question about batteries

[Special K]

The LCD display on my cd walkman has a little indicator that tells you (approximately) how much life your batteries have left. I was just wondering how it determines this. Does the current/voltage output by the battery gradually decline as the battery is used up (since batteries are not 100% ideal voltage sources), and then the circuitry of the cd player can detect this and use it to display the batteries' estimated level? If this isn't HT enough I could move it to OT or something.

 

[Shalmanese]

My guess would be the voltage declines. My palm battery is at 4.10V fully charged and 3.72 when its empty.

Different battery technologys might have different ways of measuring however, I think voltage only works for LiIon.

 

[Jeff7]

Voltage declines differently for different battery types. This is all based on memory, but I'm fairly sure of its accuracy: Alkalines have a fairly linear voltage drop, from full to dead. Nicads and NiMH drop slightly initially, then hold fairly steady for most of the charge, then the voltage suddenly drops off to nothing. I don't know what LiIon's voltage curve looks like.
How the devices actually measure the content, the voltage does seem to make sense. I have some handheld radio transmitter things - walkie talkies with a "2 mile" range. They are using NiMH batteries; they last quite awhile, but I have about 1 minute warning that the batteries are dying. The little icon on the screen says the batteries are running low, then less than a minute later, it beeps loudly and shuts off. I haven't tried this with alkalines though, so it's kind of one-sided evidence. I only use alkalines for things like clocks and remote controls anymore; everything else uses NiMH.

 

[zephyrprime]

Voltage. It's easy to do. You hit the nail on the head. Batteries are far from perfect voltage supplies and voltage declines with use.

 

[RossGr]

Since batteries produce electricity by a chemical process they have a fixed voltage output determined by that chemical reaction. They also have a internal resistance which infact determines the output voltage. Actually the voltage remains constant and the internal resistance increases as the battery drains. If you measure the voltage of a battery with a DVM you should see very nearly the constant output voltage irregardless of the condition of the battery this is why a good DVM should not be used for checking batteries. To see the internal resistance you need to draw current from the cell. All battries can be seen as a perfect voltage source in series with a resistor. To the outside world as the battery drains the output voltage appears to drop because the internal resistance increases dropping more of the cell voltage. All a device need to is moniter the output voltage to determine the state of the charge. It does not matter if it is linear or not, as long as it is repeatable over time.

 

[dejitaru]

The voltage of 'AA' batteries is rated 1.5V. Companies have increased this to about 1.7V to hack some extra life out of their batteries (since 70% of 1.7 is greater than 70% of 1.5). Some even to 2.1V for the "high tech" cells.

Ah, but the device manufacturers aren't falling for that! They've calibrated their gear for this new energy source, changing the very value of 1.5 and rendering your 1.5V cells useless, if you can even find some these days.

I guess that was more of a rant.


But yeah, there's a voltage drop.

 

[Special K]

If there is a voltage drop over time, then shouldn't I be noticing some decrease in the quality of the music that is being played by the player? Wouldn't a lower supplied voltage have some effect on this?

 

[Mark R]

Wouldn't a lower supplied voltage have some effect on this?

Which is why there is a voltage regulating power supply inside the player, providing a constant voltage to the CPU and memory (usually about 3.3 V). As the battery voltage goes down, so the power supply tries to draw more and more current from the battery - there comes a point where the battery voltage gets so low, that the PSU can't operate and so shuts down.

 

[silverpig]

Originally posted by: RossGr
Since batteries produce electricity by a chemical process they have a fixed voltage output determined by that chemical reaction. They also have a internal resistance which infact determines the output voltage. Actually the voltage remains constant and the internal resistance increases as the battery drains. If you measure the voltage of a battery with a DVM you should see very nearly the constant output voltage irregardless of the condition of the battery this is why a good DVM should not be used for checking batteries. To see the internal resistance you need to draw current from the cell. All battries can be seen as a perfect voltage source in series with a resistor. To the outside world as the battery drains the output voltage appears to drop because the internal resistance increases dropping more of the cell voltage. All a device need to is moniter the output voltage to determine the state of the charge. It does not matter if it is linear or not, as long as it is repeatable over time.

Score another one for Ross...