Adapter for charging 3v cells in 3.7v charger

[TaschenlampeMann]

As my collection of 1x RCR123 lights grows I have some lights that require the 3 volt cells and some that will take the 3.7 volt cells. Having two different kinds of chargers was a pain, especially when traveling.

So.....

I thought of building an adapter that would drop the 4.2v max voltage from my standard Nano charger down to around 3.7 volts max to charge the 3 volt cells. The adapter consists of a germanium diode (for the slightly lower forward voltage drop than a silicon diode) soldered to a circular piece of double sided copper-clad circuit board.

When charging 3v cells in the 3.7 volt Nano charger, I simply slide the diode assembly between the charger terminal and the battery + terminal...being careful to observe the polarity of the diode.

At the end of the charge cycle on a 3v cell. The Nano charger is putting out about 4.1 volts and there is 3.7 volts on the battery + terminal.

Any thoughts or comments? Is this a valid way to avoid buying and carrying two different chargers?

 

[VidPro]

is that just like the adapter the battery people are selling to do the same thing?
one i saw was a wire, some component and a wire, i ASSUMED they were selling a 15c diode for $3 , but i wasnt sure.

you made a nice looking wafer there.

Just to clarify for anyone reading this, is this Li-Fe-Po type of batteries , not some regulated or already dioded down li-ion 3.0 cell or a primary ? meaning very specific to the battery chemistry type your trying to charge.

here is some sort of adapting thing, and myself i was wondering exactlly what it was or could be , because it cannot be a powered curcuits, only passive.
http://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=2960
on the other hand i dont see any polarity indication on that device? a non-polar voltage drop, hmmmmm how about 2 diodes, that way it works in either direction?

 

[lctorana]

Remember you want to drop the OC voltage by about 0.5v, and you have observed that on charge, the Ge diode drops about that much. That is good, but I think you will be better off with a Si or Schottky diode.

Why?

Because as the battery voltage rises, the charge current drops, and (take a look at the I-V curve of your diode in a datasheet) the Ge diode voltage drops. Right down to 0.2v, or even a smidgin less, depending on type.

With Vbb=4.2v, that means your 3v battery will, eventually, reach 4.0v - too much.

Remember Ge diode voltage drop is not constant - it is a pronounced curve, and the turn-on voltage is very low, which in this application, works against you.

 

[TaschenlampeMann]

Thanks for the feedback. I'll give it a try with a Si diode.

Also VidPro, I like your idea about 2 diodes. No worries about which way to install the device.

 

[thekobk]

Could you tell me if any 1 or 2 of these will work? http://www.radioshack.com/search/index.jsp?kwCatId=&kw=diode&origkw=diode&sr=1
I would love to install a 3v 3.6/3.7v switch in my UltraFire Model: WF-139 so I can use it to charge LiFePO4 cells.

 

[lctorana]

Could you tell me if any 1 or 2 of these will work? http://www.radioshack.com/search/index.jsp?kwCatId=&kw=diode&origkw=diode&sr=1
I would love to install a 3v 3.6/3.7v switch in my UltraFire Model: WF-139 so I can use it to charge LiFePO4 cells.

Yes, I'd go with a pair of 1N4001, wired tête-bêche.

 

[mdocod]

UM.... slight problem here....


[disclaimer: all of this information could be entirely wrong]

3.7V li-ion cells with 3.0V regulators in them actually require something like 4.4V to get up to 4.2V at the cell.. so the chargers designed for "3.0V" cells actually run higher voltage than regular li-ion chargers.. UNLESS you are using LiFeP04 li-ion 3.0V regulated cells, in which case as I understand they charge to something like 3.5V but require about 3.7V at the charger to get the cell to that voltage backwards though the regulator. How power gets back to the cell through the regulator is not something I am specifically familiar with. so don't quote me on that, my limited understanding is that the regulator also ends up cutting voltage that gets to the cell during charging...

 

[Yenster]

TaschenlampeMann, what kind of 3V cells are you trying to charge? Are they Li-Ion chemistry or LiFePO4 chemistry? I'm asking because your adapter will only work on LiFePO4 chemistry. And I suppose if you've been charging 3.0V Li-Ion batteries with that 3.0V Nano charger, then your batteries have not really ever been fully charged.

 

[TaschenlampeMann]

Yenster, My 3v batteries are LiFePO4. The adapter seems to work fine using my 3.7v charger. I went back to the Ge diodes since the lower voltage drop allowed the end-of-charge voltage be around 3.7 volts. I checked on lctorana's warning that the Ge diode voltage drop would decrease as charge current decreased resulting in too much voltage on the battery. Turns out that at end-of-charge there is still about 30 mA flowing, resulting in about a .5 volt drop across the diode.

Thanks for the feedback.