How to start building lithium-ion battery charger?

Question

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

Answer 1

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.

Answer 2

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.