Portable Charger

Mutant_Fruit

I'd like a way to charge my MP3 player when i'm on the move, and have no power socket handy. So, considaring i'm doing electronic engineering, i decided to investigate making my own, but i just want to make certain i won't fry my player doing this :P

Basically, i have four 1.25V AA batteries, with a capacity of 2350mAh. My mp3 players charger outputs 5volts and 2A. Now, if i was to connect my 4 batteries in series, i'd get 5v's, but would that combination supply the necessary ampage to the player, or should i get another battery or two and use a resistor.

Or should i use a different kind of battery. Would i be better off with four 1.25V D-type batteries (not sure what capacity they'd be).

Or, considaring a battery is not an ideal voltage source, and the voltage will drop over time, would i be better off using 5 batteries in series, and using a resistor to drop the voltage to about 5.2Volts, so that when they begin to waste, the voltage won't drop to far below 5V.

EDIT: And no, it can't charge from USB. Its a Rio Karma.

Michael Collins

The best thing to try and figure out first would be how much current your MP3 player actually requires. You can be fairly certain the charger can supply more current that is actually necessary - so it's going to be less than 2 amps, a good bit less I'd say.

You'll need a multimeter/ammeter to do this, or if you don't have one maybe you could do it in college considering your doing elec eng. You'd need to put the meter in series with the transformer while charging your MP3 player, I'm sure you know how to do this anyway...

I'd say your 4 x 1.25V (NiMh?) batterys would be well able for the task. Using a resistor could get messy especially if the power dissapated in the resistor is above a quator of a watt. Not to mention the fact that you're loosing power this way.

Where are you doing elec engineering btw, I'm in 3rd elec in ucd?

Mutant_Fruit

1st elec, UCD. And the batteries are NI-Mh.

I'm fairly certain i can do this safely, i just don't want to have to go through the hassle of RMA'ing the item with amazon if it turns out that i break it When i'm in the labs on thursday, i'll try and hook something up and see what i can do. I'll let ye know how it goes.

Any more tips/suggestions would be appreciated.

Capt'n Midnight

NiCADs are renowned for high current and probably NiMH too so the 2A is not a problem. I've had NiCAD AA's shorted by a coin in my pocket and at first you get a strange feeling of warmth. You then have about 5 seconds to get it out of you pocket before it gets too hot to hold..

These days most devices don't use 7400 series TTL devices. They used to want 5V +/- 5% with a guaranteed melt down at about 6V. Most other devices are a lot more tolerant of supply voltage.

Measure the off load voltage coming out of the "5V" psu to see what it actually is, if possible measure while charging too.

The only thing that may be a problem is the battery inside the player - check on the internet to find out what it is - if it uses 4 Cells then it could be charged with 4 cells (slowly and not fully) - if it uses 4 cells in series with an Idiot diode then you won't be able to charge it with 4 cells.

[edit] if the charger gives off between 4.75 and 5.25V with no load then the safest thing would be to use a 7805 voltage regulator. This may need 5 or 6 Rechargable batteries to get past the voltage drop (there is a low voltage 7805 that should run form 6V IIRC) the 7805 has built in thermal shutdown etc. add a heat sink to be on the safe side - if you make it in a old fishemans friend tin ( watch out for possible shorts ! ) you could use the tin as the heat sink

blondie83

Hey this is an interesting read: Its a report written by a group of students who had to design a 12V charger. Theirs was not a portable one, but they explain a lot about how the charging process works, and give some interesting diagrams.

http://www.designthatmatters.org/k2/pubs/BatteryChargerReport1.pdf

Oh and I'm 4th elec UCD, welcome abroad! :cool:

Capt'n Midnight

Is this homework / assignment ? - http://www.boards.ie/vbulletin/showthread.php?p=2346672

AA batteries aren't 1.5V
they can range from 1.6V for brand new primary cells under low load to
0.9V for an almost discharged NiCAD

http://www.radionics.ie/cgi-bin/bv/browse/Module.jsp?BV_SessionID=@@@@1439816564.1107635524@@@@&BV_EngineID=ccceadddkiljgdecfngcfkmdgkldfhl.0&cacheID=ienetscape&3285088340=3285088340&stockNo=0296756&prmstocknum=0296756&prodoid=2740313
http://www.radionics.ie/ - search for 7805

http://www.donberg.ie/ - handy since they only charge €4.50 for delivery.

Mutant_Fruit

Nope, i have no idea who that guy is. This is purely for the fun of it. I know AA batteries aren't 1.5V, its why i specifically said i'd be using 4 x 1.25V batteries Besides, i know the easiest way to drop voltage is to use a resistor

Anyway, i'm going to do some more reading up on this and see what i come up with. If i get really adventurous i'll see if i can work out a way to charge my "battery pack" from a USB cable (500mA output @ 5V if i remember correctly). That way i could top up my batteries using UCD electricity, and charge my player from them when it wastes :P

Plus, it'll be fun

talking_walnut

That was me in the other post. Didn't explain very well what I wanted to do but it's ok I got it sorted. Turn's out the iRiver can take 6v without any hassle.

I think you'll be fine with what you want to do. Don't have any experience with Rio's but the main thing that has to be takin into account with the battery pack for the iRiver is that the current has to be over 1800mA. 1800 or less isn't enough to charge it.

Here's some links to info on iRiver stuff:

Synch/Recharge Cable
Battery Pack

Or check out Boxwave. They might have some premade products you can get. It was cheaper for me to order the synch/recharge cable from them than buy a cable and hack it up. Plus it is retractable

P.S Any iRiver owner check out Rockbox's progress. Its coming along quite nicely