Battery recharging physics question

I have an MP3 player that I’m going to be using in the car quite a bit in the near future, and I’d like to get a car adapter to recharge the battery.

I looked at the plug on the regular recharger that I use at home, and it says the output is 5 volts DC. Poking around in stores, I’ve seen car adapters that give 12 volts, 9 volts, 6 volts, and 4.5 volts. How close to 5 volts to I have to get to avoid hurting my battery?

4.5 volts is pretty darn close, and bear in mind, too, that the voltage rating is the output at the listed currrent draw. At lower currents, the voltage will actually be higher, depending on the regulation of the adapter. Do make sure you match proer polarity, though. There will be a symbol like this:

  • ------(o------ -


  • ------(o------ +

The first one means the center tip is negative, while the sleeve is positive. The second one means the opposite. Look for this diagram on your MP3 player near the power jack, or on the AC adapter.

The 4.5 should be just fine. 6 is a maybe. Any higher is apt to result in a dead player. Can’t say for sure as I don’t know what the player’s power input circuitry is.

Some clarification:

Q1: Are you trying to charge batteries only in your car or are you trying to play the player in your car? (The later will require a little higher voltage and more stable voltage.)

Q2: You mention output of your charger. Do you actually mean the output of the adapter that plugs into your charger?

Note that a 4.5V adaptor is “nominal” for a given 12V car system. Since your alt. is typically putting out a couple volts more than that, you are better off with a 4.5 than a 6V adaptor.

How many cells does the unit use? Three or four?

If the unit requires four cells you might have a problem. Assuming NiCAD or NiMH batteries are used, the batteries will source 1.25 x 4 = 5 V when fully charged. This means an adapter that sources 5.0 V will be adequate for powering the unit but inadequate for charging the batteries. This is because NiCADs and NiMHs typically need 1.5 to 1.6 V per cell during charging to overcome their internal resistance.

If the unit requires three cells you’re probably O.K. In this case the batteries will require 1.55 x 3 = 4.65 V for charging. This means an adapter that sources 5.0 V will probably be adequate for both powering and charging the unit.

Also keep in mind that, even though it says “5V” it may be putting out a higher or lower voltage. It may also be unregulated.

As to your questions, well, hell if I know. All’s I know is that there’s a doohickey I stick into the hole over on the side. OK, the player itself doesn’t have any battery info on it. The charger is has a plug on both ends - one end sticks into the wall, the other end plugs into the player. It’s the charger “brick” at the wall end that has the info I’ve provided above.

And I will be playing it in the car. I’ll charge it at home, then I want to have it plugged in while I’m using it during the drive, so that it’ll still be charged up when I get to work. That way I can listen to it at work, then again on the way home.

Get the 4.5 then, for best safety. Most electronics that use an AC adaptor have some voltage regulation and overvoltage protection, but unless you know for sure, best not to risk it. You’ll also want to be sure the current rating on the car adapter meets or exceeds the current rating on the AC adaptor (although it’s probably overrated, but you can never be sure).

Many MP3 manufacturers sell car adapters for their MP3 players. I suggest looking at the manufacturers web site or going to the store from which you originally purchased the MP3 player.

A famous electronics store, rhymes with Radio Smack, sells an adjustable car charger that has, a “Universal Vehicle DC to DC Adapter” that has all the ranges you mentioned and various tips to fit your player. There are two styles, the 300mA and the 1000mA. I suggest the 300mA version.

Good Luck

I did that. They have a car adapter set for sale, but it says specifially that it should not be used with my model. They don’t have one for my model.

I went there the other day, and the closest they had to that was one that you could set to any of the abovementioned voltages - 4.5, 6, 9, or 12, IIRC. It didn’t do 5 volts. At least the one I saw.

It sounds like the 4.5 volt is going to be my best bet. I’ll have to think about this. I really don’t want to screw up my machine. I’ll have to see if I can figure out a way to recharge it at work, maybe.

Smeghead: Does this MP3 unit have a built in charger or is there a separate charging unit?