18650 charge and discharge project - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 26, 2017, 09:40:19 am
content: Hi all,I get frustrated and tiered of long waiting for recycling salvaged 18650 cells. I was thinking of design a circuit that can charge and discharge a cell (1 cycle). I see there are a lot of IC that are capable of charging. There are the cheap TP4056, but would not have temp sensor or i could use a thermistor to check that.I am hopping to use an Arduino Uno and if somebody could please point me to an IC that can basically charge Li-ion cells at cc and cv and if possible have the following features as welltemp sensor to cutoff charging if temp increasesdisplay current send into cell

author: Rerouter
author_rank: Super Contributor
timestamp: August 26, 2017, 10:40:49 am
content: The simplest way would be to use your TP4056 module for the charging.You would control the "Charge Enable" Pin on your micro to switch on or off charging, and you could fit a small resistor and an op amp amplifier to measure the charging current on the negative lead of the battery, you would measure it with the arduino ADC, and count it yourself, for almost the entire charge it will just be "Charge Current x Time" its only in the initial trickle and the final constant voltage phase where its not static.Equally the TP4056 does have a temperature sensor input, i would just assume the ebay modules dont wire it up. though probably not a bad idea if you hook it up to have that temp signal feed a comparitor, should it overheat the micro can switch off that charger and beep an alarm, or however you prefer.

author: sleemanj
author_rank: Super Contributor
timestamp: August 26, 2017, 11:50:58 am
content: A while ago I made this spreadsheet to help pick appropriate resistor values for connecting an NTC to a TP4056 (see the TP4056 datasheet for wiring)...https://docs.google.com/spreadsheets/d/1tpKQsnLmb9Q7FumlHdIIN5MnNgDuT0CBUhXPiXdU9wU/edit?usp=sharingUse "File > Make A Copy" and edit the yellow fields to your desire.I built a 4-way charger from TP4057 (500mA version) with switches so that they can be set to 50mA or 500mA, on the first charge I just stick them on at 50mA and bring them up to about 3.7v, then I kick them into 500mA to finish off. I don't count charge.I also built a discharge tester, 2 "channel" (but I actually only finished one channel) using an Arduino for the brains, it is fairly dumb, using resistive load (two 5 watt resistors in parallel but the arduino can switch either or both on to adjust the load somewhat) and counting Amp-Seconds and Watt-Seconds (and time elapsed) during the discharge, but it gets the job done.Neither of my setups monitor temperature, but they probably should. The charging is done in ametal laundry powder tinthat I put a 5.5mm DC jack on although with only a 50mA charge current I think it's most likely overkill (famous last words). I should probably do the discharge tests in the bin as well.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 26, 2017, 03:07:19 pm
content: TP4056 should get the job done for me for charging side and is there a better way to adjust the charging current other than adding resistors. I could have two resistors for 500mA and 1A and use jumpers to choose. I was hoping to control them electronically if it's not very complicated.The discharging with a resistor seems a bit bulky would it be possible to do it electronically like using a MOSFET or anything similar.Sent from my iPhone using Tapatalk

author: Rerouter
author_rank: Super Contributor
timestamp: August 26, 2017, 09:38:55 pm
content: Have a high value resistor for the low charge mode. Then have another or multiple in parrellel with an n channel mosfet to switch it in or out.

author: sleemanj
author_rank: Super Contributor
timestamp: August 26, 2017, 10:01:06 pm
content: You can create a dummy-load circuit (beefy fet+sense+opamp+heatsink+fan....) and I considered it, but decided it was a waste of time, at most I'm interested in either 500mA or 1A rates, my resistor selection allows for this,with just a couple of fets used to switch that don't need heatsinking, no dealing with low-ohm sense resistors (just read the cell voltage under load and you know the current because it all went through your known resistors), even your resistors really need no heatsink - but they will get ouchie-hot (the change in resistance won't be enough to care about for this purpose).You also have the safety of knowing your load isn't suddenly going to fail short circuit like a dummy-load mosfet potentiall could (or just through software bug!), big wire wound resistors just are not going to do that :-)

author: edavid
author_rank: Super Contributor
timestamp: August 26, 2017, 10:01:54 pm
content: You can also buy an Lii-500 4 cell charger for about $14 on AliExpress... pretty good deal for $3.50/channel.