Whats the simplest way to charge NiMH batteries ?

[pkshima]

Hi all,

I wish to charge my two 2300mAH NiMH cells which I bought for my digicam. I dont want to spoil them so I am not using the charger that i bought locally because it doesnt look like a smart/safe charger.

I would like to build a safe yet simple charger which can keep my batteries charged and safe. I am aware of the circuits built around MAX712
but I am having trouble acquiring that IC here in India.

Is there a circuit that can be built using discreate components, op-amps etc. or some IC thats low cost and easily available ?

Thanks in advance,
Pradeep

 

[audioguru]

Just make a 230mA current source circuit and turn it off with a 14hr timer.

 

[pkshima]

wow!!! is it that simple !!!
is it safe ? what if the batteries are already charged or half charged.
and how did you derive the figure 14hrs.
Also what should be the voltage for the 230mA power supply ?

 

[audioguru]

wow!!! is it that simple !!!

That's how cheap chargers like mine work.

is it safe ? what if the batteries are already charged or half charged.

I charge mine only when dead. If they are already charged or half-charged then my simple charger will over-charge them which will shorten their life.

and how did you derive the figure 14hrs.

My charger and battery labels say so and I've read about batteries and chargers.
A 2300mA/hr cell needs 230mA for 10 hrs plus 40% longer because of inefficiency.

Also what should be the voltage for the 230mA power supply ?

About 1.4V for each cell in series plus enough for the current source. The charging battery cells limit the voltage.

Go to www.energizer.com and click on Technical Info. Select Ni-MH and look for the detailed Application Manual.

 

[pkshima]

great help audioguru
so what would you suggest to make a smart charger.
If not smart, atleast safe on the batteries ?

 

[audioguru]

I would use an inexpensive Maxim IC in a simple circuit and you would use a big pile of parts in a very complicated circuit.

 

[pkshima]

:lol: nice one.
but i wonder how one could use an IC without having it :!: :roll: even if he/she wished :roll:

 

[audioguru]

Simply c'mon over here or find someone here to send it to you.
You might have a distant cousin over here.

 

[pkshima]

yeah lets c if the company has any onsite appointment for me :wink:
my brother is there (US) but I was just looking if I could solve the problem without the MAX712.

 

[audioguru]

Use Energizer's Lithium disposable AA cells. It is claimed that they take about 56 times more pics in a digital cam than alkaline cells. They have an extremely long shelf life and flat voltage during discharge.
Although they are very expensive, I got free samples by promising to do a survey about them. Energizer wanted to know how much $$$money$$$ they should sell for. I told them they are crazy if they price them at 56 times more than their over-priced alkaline cells! I also told them they are crazy to sell a bunny toy that doesn't use their batteries! :lol: :lol:

 

[k7elp60]

I have been charging NiMh batteries for years using a simple charger with the current limited to less than .095 of capacity. I use a LM317 in the current limit mode. No need to time them as they get slightly warm when charged and you don't need to take them off the charger.
In your case 218mA would be the charge current. But in practical matters if you use a 6.2 ohm 1/2W resistor the charge current will be very close to: 1.25/6.2 =200mA.

 

[Exo]

what if the batteries are already charged or half charged.

NiMH batteries don't have the 'memory effect', so there should be no problem charging half-charged batteries.

 

[audioguru]

How do you know when they are fully charged and are having their life shortened by overcharge?

 

[Nigel Goodwin]

How do you know when they are fully charged and are having their life shortened by overcharge?

As long as you only charge them slowly it doesn't do any harm, in fact you can leave them on 'trickle' charge permanently! - problems mostly occur when fast charging, one solution is to monitor the temperature of the cells, they get far hotter once they are fully charged. This is why it's common to have temperature sensors inside laptop batteries!.

 

[audioguru]

For a simple charger, Energizer recommends using a timer to switch from a slow overnight charge to a very low current trickle charge for the longest battery life:

 

[Oznog]

How do you know when they are fully charged and are having their life shortened by overcharge?

As long as you only charge them slowly it doesn't do any harm, in fact you can leave them on 'trickle' charge permanently! - problems mostly occur when fast charging, one solution is to monitor the temperature of the cells, they get far hotter once they are fully charged. This is why it's common to have temperature sensors inside laptop batteries!.

Trickle charging is pretty detrimental over long periods and/or using significant current. Trick is, if the current is that low it'll take forever to charge.

Monitoring cell temp is extremely effective IF you use a constant high charging current. The cells do not generate much if any heat when charging, in fact, due to the exact nature of the reaction there is a cooling effect and they may get slightly colder. Once the current stops being stored as chemical energy it makes heat and this is observable. But if the charging current is too low, the temp rise when it begins to overcharge may be trivial and the circuit would have to be so sensitive that other temp variations- like starting with cells brought from a colder place or having sunlight hit it, may trigger the temp change sensor.

I haven't seen it written anywhere but the required charging current for thermal sensing logically has to be tied to the battery's size (heat dissipation capability and to some degree thermal mass) rather than the battery's charging capacity.