1.5V mp3 player with 3.7V Li Battery

[Menticol]

Hello guys! And first of all, Merry Christmas to all of you!!

I'm making a small electronic project for my girlfriend and I need your advice to complete it.

She have an old mp3 player powered by a 1,5 AAA alkaline cell (which I consider very inconvenient). I want to replace it with a 3,7V Li rechargeable battery (scrapped from another bricked player).

Her player have a USB connection available. What would be the simplest way to step down that 5V to the battery charging voltage (3,7V)? Maybe a voltage divider, or a zener diode?

I'm aware that there is a lot of beautiful options (low dropout regulators, etc) but I can't get them in my city

PS: I know that the battery must be equipped with a charging controller, but i think the battery already have one (a small PCB attached to the battery).

PS2: Getting a new mp3 player for her would be simpler, but she likes that player too much.

Thank you and best wishes for everyone

 

[alec_t]

powered by a 1,5 AAA alkaline cell (which I consider very inconvenient)

IMHO adapting this to work on 3.7V is even more inconvenient ;-)

What would be the simplest way to step down that 5V to the battery charging voltage (3,7V)? Maybe a voltage divider, or a zener diode?

Charging Li batteries isn't a simple matter of applying 3.7V to them. Google for advice on charging Li bats safely to prevent them catching fire!

 

[Menticol]

IMHO adapting this to work on 3.7V is even more inconvenient ;-)
Charging Li batteries isn't a simple matter of applying 3.7V to them. Google for advice on charging Li bats safely to prevent them catching fire!

Thank you for your suggestion Alec_t, I already know the importance of the charging controllers to avoid fire, explosions and other minor "nuisances"

I realized that this battery already have one. But stepping down from 5V is still a problem.

In the other hand, I checked a Chinese bug zapper and a flashlight and the charging controller is very crude. Just a bridge rectifier and couple of resistors. Why it doesn't catch fire is a mystery for me (perhaps an embedded charging controller into the battery?)

 

[alec_t]

Are you sure the controller pcb packaged with the 3.7V cell isn't just for preventing over-discharge of the cell? A charging controller would more likely be associated with the charging source.
Do you have room in the MP3 player to house not only the 3.7V cell and its pcb but also a voltage regulator to get from 3.7V down to 1.5V?

 

[Menticol]

I suspect you are right, the PCB only consists of a 6 pin IC and a transistor. Both surface-mounted and marked with obscure codes.

However I think the charger doesn't require to be too smart. I can instruct her to charge the battery below the full capacity (optimum performance and battery life are not critical for this application)

Physical space is not a problem, but the only regulators I can get are the LM78XX series, which are very inefficient.

 

[alec_t]

Physical space is not a problem

That's good. Perhaps just fit a battery holder for an AA cell instead of the AAA ? Much longer life at about the same price.

 

[lebevti]

use a buck converter to step it down

 

[alec_t]

I can instruct her to charge the battery below the full capacity

But you won't know the state of charge, so she can't tell how long to charge the cell. That means there's a risk of overcharging (BANG!) unless you have a proper charge controller.

 

[audioguru]

You don't want your girlfriend to be exposed to an extremely hot Lithium fire. It burns as hot as a magnesium flare. If it is tried to be extinguished with water then it burns hotter and keeps burning. Its smoke is toxic.

A Lithium battery must be charged and discharged properly.

 

[Menticol]

Crystal clear now. Thank you very much Alec and Audioguru