How hard or easy to build a Nimh charger to charge 12 cells

[nour]

Hi,
I have recently bought a lot of Nimh I think around 22 cells 
12 eneloop(1900 mah) 
6 energizer(1400mah) 
4 energizer(2650mah)

I have a lot of gadgets that require batteries   

I want help regarding to economically charge them
I will need to charge all the eneloop at one go
I thought about buying a dedicated 12 cell charger but I said why not trying building one myself

I am not sure whether or not it is a good Idea but ha what would I lose if I tried
So the 10000000$ Question, How easy or hard to build a 12 cell nimh charger?

I want to build a reasonable charger that can do the charging cycle in a reasonable time(not taking for ever)
for starter I happen to have a lot of LM317 (around 15)can I use it in efficient way for this project
by the way I don't have any idea how Nimh batteries being charged
any guideline, open source projects anything!!


I hope I wouldn't end up buying new one, it will be disappointing 

one little note: if a new commercial charger cost me the same amount of money of building it, I may choose to build it rather that buy it  (I know weird choice)

[IanB]

There are commercial chargers that have 4 or 8 slots for parallel charging of cells. They manage each cell individually, so that each cell has separate charge monitoring and end of charge detection. This is typically multiplexed with microprocessor control. It would take you weeks or months to design and build and test something equivalent. So I would consider this to be impractical.

The "simple" way to charge NiMH cells is to put them in series and charge with a low constant current for a fixed time. For example, for your 12 Eneloops, you would choose a charge current of 190 mA and charge for 16 hours.

However, my recommendation is to buy a good quality 4 slot charger designed for Eneloops (for example from Panasonic), charge your Eneloops when they need recharging, and put them in the "charged and waiting for use" storage area. Eneloops stay charged for a long, long time, so you can keep them charged and ready for use.

As for the Energizers, probably they are not of much use, so unless you have a battery analyzer like a Maha C9000 to test them with, I would not bother with them.

[nour]

I haven't bought them all at once, I have bought the eneloop very recently for clean and low noise power source stuff the other energizer batteries is for the gadgets use
so when using the eneloop I use them all as power source and to try to charge them in 16 hours that is a very long time 

as I have mentioned buying is my last option (it is not like I am not considering it), I just want to try building a charger first

right now I have 4 cell energizer wall charger that I am using to charge all my batteries, but think about  22 batteries on 4 cell charger 

adding microcontroller in the project would be nice
hope someone will give me a startup point or some basic instructions that can help me doing this
if someone can help with some instructions about how the charging cycle is happening, for example how do I know the amount of time required, can I measure it, how would I know if the charging complete, should I monitor the batteries temperature while charging, should I consider a certain amount of current for every battery type, Is there a specific voltage that should be applied, is the whole process is actually linear or it has some curve for the current or/and the voltage applied?

excuse my ignorance regarding that topic
thanks
 

[Audioguru]

The website www.batteryuniversity.com describes details and rules of charging and discharging all kinds of batteries.

Ni-MH battery cells can be charged separately or in series. If they are in series then they should all be about the same age, have the same amount of discharge, have the same manufacturer and the same mAh capacity. Charge them with a constant current for a certain amount of time controlled with a timer like my Energizer charger. It charges two pairs of AA cells at 360mA or AAA cells at 120mA for about 7 hours then shuts off to prevent over-charging. It is so stupid that it does not detect when the cells are fully charged so it severely over-charges cells that already have some charge.

Today's Energizer AA Ni-MH cells have a capacity of 2300mAh and hold a charge for 1 year like Eneloop cells. 6 or 7 years ago they were 2500mAh and the charge did not last long. I can't remember them ever being 2650mAh but maybe a long time ago.
   

[uncle_bob]

Hi

If the cells came from all over the place, (and you have a choice) do not charge them in series. The gotcha is the variation in cell capacity. You are quite likely to overcharge one while under charging another. People in the battery pack business struggle with this problem a lot.

Simple charger:

Monitor the current into the battery to whatever resolution you can get. A few milliamps is a good goal.

Set up a variable power supply that can step off in fairly low steps. Tens of mv is a good goal.

Rig a temperature monitor on the charger. Even better rig a monitor on the battery it's self.

Rig a volt meter / ADC on the battery. Something that will show you a few millivolts is fine.

Write up some code to run the whole thing.

You start out at a bulk charge current that is appropriate to the battery you have. The supply ramps up while you watch current climb to the level you are after. You keep adjusting the voltage to keep the current right (Yes, you also could use a constant current supply).

The voltage on the battery will increase as you do this. You watch the voltage and decide that "that's enough".

You now drop into a mode where you charge (at a lower current) for a while and then discharge for a bit. Watching the voltage as you discharge gives you an idea of how fully charged the cell is. Hopefully your power supply design can sink current as well as source it.

The charge / discharge stuff (and temperature and voltage) will get you to another "that's enough" point.

From there you drop the current still further  and trickle charge the battery. After some amount of time, you may even stop doing this. Most chargers keep trickle charging.

Lots of fun.

Bob

[Audioguru]

Most chargers keep trickle charging.

For lead-acid and very old Ni-Cad cells, not for Ni-MH cells. Energizer, Panasonic (they bought Sanyo) and others advise not to trickle charge since it is damaging and is not needed with today's cells that hold a charge for one year. Even my very cheap timer controlled charger shuts off.

[uncle_bob]

Most chargers keep trickle charging.

For lead-acid and very old Ni-Cad cells, not for Ni-MH cells. Energizer, Panasonic (they bought Sanyo) and others advise not to trickle charge since it is damaging and is not needed with today's cells that hold a charge for one year. Even my very cheap timer controlled charger shuts off.

Hi

I miss read ... I thought we also had some old NiCad's in the mix along with the others.

Sorry!

Bob

[Lightages]

I would think that building a charger to handle that quantity of batteries is gong to cost you much more than just buying a good charger. The biggest problem is the mechanical part. All the holders and connections, plus a reasonable case, etc.

[uncle_bob]

I would think that building a charger to handle that quantity of batteries is gong to cost you much more than just buying a good charger. The biggest problem is the mechanical part. All the holders and connections, plus a reasonable case, etc.

Hi

I think we all agree on that. The OP seems to be looking for a project. This is a very do-able thing and not outside the range of a home build. It's a good way to play with some code and learn a lot about the insides of batteries. It also is not likely to break the bank cost wise as a project or involve setting the house on fire.

Like a lot of projects, Bob's idea of a fun build may not be anybody else's idea.

Bob (... the one with the crazy projects ...)

[nour]

I would think that building a charger to handle that quantity of batteries is gong to cost you much more than just buying a good charger. The biggest problem is the mechanical part. All the holders and connections, plus a reasonable case, etc.

I already have batteries holders, bought some of them from china, so no worry about the mechanical parts 
I think I will try to read first about the charging cycle of the NIMH and after that decide whether or not I will be able to build that thing, also will try to search about some open source projects doing the same objective