Replacing embedded battery on MP3 player.

[neckilljo]

Hi Everybody. I'm from Brazil. Not 100% english speaker.

I'm new to this forum and I'm not used to deal with electronics (though i know some basics) and I'm very concerned about the safety of some battery adaptation.

I have a Coby MP550 MP3 Player:
**broken link removed**

Since I've bought it, the battery always ran out after ~40 minutes of continuous playing. I decided to abandon it like 6 months ago. I tried it today in the morning and the battery was dead. I tried to charge it for 1 hour but it didn't charge at all.

I opened (broke) the case and noticed the battery was bloated, and had no voltage (according to the multimeter) so i took it off.

It is a GSP031525 80mAh 3.7V. According to Coby website, it is a Li-Pol unit. Here is a picture of it.

**broken link removed**

I took a "T828"'s (chinese multifunction mobile phone) battery, 1800mAh Li-Ion 3.7V and soldered to the MP550 board.
**broken link removed**
**broken link removed**

But, doing some research about it, I learnt that Lithium batteries need to be managed/monitored while charging and discharging. Both overcharging and fully-discharging are harmfull and can cause explosion, fire, leaking, shorting etc.

As you can see, the original MP550's battery doesn't seem to be a "smart batery". It looks like there's nothing like a circuit doing some kind of load management. I disassembled the plastic case of the T828's battery and found that it has only on a metallic block, having one spot being the (-) and the whole body being the (+). Disassembling beyond this would only expose the chemicals inside it. I mean, unlike the Playstation Portable battery, looks like there's no embedded circuitry on the MP550's or the T828's ones.

Can we assume that what will monitor/manage the load of these batteries have to be whatever charge or drain them?

The T828 wall charger has a indicator led that warns about the charging proccess. Red for charging, orange for half charged, green for fully charged, but, from now on, it's impossible to use it to charge the fankenstein MP3 player. It will have to be charged the default MP550's way (via USB).

I know these are very cheap devices (not worth all the efforts) but my concerns are only about the safety of using the T828's on MP550. Even having it working normally and playing for more than 8 hours continuously so far, I havent tried to charge it to it's fullest yet.

Can I trust that the MP550 board has the needed "things" to manage/monitor the 1800mAh T828? Would it act like it did with it's own battery, charging only to the safe limit and switching itself off before the battery is runs completely out?

The filnal work is this, with the new insulating-tape-"case" .
**broken link removed**

Thanx for any reply.

 

[Hashime]

Those batteries are used in many, many devices. All of which are made in china. You are right though, Li-Pol can be nasty if they get over charged.
The only thing I can suggest is charging the thing slowly, and using something to monitor the temperature.
I am not sure about anything else, though your device charged the old battery fine, so it could conceivably charge a similar battery safely. Also consider using a digital camera battery, most have external chargers that are safe.

 

[neckilljo]

Well, I left it playing at the highest volume level and it powered itself off approximatelly 13 hours after all, only with the energy it already had before I soldered them together.

It's taking forever now to recharge via USB. It's been there since yesterday and haven't finished yet (10 hours so far).

Looks like it's charging very, very slowly because it's not getting any heat. With the wall charger it used to heat after 2 hours.

 

[Sceadwian]

Charging a lithium battery by hand is actually a lot easier and safer than you might think if you're willing to dedicate the time to monitoring it.

You need a constant current source which you can use a variable POT for if it's rated for the power dissipation. You can generally charge the battery at 1/2 of it's rated capacity (so if it's a 1000mah battery you charge it at 500ma) While it's charging you monitor it's voltage, when it hit's 4.1 volts you stop. At that point the battery is 70-80% charged, you can call it quits right there and use the battery if you want, that's the easiest way. If you want to finish charging it you have to lower the current so that the voltage never goes above 4.1 volts. until the charging current drops to about 10% of the initial charge current, but this is annoying because you have to constantly adjust the charging current to prevent the voltage from going to high. If you want to seriously extend your battery life only charge it to about 3.9 volts, this will give you maybe 60% of the total maximum capacity of the cell however the battery will last a LONG time.

The next most important thing is that Lithium batteries should NEVER be discharged bellow about 3.1 volts, the longer the battery stays in this discharged state and the lower the voltage it's discharged will dramatically shorted battery life. The battery should be kept topped off at all times if possible.

Lithium batteries should NOT heat appreciably during charging, their charge efficiency is really high, if your lithium batteries are heating up during charging you're probably charging them incorrectly.

I would trust manually charging a lithium battery over a charger of unknown quality. Mind you the battery has to be monitored constantly during charging.

 

[neckilljo]

Sceadwian, thanks for the info about charging it safely, but manual charging would make it an annoying gadget.

Considering the only mod I did on the MP550 was trading it's 80mAh battery for a 1800mAh one, would it be a good idea to trust the MP550's ability to do the usual self management while charging?

Trying to answer myself, all this technical info about lithium batteries makes me guess that "time" is the only variable, since the behavior of the chargers should be the same: charge using safe rage voltage, monitor output voltage and current, cut the power when it comes to a default value, etc.

 

[Sceadwian]

No it would absolutely not be a good idea, that should go without saying. If the battery is bloated as you said then it was likely because there is NO charge control circuitry on the MP3 player it's just feeding the battery a constant current and not watching the voltage or heat. The battery is also 23 times the capacity of the original, so even if it did work it would probably take a day and a half to charge it.

Making a simple quick charger for lithium cells shouldn't be complicated, just make a constant current source rated for the capacity of the battery and charge it until it reaches 4.1 volts using an analog compartor, then shut it off, it'll take about 45-50 minutes from a discharged state to get to 80% of battery capacity. That additional 20% capacity would take another hour or two to achieve with more complicated circuitry.

 

[neckilljo]

thanks everybody for the information provided... i had to come back and tell that it's playing very safe and the full charging takes 4 hours on USB... it plays for roughly 110~120 hours continuously

this cheap mp3 player consumes very little power... this adaptation turned it into a high reliable source of music