# Does a battery becomes open circuit when it's full? capacitors does?

1. Apr 20, 2014

### ChrisToffer

When a batter is charging, lets say 3V battery.. when it's full, does it stops there or continues to take in charge? Like for example, capacitors charges up, untill it reaches the voltage which was charging it, when both becomes equal, the cap becomes open circuit.. i wonder if batteries also do the same..

2. Apr 20, 2014

### Baluncore

Welcome to PF.
The “open circuit” you refer to is actually a zero current situation. That occurs because device voltage equals the charging voltage, there is no differential voltage, so there is no current. The open circuit concept is equally inapplicable to both capacitors and electrochemical cells.

The difference between a capacitor and a cell is that the cell voltage is fixed by it's chemistry and largely independent of charge. A capacitor has a voltage that is proportional to stored charge. If the charger provided a voltage that was clamped at or just below the electrochemical cell voltage then the zero current situation could be attained.

After a cell is fully charged, any additional charge provided will cause an overvoltage and destructive chemical reactions will occur.

3. Apr 20, 2014

### sophiecentaur

I think that's where your problem lies. The capacitor doesn't "become" an open circuit because, if there is any change of PD across it, a current will flow (until a steady value of Q=CV again). For an open circuit to be said to exist, there will be no current, however the PD is altered. (Effectively, the capacitance of an open circuit is zero.)

4. Apr 20, 2014

### meBigGuy

When a recharable battery reaches an internal voltage that equals the charging voltage the current would go to zero plus whatever leakage current (self discharge, etc) the battery draws. The same is true of a capacitor, which also has a leakage current in the real world.

As others have said, the concept of "open circuit" does not apply in the cases you cited.

5. Apr 21, 2014

### ChrisToffer

i got this small rechargeable battery that is from an mp3 player.. i would like to charge it up and use for other purposes, but the problem is that there is no specifications written on it.. i measured the voltage across and i found 4.7 V. how do i determine it's voltage, i don't know how long the battery has got discharged, because i want to make a circuit to charge it up..
Or if you know a circuit which could charge this up without damaging it would be very helpful..

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6. Apr 21, 2014

### Baluncore

I would guess that it is four cells of NiCd. Each is 1.2 volts which makes 4.8V total.
It could probably be charged from a 5 volt regulated supply.

7. Apr 21, 2014

### sophiecentaur

I would imagine that you can get hold of a suitable charger for that particular mp3 player. You should be careful if you try your own charging circuit because it is easy to charge at too fast a rate or to overcharge and that can cause the battery to explode. Nasty - be careful.
If you use an 18 hour charger, there should never be any problems.

8. Apr 21, 2014

### mjhilger

The attached photo looks to me as a Li-Po; is that all aluminum around the pack? If so almost definitely Li-Po. If it looks like only one item, then it is a single cell, max voltage after charge should be 4.205 volts. If it has a higher charge, it has probably been damaged. Please KNOW before you try to charge without the proper charger as Li-xx can cause a fire unless properly charged.

9. Apr 21, 2014

### ChrisToffer

it's aluminum around the pack

10. Apr 21, 2014

### mjhilger

Then almost certainly Li-Po. If it is reading 4.7, then it is probably already damaged and if it holds a charge will not preform anywhere near its original ratings.

Li-Po charging is complex compared to NimH or NiCd; Li-Po ideally requires constant current until a cell reaches 4.205 volts,then a constant 4.205 volts until the current drops to some small threshold determined by the rated current output of the pack or cell. Since this is from an MP3 player, its threshold is probably around 10 - 20 mA.

So if you have a lab power supply with max voltage and current controls, you can charge from there. Otherwise get a pre-built charger.