Charging NiMH Batteries

[Gregory]

I am looking for a circuit to build a charger for NiMH Batteries That will not overcharge or over temperature and Veriable voltage.

Can you help
Greg

 

[crutschow]

Seems like you're really reinventing the wheel here. Do you have a good reason to build your own, rather than buying one?

 

[audioguru]

If you want to make your own Ni-MH battery charger, Maxim-IC have some good battery charger ICs.

How do you know if a charger is any good??
I bought a cheap charger that came with 4 Ni-MH AA cells. It was imported from China by Energizer who import the battery cells from Japan.
The charger is stupid. It is just a timer. It charges fully-charged cells the same as dead ones and overcharges them. They get extremely hot!

The Maxim-IC charger IC is smart. It detects a charged cell and refuses to overcharge it. It gives a slow charge to a cell that is too hot or too cold.

 

[Gregory]

By building My own battery charger it helps me understand more about Electronices. I only have a basic knowledge of Electronices and the more help I can get the better it is for me to learn.
Thank you all for your help.
I am still learning.
Greg.

 

[jbarnaby2000]

Greg,
I think building your own charger is a great idea. Audioguru is right about Maxim too, they have some IC's that are really tailored right to what you want to do. Are you charging individual AA or AAA cells, or the sub-C RC packs??

It looks like Maxim has some new chips out for those single cells the DS2714 looks like a really good one for like those single cells. I was looking into the MAX712 and MAX713 for my RC packs. Just search on the Maxim site for these part numbers and they will even give you schematics as well to try out. Just a little word of caution, which you probably already know, it does not take much for the NiMH cells to start to vent. So I would be very conservative on your charge currents, and don't charge at really high currents.

Hopefully this helps out a little.

 

[Gayan Soyza]

Here is my NIMH charger,I charged many NIMH cells without any problem.

If you charge your batteries at a recommended rate then no problem.

I have no hurry to charge my batteries.So I don't need any advanced detecting methods or any expensive IC's to terminate the charge.

This simple constant voltage current limited type charger gave me good results.

If you are hurry to charge your batteries or if you need to charge your batteries withing few minutes do not use this.

 

[Boncuk]

Hi Greg,

if you are about to learn about battery chargers I suggest to start out with a charger for lead-acid batteries.

NiMH batteries can turn into little bombs if not treated correctly.

Lead-acid batteries are not that touchy, mainly if you use the charger for "wet batteries". If they start "cooking" there is still no problem. They just lose water which can be refilled.

If an NiMH starts cooking (which you probably won't notice) you better look for cover.

Hans

 

[k7elp60]

I have built a number of NiMh battery chargers using a constant charging source. I limit the charging current to less than .095C. I have used the LM317T in the constant current mode. I had a digital camera that used 4 AA cells. I charged them for several years this way. One thing to keep in mind is that the contact potential of each cell reaches about 1.5 volts when they are fully charged. So if the cells are inseries the charger must be capable of producing 1.5V times the number of cells in series.
Using this method the cells never overcharged at normal room temperature.

 

[audioguru]

Using this method the cells never overcharged at normal room temperature.

Energizer and other battery manufacturers say that a trickle charge current must be C/40 or less or the life of a Ni-MH battery is reduced.

 

[Gayan Soyza]

Hi k7elp60 I have noticed that when charging with constant current mode (ignoring constant voltage) the batteries will increase their voltages above 1.5V. So the batteries getting hot.Its exceeding manufactures maximum limits.

 

[audioguru]

The batteries are not supposed to get hot. They get warm then they are fully charged and then charging current must be reduced a lot or turned off.

A battery charger IC monitors the voltage of the charging battery and shuts off the current when the voltage drops a little when it is fully charged and is beginning to get warm.

When a battery gets too hot then it "vents" and releases some of its chemicals. It vents instead of blowing up.

 

[Gregory]

Thank you for all your input . It is very refreshing to see the amount of effort you have all gon to.
I have learnt a lot from your Input.
The Charger I would like to build is one that I can adjust the voltage to match the number of cells with the matching current.
I rebuild Battery packs and I do not always have a Charger.
EG 1 AA
or 5 Sub C
4 D cells
12 Sub C
15 Sub C ar the most I have rebuilt.
Can youy tell me where I can obtain a circuit for this multi-purpouse operation.
I am also intrusted in electronics
Greg

 

[3v0]

The problem I have with NIMH's is that they die from not being used. I am thinking that storing them in DIY trickle chargers would prevent this. Is there any downside to it?