Safe battery charging?

[keymuu]

Hi there!

As known, when charging rechargeable AA batteries (in my case 6 * 1,2V) they will produce heat when they cannot accept any more current. Not only AA but all batteries. This brings up an idea. Knowing that they heat up when full, so you could just monitor the temperature of them.
Would that really be safe? What when they get old? :confused:

One could think that you should monitor the current and voltage also... But again, if the mechanism is that the charging current will produce heat when the batteries can not accept more current.:rolleyes:

Any ideas around such a thought?

Are you familiar with these pages:
http://www.maxim-ic.com/appnotes.cfm/an_pk/426
http://www.maxim-ic.com/images/appnotes/426/426Fig01.pdf

Is that charger consept reliable?
How about Q1 and Q2? Seems to me that symbols are not correct...one should change them with each other, pnp up and npn down. Correct?

[Ingvar]

I would say this is not a very good method, the heat is produced when you are overcharging(read damaging) the batteries. The whole idea is trying NOT to overcharge. Go with the delta V method, it also works by detecting overcharging, but a lot faster, cheaper and more reliably. Monitoring the temperature is done in "high-end" chargers for safety reasons, don't want the batteries to go "kaboom".

[keymuu]

I would say this is not a very good method, the heat is produced when you are overcharging(read damaging) the batteries. The whole idea is trying NOT to overcharge. Go with the delta V method, it also works by detecting overcharging, but a lot faster, cheaper and more reliably. Monitoring the temperature is done in "high-end" chargers for safety reasons, don't want the batteries to go "kaboom".

Thank you Ingvar!

So, when you have PIC18F4550 and you monitor your batteries in every case, it is easy then also to follow the delta V and wait for the slope first to go to zero and and then turn slightly to negative, or perhaps just wait for the moment when your voltage is not more growing. One could check also the temperature and why not put a time window also, to be perfect :D

There is no need to monitor the current during the charging...
Or what could one gain by doing so?

However, there are better(???) systems. Found one that is called Burp charging, also called Reflex or Negative Pulse Charging, for example
you can find:
http://www.powerdesigners.com/pdf/Negative%20Pulse%20Charging%20Techniques%20Myths%2 0and%20Facts.pdf

This raises another question.
If your application has batteries (nominal 7,2V) and you will charge them by plugging a 12V/800mAh charger to the batteries. If you would like to build a Burp charging system you would need to build a discharge system also to your application... how could one do/build that, a discharge resistant or perhaps a discrete circuit that could do it. With how big current should one do that? Is Burp better or not than just delta V, temp and time window. One would think that you, to be perfect, need temp. and time also whatever charger (if not slow C/10) you use... :cool:

There are a lot of information floating around,
the only problem is that you have not build before such a charging(/discharging) systems,
and so you lack the practical experience...:(

PS. how empty can a new 1,2V cell be discharged before permanent damage? Is it the same for an old one, near the end of it's life? Any differences between different battery manufacturers?

[keymuu]

I though that this would bring up discussion among different parts here (Bruce, Melanie, skimask and others), but then apparently not! If this is not the right place to discuss such a matter, then could you please advice where to ask such a question. Please!

With kind regards
Key

[mackrackit]

I though that this would bring up discussion among different parts here (Bruce, Melanie, skimask and others), but then apparently not! If this is not the right place to discuss such a matter, then could you please advice where to ask such a question. Please!

When Ingvar replied and your response had this in it


There are a lot of information floating around,
the only problem is that you have not build before such a charging(/discharging) systems,
and so you lack the practical experience...
I guess others did not want you telling them they do not have enough experiance to answer you question.

You could search the forum:
http://www.picbasic.co.uk/forum/showthread.php?t=7624&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=6985&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=4958&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=2944&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=642&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=2639&highlight=battery+charging

Not all of the above talks about your battery arrangement...but you have not told the type of battery you are using, you only told the package size.

But to answer your first question about charging.
Use a constant voltage with an amperage limit for the amount of cells in the system. As the voltage of the battery increases the amperage will naturally drop, giving a taper charge to finish. When the voltage is reached have the system shut off. That simple. All the other methods are just a bunch of hooy.

[keymuu]

When Ingvar replied and your response had this in it

I guess others did not want you telling them they do not have enough experiance to answer you question.

You could search the forum:
http://www.picbasic.co.uk/forum/showthread.php?t=7624&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=6985&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=4958&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=2944&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=642&highlight=battery+charging
http://www.picbasic.co.uk/forum/showthread.php?t=2639&highlight=battery+charging

Not all of the above talks about your battery arrangement...but you have not told the type of battery you are using, you only told the package size.

But to answer your first question about charging.
Use a constant voltage with an amperage limit for the amount of cells in the system. As the voltage of the battery increases the amperage will naturally drop, giving a taper charge to finish. When the voltage is reached have the system shut off. That simple. All the other methods are just a bunch of hooy.

