Charging nimh AAA batteries in series

[ramonegumpert]

Dear Gurus,

I bought a pair of AAA nimh batteries rated at 1.2V 900mAh. Fast charge can be done in 5 hours @ 270mAh. Standard charge can be done in 15hours @ 90mA.

My doubt is assuming both batteries have different impedence, can they be charged in Series at say between 1.4v to 1.6v using a 5v dc source rated at 500mA ?

I intend to limit the current to 270mA and use a resistor to reduce the input voltage to 1.6V.

I am not sure would there be safety issues as i am not using any battery controller but just a direct connection to power. I intend to cutoff the power after 5 hour max

sincerely
Ramone
:!)

 

[sophiecentaur]

If you can keep count of the total charge left in the batteries and the number of Ah you're giving them then there should be no trouble oing it 'manually'..
But I would hesitate to rely on a 'smart charger' to charge batteries in series unless they are clearly of the same manufactured batch and have the identical history of use. Clearly, it is safe to charge a number of cells in series in this way because all higher voltage batteries have multiple cells inside but a duff discharged battery in series with a new, well charged one could present a problem, with the new one being grossly overcharged /cooked / knackered by the end of the exercise.

 

[ramonegumpert]

Hello sophiecentaur,

Thank you for your kind advice.
The 2 rechargeable batteries are new ones I bought 1 year ago :P
They have never been used before and come as a pair.
I measured the voltage for both are similar at about 0.5volts.
It seems that there is 0 micro amps inside the batteries.

I think i will give it a try and charge it 'manually'. :)

Have a great day.

sincerely
Ramone

 

[vk6kro]

If you bought the batteries together and you fully discharge them before charging, there shouldn't be a problem charging them in series.

If they are being used in a digital camera, they will still have a lot of charge in them when the camera stops working. So they have to be discharged carefully.

In this circuit, the diode plays an unusual role. It limits the final voltage that the single NiMH cell drops to. The current is limited by the 4.7 ohm resistor. When the battery voltage drops to 0.6 volts, the current will stop and the battery is discharged as far as it is safe to do.

Then you need a timer to make sure you do not overcharge the batteries. If they overcharge, they will get hot and may be destroyed. This is not really dangerous but it is expensive.

 

[sophiecentaur]

Hm.
They may have suffered from spending so long in a discharged state. I read somewhere that they shouldn't drop below 0.8V. If you can't detect any current from it through a meter, that may be bad news. But just go for it and charge them for a while. See how they behave.
Good luck.

 

[vk6kro]

Try charging at about 30 mA for a start and measure the voltage while you do it.

If the batteries are OK the voltage should start to rise in a few minutes.