18650 2s bms with charge LED

Ya’akov

Joined Jan 27, 2019
6,260
This question doesn't include enough information to give a useful answer.

First of all, we need to know which BMS you are taking about. Then we'd need a schematic, or at least a set of sharp, well lit photos of both sides of the PCB(s).

Given those one or both of those you might get and answer.
 

Thread Starter

liteace

Joined Mar 7, 2012
158
Ive not got BMS yet, I dont know whats the best one to go for is, Ive seen lots on China dot com, are these safe to look after Panasonic or Samsung cells?
 
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bassbindevil

Joined Jan 23, 2014
499
Nearly all the things sold as "BMS" are purely protection modules (over-charge, over-discharge, over-current), and some include balancing. They do not manage charging at all, they just prevent unwanted indoor fireworks if you hook up the wrong kind of charger.
The simplest option for charging is to buy a wall wart or power brick with the appropriate voltage (7.2 or 8.4V in your case) and current (1 amp or less); they'll be advertised and (usually) labeled as a lithium charger, and will have an LED for charging status. If you need to charge from a DC source, there's modules that can step-down from higher voltages (SY6912, MAX745) and some that step-up from USB-C 5V (CN3302/3/4/5).
If you're stranded on a desert island, old camcorder and digicam lithium wall (or car) chargers should work as long as the voltage is correct. I mean the kind that directly charge batteries, not ones that plug into the camera (as far as I can tell, the cameras manage the charging). Use clip leads from the charger terminals; same goes if you happen to have a universal charger like the DigiPower TC-U450.
 
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Thread Starter

liteace

Joined Mar 7, 2012
158
Thanks for the info, all of my units with rechargeables in have some sort of charge indicator, my laptops, my phone, my worklamps, my drill, my impact driver, my pressure sensor. I cant understand why there's a 100'ds of different, from cheap to not cheap, charge boards/pcb's out there non with any sort of charge indicator
 

Ya’akov

Joined Jan 27, 2019
6,260
Thanks for the info, all of my units with rechargeables in have some sort of charge indicator, my laptops, my phone, my worklamps, my drill, my impact driver, my pressure sensor. I cant understand why there's a 100'ds of different, from cheap to not cheap, charge boards/pcb's out there non with any sort of charge indicator
A SoC (State of Charge) indicator turns out not to be a simple thing. There is no reliable way to know the actual SoC of a Li cell by measuring its terminal voltage. Instead, the usual approach is a Coulomb Counter that actually watches the amount of charge drawn from the cell(s) and tracks it.

It is not necessarily part of the BMS. The BMS and/or protection circuits don't care about SoC, they rely on terminal voltage.

Depending on what you want to do it might be easiest to use a very small MCU and something like the LTC4150 driven Analog Devices. This SparkFun breakout board is an easy way to use it.
 
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