Changing LiIon charge cutoff from 4.0 to 4.2v

[Streak]

I have received a R123 and charger. The charger is based on the MC34063 DC-DC step down regulator. The datasheet says that the Vout is set by adjusting two resistors whose values are currently 2.2k and 1k giving an ouput voltage of 4v. Indeed when the charger shows charge complete the battery measures 4.0v.

I would like to get a bit more into this cell and get the charger to cutoff at say 4.2v. This would mean changing the higher resistor value to about 2.36k or the lower one to about 932 ohm.

Firstly is it worth the excerise?
Would the cell be OK with the higher cutoff voltage?
Could I use a 1/8w carbon resitor instead of the SMD one on the board?

TIA

Streak

 

[vcal]

[ QUOTE ]
Streak said:

I would like to get a bit more into this cell and get the charger to cutoff at say 4.2v.

Firstly is it worth the excerise?
Would the cell be OK with the higher cutoff voltage?
Could I use a 1/8w carbon resitor instead of the SMD one on the board?
TIA
Streak

[/ QUOTE ]
Conventional wisdom assumes that you will get fewer charge cyles over the life of the cell if you charge to 4.2V.

OTOH, only charging to 4.0V means probably only about 75-80% of the cell's design capacity is being reached.

A good compromise terminal voltage for many seems to be about 4.1V.

 

[shiftd]

i like mine charged up to 4.15 V. going to 4.1 or 4.0 is a bit too low IMHO. 4.2V is the upper limit and you must be careful since there is variation in the DMM and resistor, so it might be more than 4.2V, which overcharge the battery.

 

[cy]

foursevens sold a modded li-ion charger a little while back with an added pot. allowing you to dial in volt desired. That was the best deal, I've seen so far on li-ion charger offers.

my Li14430 li-ion charger by DougS terminates at 4.18volts. charge rate should not exceed 1C. The more intelligent chargers will charge at .1C until voltage reaches aprox. 3v, then steps up to 1C for rest of charge.

li-on cell is fully charged at 4.2V and depleted at 3v. so why not use the full potential of your cell?

if you want to extend the life of your cell, yank at first sign of dimming. take a voltage reading for a few cycles to make sure you are not depleting below 2.5v. this will kill your cell.

 

[PeLu]

I do have numerous single cell LiIon chargers and also changed some.
One was changed to 4.24V by adding a resistor, one has a potentiometer already built in and the newest one has a switch which says 3.2V/4.2V and has a quite high open voltage.
These are all chargers for r123 cells.
One thing is to mention: If you start with C or C/2 as a charging current, the cell will not be nearly full at 4.2V. It needs some time to be at constant 4.2V and when the current goes back to C/10 or C/20 the cell is considered as full.
'cutoff voltage' gives the impression that the reaching this voltage indicates the end of charge, which is clearly not so (unless you charge with a very small current).
And BTW. several cheap chargers put out some ripple, which should shorten the lifetime again.
And as the rechargeable R123 cells are available for very little money, I do not care so much about their lifetime (that means if I get 100 or 300 cycles).
And it is your decision what you call a 'full' cell. You have the choice of even higher charging voltages (and tehrefore capacity) and sacrifying lifetime. Klaus showed us a nice diagram about life time in cycles versus charging voltage some time ago.

 

[Streak]

Thanks for the input.
This charger measures no voltage when no battery is in place, so by inserting the battery it completes the circuit. Charge light is orange to start with and then slowly turns green once the battery is charged.
Measuring the voltage across the battery during charging shows a steady climb from say 3v up to about 4.0v at which time the light is not quite green but almost there. Next morning the light is green but the voltage has not changed at all and is till at about 4.01v.

I would like to bump up the voltage to say 4.1v but finding the correct SMD resistor combi is not that easy.

Streak

 

[cy]

pelu, thanks for pointing out charge VS life. I've been running my CR2 li-ion at 4.24v for sometime. I don't seem to have any problems so far. all my other li-ion is topped off at 4.18V.

most common li-ion chargers are powered by wall warts. those typically have 350-550 milliamps output. so charge rate is usually not a problem.

 

[PeLu]

Streak: replacing a SMD resistor with a 'normal' one is no problem, I think I've done that. There is very little current through them anyway.
I put one resistor in parallel to another because I just had the correct value at home (5k6) and so I saved the work to remove anything.

cy: I don't know if the wall wart is the limiting factor, the charger Streak mentiones comes probably with an additional 12V cord. No limiting factor there.
Two of my three different cheap R123 chargers have a built in power supply, one a switching, one a transformer.

