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?

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.

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/appno...6/426Fig01.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

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/Ne...nd%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...

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/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+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/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+charging
http://www.picbasic.co.uk/forum/show...ttery+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

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
They are NIMH, Sanyo AA (HR6) batteries 2700mAh each.