That why is a 5 volt regulator the best solution to load a battery 3.7 volts! It’s just great and it will charge ur battery fully! Try and learn more from electronic everyday! Good luck!

Welcome to AAC.

Please take this in the spirit in which it is meant: Your advice is terrible. It is basically wrong and it is dangerous.

Yes, the target voltage when charging a cell (or battery of cells) is necessarily higher than the average voltage of the cell or pack. But it isn't arbitrary, and while some battery chemistries tolerate overvoltage better than others, the result will at best be reduced battery life—and at worst, a devastating fire.

And Lithium chemistry cells are prone to the worst outcome. Overcharging cells causes heat, and heat destroys them. This is true for any cell but in the case of most Lithium cells, the heat can also ignite the very flammable electrolyte (LiFePO₄ is an exception to this, and so is safer).

Lithium cells require a CC/CV charge strategy. The idea is simple—

Charge at a constant maximum current of 0.5~2.0A depending on the particular
cell and application, then when that would require >4.20V switch to a constant voltage.​

A fully charged Lithium cell is at 4.20V. Applying 5.0V will overcharge the cell, destroying it over a short time by heating it up, and in the worst case causing a fire and a thermal run away that will cause all the cells in a battery to vent their electrolyte which ignites and sets fire to whatever is around it.

There is a ubiquitous and dirt cheap cheap, the DW01, which is designed to protect against overcharge, over-discharge, and overcurrent. Combined with proper MOSFET(s), it Iis a cheap and easy solution protecting individual cells. There are also a number of inexpensive and simple to use charge controller chips, as well as the even simpler option of buying a cheap module based on one of these chips.

Your suggestion of a linear regulator for this application is very wide of the mark. You don't seem to have a lot of practical experience—which is not an insult, just an observation. And you seem to have no theoretical knowledge—again, no insult intended. You can fix both of these things with a little effort but giving advice from ignorance is not good.

And, in this case and in others in electronics, it can be dangerous. Please spend more time reading the advice of the very experienced and knowledgeable members here, many of which are working professionals, or retired from long careers as electrical engineers and technicians.

Please don't take it as hostile if you find some of your messages removed should they contain bad, or dangerous, advice,

Again, welcome to AAC—I hope you settle in and find this a good place for you.

 

THAT ADVICE, to "throw away all the transformers" is the worst advice I have heard in several years!!
Certainly there are in many cases other, possibly more efficient, ways to do things.
There are also a lot of situations where an adequate transformer is very much the best choice. Just not everywhere.!!!!!!

To charge the 3.7 volt batteries you can use a step-down switching regulator and a battery charging control circuit. THAT will be the efficient scheme.

A buck converter uses a transformer, in fact.

 

use a 5v reg to step it down from 12v, then connect the output to a usb powered 3.7v charger. This will allow safe charging and prevention of overcharging of the cell. Hopfully the charger is a type that has a load output to prevent over discharge of the cell.

A linear regulator dropping 7V at ~1A is going to have to dissipate 7W. That's not good at all.

 

By the way!!! Mains frequency transformers usually are not useful for most DC circuit applications. So unless one has much more money than they know what to do with, hold those transformers for when you know what to use them for. ALSO, ignore fools.
Pay attention to advice from "Y"! It seems that he has access to vast amounts of information, PLUS he knows a lot.