ni-cad to li-ion

[new guy]

I have an old NI-CAD rechargeable battery recharger. Can I use it to recharge new LITHIUM ION batteries? For "AA" or "AAA" batteries?

[MrAl]

Hi there,


Unfortunately, those batteries are VERY different and require a VERY different charge algorithm.
It would actually be dangerous to charge Li-ion with a NiCd or other charger like that.

Fortunately, it is not hard to make your own Li-ion charger if you have a DC wall wart laying around
of say about 9vdc at maybe 300ma or higher. If you build your own circuits this would be easy,
only requiring one IC and a couple resistors.
If you would like to see the circuit let me know. It's the simplest Li-ion charger you can make
and cost is about 2 or 3 dollars USD.

[dacflyer]

i'd be interestedin it, i have a handfull of cellphone batteries i could put to good use. they are
3.6 - 3.7 volt sizes.

[MrAl]

Hi,

Note that this charger charges at about 150ma but you can go higher with many cells out there.
To get a higher current, lower the first resistor R1.
You can leave out the transistor and diode and LED and associated parts if you dont want
an indicator light that lights roughly when the cell is done charging. If a blue or white LED
is not available then use two Red LED's in series (need higher forward voltage there).

[Robert Reed]

MrAl
Two questions:
When the cells terminal voltage is under 2.5 volts, does that means it's shot?

I could add a simple circuit in place of the LED voltage to terminate charging when the charging current drops to the same point as indicated by the extinguished LED, also with capabilities to light a different LED to show charging cycle completed. Do you see any problems doing it that way?

[MrAl]

Hi Robert,


Yes, everything i have read since around 2001 says that the cell is to be considered 'damaged' and no
attempt should be made to charge it again after it drops under 2.5 volts as that could result
in fire or explosion. If it is 2.5v or above but less than 3v it is supposed to be charged with a lower
than usual current to get it up to 3v or so and then regular charging is ok again.
Of course the charger should not be connected with no power input either for long periods of
time.

I suppose you can build a circuit in place of the LED yes, no problem. The cell really should not be
left alone though while charging just in case something goes wrong. If you stay in the vicinity of
the charger while it is charging then you can get away without a circuit to force termination.
Many people are worried about this too, but it doesnt really matter as much as they make it
sound sometimes. As long as it is not left on for hours on end you're ok.
I make sure i am always around when i do my cells anyway, so i know just when they are done.
I use a uC based monitoring circuit along with my charger circuit with blinking LEDs to indicate
the charge status (voltage, charge current).

If you build one let me know how you make out with it.

[dacflyer]

not to make it complicated more, but how aboout charging termination with a temp sensor ?
this way if the battery starts to get too warm, the charge will terminate ? good idea or ?

[MrAl]

Hi there dac,


Well, if you are looking for a termination method that will help protect against overheating then that might work ok.
On the other hand it will not help to determine the end of charge in a 'normal' cell that does not have a problem because toward the end of charge there will be almost no heat generation. The only way to detect that (normal end of charge) is to look for a very low current like C/40 or something like that.

[CeaSaR]

MrAl,

Couldn't you use the 2 red LED option, coupling one of them to a phototransistor setup to keep
the cell under charge connected to the circuit while the LED's are lit, then disconnect the cell
when the LED's extinguish? I see a momentary SPST would be needed to start the cycle. That
way, the same circuit would be used with the added benefit of automatically ending the charge.

Just a thought.

CeaSaR

[MrAl]

Hi Ceasar,


Yes, you could arrange something like that too. That does however imply that you will somehow not be able to keep an eye on the circuit which isnt a good idea in general unless maybe kept away from flamable stuff.

In any case, many people want to have that feature too, but this charger was designed with the intent of producing the absolute simplest circuit that can possible be devised that could still charge an Li-ion cell safely. The LED and other related parts were added later. The basic circuit is comprised only of the following:

1. The LM317
2. The input resistor R1 (limits max current during charge)
3. The two voltage set resistors R2 and R5
4. A DC wall wart of course is also needed to supply power, but a wide range of voltages are possible

Also note that some wall warts will limit the max current so R1 may not be needed. The user would have to
test for that though. Many people have more than one size cell though, so they may want to use two different
values for R1: one low value for big cells and one bigger value for small cells, unless they dont mind if the
bigger cell charges slower.

Whatever we add later just makes it better, but also makes it more complicated.
In my original version i built for myself i added a transistor and a couple small resistors to limit current
instead of using R1. That allowed resistors that are not as high powered to be used for current limiting,
but yes the circuit became more complex.