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Introduction Wanted to start with my level in electronics. I am very beginner hobbyist & the last thing related to electronics I did in college was creating an 12V AC to DC Converter that was 15 years ago. I a Software Engineer by profession.

Problem I have a Ecovacs vacuum cleaner that runs on Lithium-Ion Battery (Li-ion Volts:14.4 Capacity:6400mAh / 92.16Wh). The charging mechanism is not working - robot does not stay on the charger, but if a battery is is charged it works fine, does what it is supposed to do. A fully charged battery works for about 4-5 cycles (about a week).

Solution I wanted to create an external charger for the battery so that I can swap the uncharged battery with a charged one every week or whenever it needed.

Question I wanted to create my own circuit as I wanted also learn/play around with electronics as an hobbyist; the question is where do I start?

Update #1 After taking advice from people, Li-on charger may a little risky to start a foray into electronics. I bought Charger & Safe Bag from Amazon.

Update #2 (Warning) As the suggestion from Cassie Swett, please do not attempt to charge a lithium battery with a homemade charger as it may cause explosion or produce a fire that can't be extinguished

Kunal B.
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    Lithium-Ion Battery charger is not the place to learn electronics ... too risky – jsotola Jun 11 '23 at 03:08
  • Oh, you mean hazardous because they catch fire? There are pre-built circuit available on Amazon - https://www.amazon.com/HiLetgo-Lithium-Battery-Charging-Protect/dp/B00LTQU2RK but does not meet my needs – Kunal B. Jun 11 '23 at 03:20
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    _"...but if a battery is charged it works fine"_ - since your charger is not working, how do you know this? – Bruce Abbott Jun 11 '23 at 04:15
  • I had initially thought the battery was bad, I got new fully charged one and vacuum worked. Also, I have a voltmeter. – Kunal B. Jun 11 '23 at 04:46
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    @KunalB. If the task isn't crazy like, *"I want to design and make a cell phone"*, then I think it's a good idea to use strong motivations as a driver to learn. Performing a battery charger for Li-Ion, while not so good as an early-on project, isn't so far afield that it cannot be achieved if you drill in and study and break the project up into a few steps. And there are a number of pages and Youtube videos, as well, some of them tearing down bad chargers that exist. [Start here for an overview](https://www.digikey.com/en/maker/blogs/2021/charging-lithium-ion-and-lipo-batteries-the-right-way). – periblepsis Jun 11 '23 at 05:45
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    As a software engineer, you may be able to program a PIC or Arduino to perform a proper charging algorithm. That may involve some tricky analog design and a way to interface voltages and currents to the processor. Just remember that lithium battery packs come with some significant danger, so be careful, and ask many questions during the design process. – PStechPaul Jun 11 '23 at 07:34
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    "I bought charger ". Wise decision! – Davide Andrea Jun 11 '23 at 12:53
  • This is one of those rare things where if you need to ask where to start you shouldn't be doing it at all. Get some (years of) related projects under your belt and then reconsider, if you really insist on doing it yourself. I have a computer engineering degree (similar to an EE as far as this question is concerned) and I am not willing to design anything related to liion or lipo charge controlling for my own hobby use, because I don't know what I'm doing in that area and there's no oversight in hobbies. I buy off the shelf (but not from Amazon!!!) parts for that. – cat40 Jun 12 '23 at 01:53
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    Also, what you linked is a charging bag. One is a good idea, but it won't charge your battery itself and I still wouldn't buy it from amazon because you have no idea how or if it was tested (this one is patently nonsense - "explosion proof"? Really?). Buy a charger and a bag from a **reputable distributor** in your area. If it doesn't come with a datasheet that matches the advertised specs, don't buy it. Real distributors will provide the datasheet before you even buy. And, make sure you buy one with the proper specs – cat40 Jun 12 '23 at 01:56

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Answer is, you don't. Those types of batteries are extremely dangerous if mishandled, can catch fire and explode violently upon overcharging. Rolling your own, custom charger is a perfect recipe for disaster, especially considering that you aim not to charge a small smartphone battery, but a nearly 100 Wh battery of a vacuum cleaner. More about this in the relevant Wikipedia article.

What is more, even if overcharging won't result in a spectacular explosion, the battery could still harm you in other ways; the electrolyte in those types of batteries is somewhat toxic and highly corrosive, and angry battery could baptize you and your surroundings with said electrolyte. Those types of batteries are similar to pufferfish. They puff up if they feel danger, whether it is a predator's presence, or an overcharge condition.

Don't do it.

Another reason not to do it was provided in the comments by the user Uwe:

If the charged batteries overheat and start a fire, the fire insurance would refuse to pay if they find out a self built charger was used.

Chad Branzdon
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    If the charged batteries overheat and start a fire, the fire insurance would refuse to pay if they find out a self built charger was used. – Uwe Jun 12 '23 at 09:29
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WARNING!
Li-Ion batteries are prone to catching inextinguishable fire or explosion if ALL of the following parameters are not kept in check: maximum cell voltage, maximum charging and discharging current, cell-balancing (making all cell voltages equal) and minimum and maximum temperatures.

Basically, to charge a Lithium-Ion battery, you need a constant voltage, constant current power supply.
The easiest thing to do, which even experienced electronics technicians/engineers sometimes do, is to either get a current-limited power supply which gives you 4x4.2V=16.8V and no more than 3A, or to get a power supply which gives you from 17V to 24V and add a CC/CV module and set it to 16.8V and 3A. There are plenty of such modules around, just search for "CC/CV module".
Or you could modify an existing 16V to 24V power supply to give you 16.8V/3A, but that is much more involved and depends on a specific adapter you have.