I tried to charge the R123 cells on a more sophistcated charger with higher initial currents, but they went pretty fast to the max voltage and charging time was not much shorter.

BTW, there are other Li Ion cells with graphite electrodes which need a max. voltage of 4.1V anyway. But these are almost extinct (I do use a few 5.5Ah cells).

Maybe somebody remembers were Klaus postetd this graph....

 

[SilverFox]

Hello Peter,

Klaus referred to this article and this one as well.

I can confirm one data point on the curve. I tested a cell charged to 4.5 volts and got 6 cycles... /ubbthreads/images/graemlins/frown.gif I believe it matches the curve precisely.

Tom

 

[djpark]

Streak, your resistor calculation for the FB divider shows you know your stuff. I will say it is worth increasing the voltage to 4.2V.

But I'd like to mention a couple of things about this type of charger.

I've done MOD on JSB charger which is very similar design (though the size may be different) and you can see my post here.

http://www.candlepowerforums.com/ubbthreads/showflat.php?Number=832073#Post832073

There is another resistor sensing the current and mine limits it to 200mA. There is no inteligence in the charger and the current flow is determined by the difference between the charger output and the battery voltage. The led is simply showing the current flow status, but with clever setting of bias, led turns off at low current even though the current is still flowing to the battery.

Here is one of the charging status logging done with 4.25V output set by the original charger design.

As you can see, at about 4 hours the charging current was reduced to 10% of original 200mA, but it didn't stop and continued to charge with the led still on. You will never really know when the charging is completed to reasonable capacity.

If you are setting the charger output to 4.2V exactly or 4.22V with 12k parallel to 1k, it would take shorter time but less capacity. I wouldn't mind leaving the battery in the charger in this voltage setting, though.

What I did to this charger was adding LTC4054 charger chip to control the charging at 300mA and use the original charger as buck regulator and battery holder. A bit of wasted resources, but I am pretty happy about what I got. You can see the pic on the above post.

On the other hand, many gave warning on the charging li-ion at high voltage and I totally agree with them.

I had a 14500 charged to 4.5V before (just once only) and it gave me really a lot of discharge capacity initially. But now when it is charged to 4.2V only, it drops voltages very quickly and also can't last as long as others. If a single charge at this voltage can affect so much, it is not difficult to imagine how fast repeated charging cycles at high voltage can kill the battery.

-- dj

 

[Streak]

Great post DJ, thanks very much. My charger is the same unit as the one you have pictured in your other thread just under a different brand name but identical PCB.
I contacted the manufacturer and found out that they do make two R123's one with and one without protection. $1 difference in price between the two.

I would imagine the battery that comes with the kit is the unprotected one.

I will place another resistor in parrallel with the 1k and try and push the voltage up to around 4.15.

I am evaluating the kit for the importer, as such I ran the battery in my Q3 until the Q3 would not operate anymore. Voltage at that time was less than 2v, will see how quickly the cell dies as a result of this. I guess what I should experience is either reduced run time or fewer charge cycles.

Cheers

Streak

 

[Streak]

Further information from the manufacturer is that they supply the unprotected cell in the kits for photo applications and the protected kits for flashlight applications.

It appears that the charger however is the same.

Streak

 

[cy]

DJ, nice post as always /ubbthreads/images/graemlins/bowdown.gif

Streak, have you taken both apart?

on the protected units I've received. the cell terminates the charge, instead of the charger. if you try charging a bare cell in protected cell's charger. it will severly overcharge, killing the cell.

yours may be different. the "3V" r123 cells comes off the charger at 3.9V if I remember correctly.

you've got PM

 

[MrAl]

Hi there,

Im not sure if this has been said already, but changing
the voltage to something higher with an Li-ion is not
recommended unless the manufacturer states the voltage
is really that much higher. The spec's on these things
are tight -- plus or minus about 1 percent -- so they
mean what they say. If you go over the rated voltage
your sure to doom the cell much sooner than normal.
Now that applies to Li-ion cells, but only if the voltage
is actually higher than rated, so the first thing you
must do is check the actual voltage rating with the
manufacturer. If they say 4.0 volts, then you better not
increase it or get ready to buy a new cell very soon.

If, after all is said and done, you decide to increase the
voltage, i think you can get away with connecting a resistor
of value 13750 ohms in parallel with the 1k resistor that's
in there now. You may wish to double check this value first.

Good luck with it!
Al

 

[Streak]

Hi Al,
Thanks as always for the input.
I added a 22k resistor in parallel with the 1k and the shutoff is now 4.16 volts.