Advice for 18650 battery charging AND testing design - Page 1
Forum section: electronics / projects

author: vdeconinck
author_rank: Newbie
timestamp: December 15, 2017, 12:44:45 pm
content: Hi, all,I'd like to build a simple circuit that would be used for charging AND testing 18650 battery cells, with monitoring to be able to trace the cell curves.I currently charge them on the cheap with TP4056 modules (0.27$ per module, shipping included, is hard to beat), and I've built a small constant current load usingDave's tutorial, an Arduino and a DAC - basic design is attached (filtering and protection omitted).That setup works OK to charge a battery, then monitor its discharge until it reaches 3V (and compute its capacity), then charge it again, but each step takes lots of time and it needs manual operations to switch setup between steps.I would like to automate the process because I have dozens of cells to process (and because well, why not ?). I'd also like to stop charging the cells at around 3.7V the second time (recommended voltage for storage).All in all, charging a LiPo cell basically requires a CC/CV setup, as explained inanother eevblog video, so we "just" need to measure the battery voltage and current (which are already monitored in a programmable load), and it looks like all it takes is another mosfet (e.g. P-channel) between 5V and battery +, controlled by the microcontroller, right ?Now, that is a bit optimistic of course, because first it needs a bit of rearranging the components (the current-measuring resistor needs to sit next to the battery to be in the circuit even if the load MOSFET is off, and then the direction of the current is also reversed when the battery is being charged, so the negative feedback loop has to be redesigned... and the end I'm puzzled...So I'd like your opinion on this. Do I need to design two independant circuits (charger vs load) and just switch from one to another, or is there a clever way to share some parts of the circuit (the battery voltage measurement seems obvious) ?Any comment/schematic/advice is welcome !Kind regards,Vincent

author: ogden
author_rank: Super Contributor
timestamp: December 15, 2017, 02:15:33 pm
content: Indeed you can connect charger and load together w/o any switching circuit. Just make sure they do not operate at the same time. For schematic search internet: "IMAX B6 Schematic". Other option - just buy Imax B6 or it's cloneAnother source of information:

author: vdeconinck
author_rank: Newbie
timestamp: December 15, 2017, 11:27:47 pm
content: HI, ogden, and thanks for your reply.Indeed, I had watched that teardown video of the Turnigy Accucell 6 by Dave some time ago but only remembered it did balancing charge. But indeed it also does discharging!It seems it is very similar (or a clone) of the IMax B6 and I found two schematics for it, the one used by Dave (and linked in its video description) and another, annotated in Russian. If it's of use to anyone, I translated the annotations and I'm attaching both versions here.Buying an iMax is an option but it is aimed at cells in series while I'm looking for single-cell charging. I could use it for one cell of course, but that would mean testing cells one at a time, or buying several chargers...So, although the circuit looks somewhat complex at first, I have to study it more and see how I can make my own (maybe simpler) version.Thanks for the heads up, and if you (or others) have any hint or comment, it is highly appreciated, as always.Kind regards,Vincent

author: ogden
author_rank: Super Contributor
timestamp: December 17, 2017, 03:18:17 pm
content: Quote from: vdeconinck on December 15, 2017, 11:27:47 pmIndeed iMax B6, Turnigy Accucell and similar kind of chargers can be used just for single cell charging!

author: Audioguru
author_rank: Super Contributor
timestamp: December 17, 2017, 05:31:47 pm
content: Your idea of using a simple and cheap CC, CV charger is missing important safety functions found in inexpensive charger ICs.1) What happens if you try to charge a lithium cell that has been discharged too low? Maybe a FIRE!A charger IC detects a low voltage then attempts charging with a very low current. If the voltage does not rise then it stops and gives an alarm before a fire.2) A charger IC has a pin for a temperature sensor. Then charging stops before a fire occurs.3) Look at the datasheet of a charger IC to see some more safety functions.

author: vdeconinck
author_rank: Newbie
timestamp: December 17, 2017, 10:05:50 pm
content: Hi, audioguru,Thanks for pointing those dangers out.Indeed, I intend to detect and take care of drained cells in software, and to add a chain of DS18B20 sensors for temperature monitoring (1 per cell + 1 for the discharge MOSFETs heatsink to control a fan). Even for non-dead cells, temperature monitoring also gives a good indication of the health of a Li-Ion cell (high internal resistance), so recording the temperature was in my plans too.I also plan to add a reverse polarity protection and a software "time-out" to stop trying to charge a battery if it doesn't reach the end of the charging stage in a "reasonable" time. Do you see anything else that would be required ?Now I admit I'd be tempted to use TP4056 chips because they include most required features, but I'd also like to measure how much energy goes into the battery when charging, and adding a shunt resistor in series with the cell would obviously modify the voltage measured by the chip. Maybe I could measure the current getting "in" the TP4056 as its own power consumption is probably negligible. What do you think ?Kind regards,Vincent

author: station240
author_rank: Unknown
timestamp: December 18, 2017, 10:23:42 am
content: Have a look at the TP5000, which does have a temperature sensor and current shunt.The off the shelf modules don't have the temp sensor though.

author: vdeconinck
author_rank: Newbie
timestamp: December 18, 2017, 07:24:18 pm
content: Hi, station240,Thanks for the hint and for the translation of the datasheet. Highly appreciated :-).The external shunt is a real plus, as it makes current measurement available to external circuits.It's a pity modules don't have the temperature sensor. The logic is also implemented in the TP4056, but most modules omit it too... and building one's own circuit based on a bare chip is much simpler with a TP4056 (less external components required)I'm still weighting the idea to measure the current getting into the charger chip, e.g. adding a 0.1Ohm in series with the power supply... any opinion on this ?Vincent

author: Audioguru
author_rank: Super Contributor
timestamp: December 19, 2017, 12:50:15 am
content: In rcgroups.com forums, people say that a cheap LiPO takes more mAh charging than it discharges.So what, that is why it is cheap. It still works pretty well and has a fairly long life. When its discharging time is low when I use it or its ability to produce maximum power is less than I want then I replace it.I harvested old 18650 Li-Ion cells from laptops and MP3 players and they work fine. I didn't test them, I just use them.

author: ez24
author_rank: Super Contributor
timestamp: December 19, 2017, 04:02:28 am
content: Quote from: Audioguru on December 19, 2017, 12:50:15 amWhat do you do about the flat tops when some equipment requires a dome top? (my problem)

author: Inverted18650
author_rank: Frequent Contributor
timestamp: December 20, 2017, 04:36:11 am
content: A spot of solder works to convert the flat top to “dome”. I used the Turnigy Quadcore Reaktor for testing for a long time. Simply put, there is no “down and dirty” method that I have found to safely and accurately categorize them. You should charge them at/around 1C so it will always be a time consuming process.I use the Opus BC3100 model for “quick” specs now. No temp sensor but they can display internal resistance via the “quick test” mode. It’s not extremely precise but for my projects (powerwalls, EV/E-bike packs) I do not need it to be exact. For the money, they are solid little testers.