Simple way to drop 5v charger to 3.6v??

[veloman]

I want to use single cell chargers to balance my large lifepo4 pack, but not at the cost of $15x 20 cells for the Volt Phreaks lifepo4 chargers. Cell phone chargers put out 5.05v or so and can be had for free or next to nothing. What is a simple way to drop the voltage to 3.6 - 3.8v ?

I am testing a LM2596 dc convertor right now, but think the input needs to be > 2v than the output, so it may not be working properly as is. When I was adjusting the voltage on the pot, it didn't seem to react properly.

I tried a diode, but it didn't drop the voltage at all.

 

[amberwolf]

Any pair of diodes with a voltage drop of 0.7V each at the current levels you will be drawing would drop 1.4V total, to get you 3.6V out of 5V.

Other than that, you'd need a regulator of some type. The LM317 might work, though there are much more efficient ones around these days (might be more complicated to build/use).


EDIT: if the diode you tried didn't drop the voltage at all, there might either be something wrong with it (internal short) or it might be designed for low voltage drop at the current level you were pulling thru it, etc.

 

[veloman]

The cell phone charger was 550ma. I got no voltage drop on open circuit. If I hook it up to battery to charge, then it will go to just about the same voltage as battery, as that is pulling it down. I guess I could let it sit and see what it stops at, while connected to cell.

Does that make sense?

I mean, if there is no voltage drop at 0 current, then theoretically it would charge up to full voltage of 5v.

I'll try it again, maybe I had the diode the wrong direction or something.

 

[amberwolf]

If the diode was wrong direction, you'd get 0V cuz it blocks in that direction. To work, anode goes to V+ and cathode to Batt+ (or the other way around if it's on the negative side).


If there's no current flow, then there's no significant voltage drop--that's the problem with all the "simple" voltage drop methods--they all require current flow at some relatively constant level to make a constant voltage drop, AFAICR.


If you wanna test it's voltage drop, measure across it with a resistor from it's cathode to the charger's V-. The resistor needs to be sized for the current draw you expect from the battery, to emulate it charging. But at max current draw it will be different than at full charge, so the voltage drop will be different, too.

Differnet kinds of diodes have differetn voltage drops, too, and different curves along which that drop changes.


If you want it to be a constant 3.6V output, you're gonna have to use some sort of active regulation.

It is probably going to be easier to use a higher voltage AC adapter, like the bajillion 12VDC wallwarts I see at Goodwill all the time, and the chip you started with, or the LM317, or something similar, than to try to drop just a tiny amount of voltage.


But there *are* ways to do what you want--it's done inside celphones and other devices all the time. I just don't know how complex it is. You could open up an old celphone and trace out from the battery to the chips feeding it and see what they used.


Alternately, if you don't care how big a mess it all ends up to be, you can get old celphones cheap (maybe free if you ask around on freecycle or craigslist) and use THEM to do the charging for you.

 

[veloman]

As for using old cellphones to do the charging, I don't think that would work for lifepo4, since they charge to 4.2v (li-ion).

I have an assortment of 12v chargers ranging from .5a to 4amps. As far as I know, the LM2596 doesn't have a current limiter, so I would need to make sure it doesn't push too many amps and kill the converter.

I am going to move away from the diode method. I was charging with them on the circuit and they did get hot, but as you said, not too predictable.

I know others have used the Volt Phreaks chargers to balance charge a lifepo4 pack. Plug it in and forget it. That's what I'm aiming for. I already spend too much time monitoring charging batteries.

 

[skeetab5780]

If it has a 5vdc output it is probably just a PSU not a charger. Most of the old cell phones have a built in circuit to charge off of that. Although they do sell very cheap 4.2v lithium single cell chargers for under $5 online.