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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

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    \$\begingroup\$ Lithium-Ion Battery charger is not the place to learn electronics ... too risky \$\endgroup\$
    – jsotola
    Jun 11, 2023 at 3:08
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    \$\begingroup\$ "...but if a battery is charged it works fine" - since your charger is not working, how do you know this? \$\endgroup\$ Jun 11, 2023 at 4:15
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    \$\begingroup\$ @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. \$\endgroup\$ Jun 11, 2023 at 5:45
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    \$\begingroup\$ 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. \$\endgroup\$
    – PStechPaul
    Jun 11, 2023 at 7:34
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    \$\begingroup\$ "I bought charger ". Wise decision! \$\endgroup\$ Jun 11, 2023 at 12:53

2 Answers 2

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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.

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    \$\begingroup\$ 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. \$\endgroup\$ Jun 11, 2023 at 14:57
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    \$\begingroup\$ 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) \$\endgroup\$
    – cat40
    Jun 12, 2023 at 2:06
  • \$\begingroup\$ @CassieSwett I have included a warning. \$\endgroup\$ Jun 13, 2023 at 16:20
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    \$\begingroup\$ 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. \$\endgroup\$
    – Russell McMahon
    Jun 13, 2023 at 21:19
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    \$\begingroup\$ @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 :-) \$\endgroup\$
    – Russell McMahon
    Jun 26, 2023 at 9:10
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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Uwe
    Jun 12, 2023 at 9:29

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