings,  suddenly meetings weren't so interesting.  Some members are struggling to get used to new online technologies, some only have tiny screens, some discipline is required so as not to interrupt others when talking, the "whisper to your neighbour" doesn't work anymore, Show & Tells don't work as well....

Attendance at meetings dropped off, since they weren't as much fun, or as easy to attend.  Something had to be done!

Some groups didn't seem to have this problem; those groups usually had a more extensive agenda, with prepared topics, presentations, speakers.  They learned how to make better use of the presentation techniques that work well on Zoom, rather than informal methods of talking while trying to hold something up to your camera, in focus, and held still....

I'm on the executive of one group that had this attendance problem.  We decided to organize one significant presentation for each monthly meeting throughout last winter, and so we each volunteered to do one of these.  I volunteered to do one on the topic of "Batteries", since we use them extensively in this group (building and flying model airplanes).  I've attached the PDF version of that presentation here, for reference.  Questions welcome.

Hobby Battery Presentation

Making Li-Ion Packs: Battery Management System (BMS) Useage

Introduction:

 I have accumulated a small number of older laptop battery packs of various types.  One older pack contained 6 Li-Ion 18650 size cells.  These cells are still in good condition, although of uniformly low capacity (measured at ~700mAh).  Still useful, for simple applications.

A Simple Build:

I have lots of small 1S LiPo and Li-Ion cells, but not a lot of 2S packs.  2S is a good size for simple robotic and other applications.  Looked like a good project...  I aquired some 3 cell 18650 size battery packs, and some small 2S BMS boards.  The BMS boards are a good fit inside one of the battery holder cell locations, leaving the two other locations for two cells.  I wired up the cells and battery holder along with a 2 pin power connector and a 3 pin battery balance connector.  The new packs tested well on my tester, with no problems.

The Application Devil:

So I installed one of the packs into a small robot I am putting together (an Arduino based balancing robot).  I'm not much of a mechanical engineer, so the builds go slowly; I'd rather build only once, and there turn out to be lots of bits and pieces in this assembly, and balance, and robustness when falling is a bit important in a balancing robot!

Then came time to connect up the motors, their H-bridge, and battery power, instead of running testing from a 5V test supply.  But there's no power from the battery pack!  The cells seem okay, but the BMS has no output.  Just to be sure, I ran the pack through my tester / charger again; everything looked fine.  Unhook from the tester, meter says all okay.  At this point, I had to see to other things.  Several dyas later, another test, and same result - no power!  time for some investigation.

Google Investigation:

Turns out lots of people have similar experiences!  Some forum reading quickly turns up some possibilities, (apart from the obvious clutter in most forums concerning bad cells, bad wiring, bad BMS systems, no charge...!):  typical small battery management ics don't like to have a cell removed from the circuit.  They see it as a fault in a cell, and shut down input and output circuitry.  This circuitry often will not start up again until the charging connections see voltage external to the battery pack.  A strange quirk, but common across several of the chips whose datasheets I examined.

The BMS:

The unit I have is labelled "HX-2S-D01".  The protection chip is made by Hycon Technology, one of their HY2120 series. (mine is a 20CB L62L type, a small 6pin smt).  The 2 FETs used are KGA712E type, and appear to be suitable for my purpose.

The datasheet is very clear on the problem (pg 12, Description of Operation, Normal Status):

" ...Notice: Discharging may not be enacted when the battery is first time connected. To regain normal status, CS and VSS PIN must be shorted or the charger must be connected. "

I examined the datasheets of 2 other protection chips, from other vendors.  Same internal design, same Notice.  Interesting...

So, I shall implement their solution (perhaps with a 10K resistor in series with the PB I install...? ), and I'm sure we will have a practical solution.  A bit bizarre though...

Eureka!! It Works...

Adding a simple PB (and a series 15K resistor; I worry...) has solved the problem.  Tested by removing 1 cell from a working pack, and replacing it.  Pack's output is 0V, until PB is pressed and released.  Now if someone with more time than me could analyze the datasheet, to see if there is a non-PB type solution...?

References:

1. "How to use a ... BMS ...", https://electronics.stackexchange.com/questions/477821/how-to-use-a-cheap-bms-for-discharge-only

2. "Identify chip on 2S BMS...", https://electronics.stackexchange.com/questions/476188/identify-chip-on-2s-bms-balance-card

3. Hycon HY2120 Datasheet: https://www.hycontek.com/wp-content/uploads/DS-HY2120_EN.pdf

4. Common Source: https://www.aliexpress.com/item/1005001393168230.html