Hi.

My MP3 player is a Creative Zen Xtra, which has a 3.6v Lithium Ion battery, and takes a 5v power supply.

However, If I'm away from home, and run out of power (it eats batteries), I want to be able to charge it up using batteries I can buy from shops (I.e. AA, PP3 etc). However, if I use a PP3, I need to drop the voltage down to 5v. Simplest way seems to be a regulator. Looking at the mains power supply, it says it can supply 1.5A. I'm guessing a 1.5A reg would get pretty hot. And so would waste a lot of the battery power. So I may not get very much battery life out of one PP3.

I don't know what the capacity of the MP3 player's battery is, it doesn't say.

Does anyone have any suggestions?

I have seen people who put four 1.2v NiMH batteries together. But at around £6-10 for a set of decent capacity NiMH batteries, and £10-20 for a charger that can charge them, thats rather a lot.

Cheers,

Tim

 

Regulators work by turning excess voltage into heat, and will only heat up when required to pass higher currents, or convert down from high voltages.
If you are stepping down from 9v to 5v, and the current is low enough (100mA or so) you should consider a .5 or 1.0 A regulator.

Also, as this site suggests, include a reverse protection diode if using a 9v clip-type battery.
http://www.teachers.ash.org.au/jfull...regulators.htm

I have read that their efficiency is quite bad, 70-80% apparently.
So, maybe you could use 4x1.3v NiCAD or LiMH rechargable batteries instead...? 4x1.3v=5.2v...

That would save your pocket and the environment!

 

lol, I don't believe you can do both!!

yeah, i don't imagin an extra 0.2v would cause it any probs, its (as i mentioned) just high capacity batteries (around 2000 mAh) are quite expensive, and chargers capable of charging them are also quite expensive.

The other thing is, if I ran out of charge, and wasn't somewhere where I would charge even my backup batteries, I couldn't use normal batteries from a shop, because they are usually 1.5 (1.5x4=6, which is pushing it)

 

Well yes, true. It's not a good idea to push in 6v when 5v is recommended... but many of these devices have regulators internally, and the 5v input may be converted down to 3.6v anyway!
I don't believe that a 9v battery is a good idea, they are very limited in capacity, and won't fully charge the spare battery.

Did you consider a solar charger?

 

just looking at them now as it happens. Same ole prob though - a solar charger in my price range will be 4.5v or 6v (thats all I can find at the mo), so I would still end up regulating the voltage down - and considering the ammount of sunlight we get here in blightly, I need all the power I can get from it!!

The thought about the internal regulator had crossed my mind as well, but I was concerned that only some of the player may have a regulated supply, the (for example) amplyfier may run off the external supply where possible - I don't know, and I can't afford to risk it.

Thanks for the suggestions,

Tim

 

The Lithium Ion charger will almost certainly not work properly if your power source cannot supply the max current the charger tries to use. So unless you have a way to use a different charger, the 9V battery or solar panel is trouble.

Most, but not all, regulators burn up the extra voltage as heat, but the buck regulator uses inductance to lower the voltage efficiently. In this case, the current going out of the regulator can be higher than the current into it, though of course the power out will always be less than power in.

 

just thinking about this... (sorry to bring it up again)

how about 4 AA batteries in series, to give +-6v, then use a diode to drop the voltage down to about 5.3v

I reckon that 0.3v above is close enough...?

I know that when the batteries started to discharge, the voltage would drop, so I think that if I did a few tests, I could time it so I could bypass the diode at a certain time in the charging proccess, therby eliminating the voltage drop accross the diode at a certain point, namely when the voltage accross the batteries got too low.

What do you think?

How much current could i get from 4 AA/C/D cells in series?

Thanks,

Tim

 

Yes, the diode trick will work, on the other hand, there's a good chanse the device will work off 6V directly...

If you use NiMH rechargable batteries (AA) you can get about 2.1Ah out of 'em...

This would also solve your first problem, as rechargable batteries only have 1.2V, making their output 4.8 (close enough to 5)

 

not sure I want to risk it! on the other hand it is still under warrenty...

yeah, but I decided against reachargeables, because I didn't want to spend the money on a charger, and if I'm somewhere where I can't get to a power source, it won't help me (hence why I didn't just get 4 1.3v NiMH batteries, as suggested earlier in the thread). I'm talking about normal Alkaline batteries (I usually favour duracell plus)

Also, I'm slightly confused about the difference between Amps and AmpHours...?

Cheers,

Tim

 

Amphours is the amount of power a battery can give during an hour...
So, a 2.1Ah battery should be able to give 2.1A during an hour...

I still think NiMH batteries will be the cheapest option. Normal alkaline batteries don't even come near the power NiMH's can deliver...

My digital camera works 15 minutes with alkalines, over 2hours with NiMH batteries, that's the diffirence were talking about...

 

and it would be able to give 4.2A in half an hour? but presumably it stops at one point, such as I would be supprised if it could supply 8.4A in quarter of an hour...? or could it? How does this Ah rating compare with the Amp rating on the output of a mains charger?

 

Well, the maximum is the short circuit current wich is the current that would flow when you just short the batteries out...

But don't underestimate rechargable batteries. I've seen NiMH AA batteries go beyond 10A when shorted, melting the wires in the process...

(obviously, drawing too much current at once is also harmfull for the battery)

 

lol, wow, thats a lot of power! Like 15W or so...

Thanks,

Tim

 

The AH rating of a battery is given at a specific discharge rate, drawing more current from it will lower the AH rating. So drawing 4.2A from a 2.1AH battery would not last half an hour.

As Exo has already mentioned, you can draw huge currents from most rechargable batteries (for a short time) - which is why they can be very dangerous, they will provide high enough currents to destroy themselves.

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The datasheet of battery always give the discharge time for Ah capacity.
E.g. I have two 12V battery, the dimensions is same, but the Ah marking on the first say 6.5Ah, the another 7Ah.
The min. discharge time for 6.5Ah is 10 hours, for 7Ah 20 hours.
This is a trick, looks like better the 7Ah marking, but i'm almost sure, the 6.5Ah/10h equal with 7Ah/20h....