Dave, thank you so much :)

I do not know now with what words I have been searching the forum, but obviously with wrong ones :o

I have been googling around and found for ex. those MAX712 but I do not trust an application note when there seems (to me) to be errors (npn-pnp). Is it really so? Can't believe that such a big company as Maxim Dallas could have such an error. That is why I asked if somebody has any experience with such a charger (around the MAX712).

I read through all the links you kindly provided, thanks again for the links.
In one of the links there was an idea of charging each cell (battery) separately when they are connected in series. That is interesting and sound really safe, but will more complicated to build the more batteries you have, so maybe it is just a good idea to forget in normal cases.

You asked what type of batteries. And you are right, I have forgotten to tell that, sorry:o
They are NIMH, Sanyo AA (HR6) batteries 2700mAh each.

[Acetronics2]

Hi, Keymuu




I guess others did not want you telling them they do not have enough experiance to answer you question.



Also, consider it's a PBP Forum ... not a general electronics Forum ...



Dave is here "close to the truth" ...

Just add to that all batteries have a Tempco ... and you'll have to trim the end-of-charge voltage depending on temp.

Now ... also remember "slow" charge and reasonnable discharge rates give the max. life duration to your cells.

For the end ... remember each cell has its particular capacity ... so it MIGHT be charged cell per cell. and voltage shouldn't drop below .8v ( for Nixx ) ...

For series assemblies ... a Schottky diode parallelled to each element is a safe precaution not to invert the polarity of one cell while discharging, ... or in use ...

The chargers might be able to recognize a cell voltage inversion ( very quick voltage drop of the assembly ) and , as soon as, turn to slow charge ...

But ... I did not hear about that, during all these years ...

Alain

[mackrackit]

Sometimes the manufacture has some specs for charge and discharge.
http://us.sanyo.com/batteries/specs.cfm

[keymuu]

Sometimes the manufacture has some specs for charge and discharge.
http://us.sanyo.com/batteries/specs.cfm

Thank you Dave and merci Alain! :)

This conversation about this matter and the the info you can find scattered around has brought myself to to new level of understanding the batteries.

Here is how I now will build it:
My PIC has 6*1.2V NiMH batteries and it measures the voltage of the batteries. A regulator (L4940) convert it to usable 5V.

The charging will work in the following way:
You plug 12V/800mAh to the batteries via a mosfet that will be on because the gate is connected via a resistor to the battery side and from the gate directly to a port of PIC so that it can cut off the charging even that the charger is still connected. How to continue from this point is slightly unclear at this point. There could be a small charge pulse for example 1s charge and 15s not, or measure the batteries all the time and wait them the drop a little and charge then until reached the previous "full" state.

So, during measuring of the battery woltage I will measure the voltage "curve" and wait until it does not grow any more (instead it drops a little) and that is the point to stop. On the other hand I will have two NTCs, one on the battery pack and the other as a reference (not near the batteries), I can so follow the heating up of the batteries and if they heat up too much, say 15 degrees PIC could cut off the charger by pulling the mosfet gate down. And keep on the this "batteries warm" mode until then charger is unplugged.

The only bad thing with this idea is that you can not shut off your PIC and the "battery usage" when your batteries are fully charged, but keep on this "after charging procedure" .:(

If you happen to have an idea how to plug the charger so that your PIC can notice that and turn off the charge when fully charged, please let me now....:)

[Acetronics2]

Hi, Keymuuu

I remember, some years ago, digging into Maxim , Micrel , National , and Linear AN's ... I finally got very clear infos about battery charging techniques ...

But you should download :

from µChip
- AN 960
- AN 1015a

From Galaxy Power Inc
- AN 0014 ( ICS 1700-1702 Chips --- Which are Pic's !!! )
- AN 0017

that will save your last Hair ....

Alain

[keymuu]

Hi, Keymuuu

I remember, some years ago, digging into Maxim , Micrel , National , and Linear AN's ... I finally got very clear infos about battery charging techniques ...
But you should download :

from µChip
- AN 960
- AN 1015a

From Galaxy Power Inc
- AN 0014 ( ICS 1700-1702 Chips --- Which are Pic's !!! )
- AN 0017

that will save your last Hair ....
Alain

Merci Alain, vous êtes l'homme, ... qui sais, ... vraiment:)

AN 960 was really good...
I could find a lot info about Galaxy power but was not able to find those ANs or home page for Galaxy Power, however, I will proceed on the path I described in my previous "mail", hope it is not that bad...:rolleyes:

Thank you all who helped me with this matter, thank you indeed :)

[spikegomez]

Some manufacturer have the user guide for charging and discharging the battery in a right way...
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