Finally, you need a 4-cell Li-Ion balancing module (if your battery pack doesn't have one built-in) to make sure all cells are at about equal voltage. A battery pack protection may be needed, but it is most likely built into the packs.
You can see if your protection circuit does any balancing if you do some charging and discharging of your batteries; each cell should be equal to another cell's voltage, give or take around 0.01 volts.
I recommend keeping the charging current low (0.5A to 3A maximum) because all batteries prefer lower charge and discharge currents and it gives more time to the balancing circuitry to do its work.

Edin Fifić
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    This answer should be edited to include a note that attempting to charge a lithium battery with a homemade charger can cause it to explode or produce a fire that can't be extinguished. – Cassie Swett Jun 11 '23 at 14:57
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    I know you mean well, but I do think it is quite dangerous to provide advice on how to do something that the advisee has almost no hope of doing correctly and where even minor failures in execution can cause excessively energetic failures. Consider adding a strong warning at the top and bottom about the possible consequences of attempting a homemade charger (for anyone who stumbles across this). Or at the very least add some basic safety precautions (like don't charge unattended and only charge in a metal pail half-full of sand on a concrete floor with nothing flammable around) – cat40 Jun 12 '23 at 02:06
  • @CassieSwett I have included a warning. – Edin Fifić Jun 13 '23 at 16:20
  • @cat40 I have included a warning. – Edin Fifić Jun 13 '23 at 16:21
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    An added NECESSARY step in charging is to remove Vcharge either as soon as the voltage reached 16.8V or once the current drops to say 50% of Vcharge after 16.8V is reached. Just floating at 16.8V will first degrade and then destroy the battery. – Russell McMahon Jun 13 '23 at 21:19
  • @RussellMcMahon You're right. Did you misspeak on the current amount? To my knowledge, once it drops below 3% of rated capacity (30mA for a 1000mAh cell/battery), charging should be off, though charging can be cut off at any time up to 3 hours after 4.2V per cell is reached. – Edin Fifić Jun 21 '23 at 11:04
  • @EdinFifić My figure was correct. The longer you leave a cell on charge and the lower the tail current falls the closer the cell is to rated capacity (or beyond). "Road Warriiors" who demand all possible capacity for their eg laptops may use 25% or even 10%. 3% is way way overdone. BECAUSE - the more capacity you ring out of a cell the more you decrease the cycle life. At 50% current you get close to full capacity and OK life. 25% or 10% give you very little more and substantially lower cycle life. 3% is not good. Stopping at 4.2V with no tail gives you 99-95% and useful cycle life gain. .. – Russell McMahon Jun 21 '23 at 12:32
  • @EdinFifić ... stopping at 4.1V gives (from memory) 80-90% capacity but doubles the whole of life mAh capacity stored and retrieved. ie by reducing the amount you put in per charge you get much more overall lifetime capacity. || Well managed LiIon cells can reach 2000 cycles. Thats an unheard of figure in most applications. The OLPC (One Laptop per Child) people are getting that and their software is open source to see how they do it. They can also get 2000 cycles from NiMH ! – Russell McMahon Jun 21 '23 at 12:35
  • @EdinFifić Here are 180 of my answers relating to LiIon charging. No gurantee that everything I say is utterly correct (I try) but you should find some useful material there. https://electronics.stackexchange.com/search?tab=votes&q=user%3a3288%20liion%20charging&searchOn=3 – Russell McMahon Jun 21 '23 at 12:38
  • @RussellMcMahon I am aware that using 50% of a Li-Ion battery's rated capacity (30-80% charge state) will give it about 2.5 times the cycle life energy (many more cycles, but the total mAh usage over lifetime becomes about 2.5 times), as well as using significantly lower charge and discharge currents. Similar to a human or an animal working at 100% capacity - they won't last long. Checking a vacuum cleaner battery I've realized it was balanced and at 80% of its original capacity, but its internal resistance was twice as high and thus it tripped "bad battery" indicator at high load currents. – Edin Fifić Jun 24 '23 at 04:21
  • @RussellMcMahon Thank you for providing that link. I know most of those facts, very few things are new to me. Didn't know about the OLPC allowing charge level limit setting, which is something I have been upset about for many years for its lack in all laptops' power management. I would gladly set it to 80% (around 4.0V) as I rarely use laptops away from an outlet, and would rather have their batteries giving me more cycles. – Edin Fifić Jun 24 '23 at 04:26
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    @EdinFifić The older I get the more I know and the number of things that are new to me increases. The first linearly or by some mild power. The latter exponentially :-). | I'm 72 years old. The more I know the more I know that I don't know :-) – Russell McMahon Jun 26 '23 at 09:10
  • @RussellMcMahon That's something similar to what I've been hearing and saying. The less a person knows, the more they THINK they know, because they are unaware of how much they DON'T know. Once we learn enough, we realize just how vast or actually infinite the amount of all knowledge in the Universe is. I have been amazed by the latest astronomy/astrophysics and quantum physics discoveries which really makes one filled with wonder when looking at all those details from multiple perspectives. Knowledge truly is power, but also a wonder. I pity the ignorant and thoughtless. They are truly poor. – Edin Fifić Jul 02 '23 at 21:59