Charging and using LIon batteries in parallel

[Gertlex]

I've got two 450mAh 3.7V lithium ion batteries for my MP3 player. (I've got two of this mp3 player, in fact, since one died).

Said MP3 player is the Insignia Sport 2GB with bluetooth, but that's mostly irrelevant.  I use the bluetooth with my bluetooth headphones, but the mp3 player only gets ~5 hours/battery.  The headphones get 9 hours or so.

On a fundamental level, one can put two identical batteries in parallel and they will output the same voltage for twice as long as just one.

In reality, I'm sure the two batteries aren't so similar.  Problems I see:
-One battery has been used regularly for a full year. (but still had the same battery life... roughly. I've never truly measured)
-The other is "new" off ebay.  (and was manufactured more recently, going by numbers on the mp3 player)
-LIon has protection circuits. Presumably this is built into the battery and not into the mp3 player? Would the two circuits play nice, in parallel?
-The mp3 player plugs into a USB mini charger.  Would the power flowing increase for the dual load, or would the charge time be twice as long, or, (even though it's in parallel...) would the effective charging voltage be noticeably lower?
-Does bad stuff happen if the + and + are connected and the - an - are connected between two batteries? I.e. If I connected two 9V batteries like that... would bad things happen?  Any capacitors/resistors needed somewhere?  My understanding is that theoretically, no... but the only place I care to depend soley on ideal theory is my circuits exam in two days. 
-Can I do non-destructive testing to get more info?

Do I really need that much battery power? YES... Today's the third day in a row I'll be in a library working until 4am x_x

•Gert

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[TrickyNekro]

Typically you should charge the batteries remotely and one at a time...
You can use Li-ion cells in parallel if the voltage contrast is narrower that 0.05V (tough)
If you connect in parallel, yes you have more battery life and more hours of music as
The protection circuit is based on voltage and not current draw...
This is because Li-ion cells can't be discharged below 2.5V or they die...
And they can not be charged more than 4.25V or they explode... (literally)

Current draw measurement is far more expensive in mass production than voltage detection is
So everyone just prefers voltage... It's easier to play with it... period!

Consider that the charging battery is isolated and is only connected to the charger and not other battery...
Also consider getting a good charger... I have Ultimat 14... (charges everything except Sealed acid batteries) Best for everything... Unless you want to charge sealed acid batteries... then you probably need Ultimat 17... (which can charge every type of battery )

If you want to build a pack, then
First, DO NOT attempt to solder directly on the batteries... things can go boom boom... no kidding...
Second, if you want to charge the batteries as a pack check that the voltage difference of the cells
is below 0.05V...
If needed charge the batteries independently and then make the pack...
Third, if you don't know if you are right about something ask or prepare a fire extinguisher...
If you try to drop water on a bad Li-ion cell, thing will do Twin Towers.... believe me...
Water interacts with Li+ and Hydrogen is freed, and Hydrogen is 3 - 4 times more explosive than gasoline is...


Consider all these and then have fun... Li-ion and Li-Po are fun to play with and
surely are a good background for learning about many electronic systems... But safety first...

[Gertlex]

Ya...

You mentioned one thing that was on my mind but didn't make it to the keyboard last "night".

Since the batteries are of different ages, that makes it less likely that they'd stay within .05V of each other for the duration of their discharge, right?  I think that alone rules this out.

I think next I shall look into a larger capacity Lion battery and try using that

Are cellphone batteries usable for this sort of thing?... i.e. the ones I've seen have three terminals.  Do they have some special protection circuit that requires something from the phone in order to be used? (If that makes any sense at all, I grant myself a cookie!)

Thanks
•Gert

[Admin]

I don't know all the answers, but this should answer half of them:

If two batteries are connected in parallel, with different voltages, one will discharge into the other (a bad thing).

If one battery charges or discharges faster than the other, they are guaranteed to have different voltages.

Since they are connected, this voltage difference creates a short circuit.

Discharging creates heat, therefore damaging the chemical makeup of the battery.

hope that clears it up!

[TrickyNekro]

Well, it's better off with parallel LiPo cells, unless you know to use a switching regulator...
Cellphone batteries can be used for this... The middle is only sensor output, although some batteries require some voltage at that pin... This voltage can be created by a voltage divider... But I don't know much more sorry...

Nokia batteries can be surely used in whatever application you like, come to think it's the same with SonyEricsson...

Better stay off parallel use... Lipo batteries can deliver huge currents (believe me, I just burned some wire cause I mistakenly short circuited one.... And it burned in fragments of second....)
You can also get a pack of two batteries and separate them as charger and protector comes in the MP3 you will have not a problem... (You use one cell....)

Best Regards, Lefteris
Greece

[Gertlex]

Problem solved. Found out how to do a hard memory reset for the MP3 player.  It's no longer bricked and I now have the effective double battery life desired.