dead (flat) battery ??

[Marilyn de Queiroz]

I got a multimeter but I'm having trouble figuring out how to use it properly. The manual is pretty useless, so I thought I would post here because I figure that lots of people here can help me.



I understand that I need to plug the black cord into the COM plug and the red cord into the V plug to measure voltage.

I measured a bunch of "dead" 1.5 volt flashlight-style batteries. They almost all read 1.2V (I put the dial on the left side under Vm -- not the right side by V~)

I read that sometimes the battery is dead if the volts are still high, but the resistance is also high. However, when I put the red cord into the ohm plug, I either get 1 or .6

Neither of these results seems to indicate a dead battery. What am I doing wrong?

[David O'Meara]

Now you're testing me. Yes, a 'dead' battery can still register a voltage. Yes, a battery has an internal resistance. I think the problem is the definition of 'dead', as quited above, since a battery that doesn't work in a high load device (like a digital camera) might still work in a low load device like an LED torch.

[Peter Rooke]

Well - its been a long time - but I once did a good job of failing a electronic principals qualification years back - and the most annoying thing I still remember formulae that I have never used and will never have a use for

Anyway here you go: http://science.howstuffworks.com/question501.htm
http://www.need.org/needpdf/infobook_activities/IntInfo/Elec2I.pdf
[ August 27, 2007: Message edited by: Peter Rooke ]

[Marilyn de Queiroz]

It seemed odd to me that all of the "dead" batteries had a voltage of about 1.2 and a resistance of about .6 so I thought I'd try a brand new battery. I got 1.6 volts and .58 resistance. That doesn't seem like much of a difference between a brand new battery and a dead one. Maybe I'm doing the measurements wrong ...

Also, I was under the impression that the resistance could increase up to the amount of the voltage, but I can't get anything higher than 1
[ August 27, 2007: Message edited by: Marilyn de Queiroz ]

[Jim Yingst]

[MdQ]: It seemed odd to me that all of the "dead" batteries had a voltage of about 1.2 and a resistance of about .6 so I thought I'd try a brand new battery. I got 1.6 volts and .58 resistance. That doesn't seem like much of a difference between a brand new battery and a dead one. Maybe I'm doing the measurements wrong ...

No, sounds plausible enough. As a rough analogy, if you've got one person who can lift 160 pounds, and another who can lift 120 pounds, then if they both try to lift a 140 pound object, the first will succeed and the second will fail. Many electrical devices require certain threshold of voltage before they can function at all.

[MdQ]: Also, I was under the impression that the resistance could increase up to the amount of the voltage, but I can't get anything higher than 1

Nah. I'd say that's comparing apples to oranges, but it's more like comparing apples to trees. The units of the two scales don't have anything to do with each other.

[Marilyn de Queiroz]

If the units are totally different, how can you use the

current = voltage / resistance?

formula?

I'd get

x amps = 1.2 / 0.6

which comes out to 2.0

But when I measure the amps, I get 0

[Marilyn de Queiroz]

Originally posted by Jim Yingst:
No, sounds plausible enough. As a rough analogy, if you've got one person who can lift 160 pounds, and another who can lift 120 pounds, then if they both try to lift a 140 pound object, the first will succeed and the second will fail. Many electrical devices require certain threshold of voltage before they can function at all.

That makes sense. Kinda what DOM said.

[Peter Rooke]

From what I recall - you have to measure current [the flow] (amps) within an closed circuit - so connect your battery to a resistor (that you know the resistance of) and then measure the current (ie Amps) though this. You will not get any current if it does not have anywhere to flow.

Power is equal to current times the resistance squared ( P = I * R[squared] ).

[fred rosenberger]

it's been many years, but...

I thought to measure the voltage, you need to measure in parallel. forgive my crappy ascii art:

--- meter ---
|           |
|           |
+---load----+
|           |
|           |
+--battery--|

in other words, you need a closed circuit, and then you put the test leads on either side of the load (a resitor or small lamp would work).

To measure current, you need to measure in series:

+--meter---battery--load--+
|                         |
+-------------------------+

Current is constant in series, wheras voltage is constant in parallel.

and I'm not sure what good measuring the resistance of a battery is. Your meter provides a voltage. by measureing the current that comes back, you can calcualte the resistance. The meter assumes 1.5 volts (since that't what it's internal batter provides). If you measure a battery, it's going to provide SOME volts, which in series get added. So, the circuit now actually has 2.7 volts, which will mess up the calculations.

you can measure the CURRENT from a battery, or the VOLTAGE. I don't think batteries can have a resistance, since they GENERATE power. adding something with resistance would DECREASE the current for a given voltage. adding batteries would INREASE the current for a given voltage.
[ August 28, 2007: Message edited by: Fred Rosenberger ]

[Peter Rooke]

Or could could buy a Battery Tester - but where's the fun in that.

[Ernest Friedman-Hill]

(Dusting off my ancient Ph.D. in Chemistry... *koff koff*)

Electricity is the flow of electrons from one place to another. Voltage is the "force" that the electrons are pushed with. Current (amperage) is the "amount" of electricity (number of electrons) that flows. Resistance (ohms) is the ... well, the resistance to flow; how much the electrons bump into stuff.

Many chemical reactions are all about exchanging electrons, especially ones that involve metals. Metals like to give up electrons; other materials are all too happy to absorb extra electrons. An electrochemical cell is a clever device in which a metal, an electron-loving material, and something to instigate the reaction are all brought together in such a way that electrons can flow out of the metal, out into the world through a wire or something, then back into the electon-loving material.

Most of the things we call "batteries" ain't batteries at all, they're single electrochemical cells. A "battery" is a collection of such cells, wired in series. A car battery is a collection of six such cells that uses lead as the metal, lead oxide as the electron-lover, and sulfuric acid as the go-between.

Now, in any electrochemical cell, eventually the metal's going to run out of electrons to lose, and the electron-lover's going to get its fill, and then the cell doesn't work any more. For some kinds of cells, you can force electrons back into them and thereby run them backwards: lead-acid batteries are this way. But run-of-the-mill alkaline cells (filled with zinc, manganese dioxide, and potassium hydroxide!) can't be recharged. Eventually all the zinc powder turns into zinc oxide, and all the manganese dioxide turns into Mn2O3, and it's trash.

The voltage you get from an electrochemical cell is a characteristic of the chemical reaction you're using. A typical alkaline "battery" has a nominal voltage of 1.53-something volts because that's the electrochemical potential of the reaction they use. In an ideal world, you'd get exactly 1.53-something volts until the whole thing was used up, and then it'd drop immediately to zero.

But of course, we don't live in an ideal world. As the reaction progresses inside the battery, we get lumps, and bubbles, and little bits of gunk that build up; either expected reaction products, or by-products due to impurities. And the electrons have trouble getting past those bits, and so the resistance (nominally) goes up, and thus the voltage (nominally) goes down.

But the things is that there's no formula that governs this: it's not a simple thing anymore. It's dirt and glop and chaos. Yes, the resistance of a dead battery may go up. No, there's no reliable measure by which you can relate resistance to battery freshness.

I have a multimeter. I check batteries all the time. If the voltage is significantly below the rated voltage (i.e., a 1.5V battery says 1.4V) then it's a dead battery. As someone else pointed out, these are still good for flashlights, non-electronic toys (motorized things) and other stuff that doesn't really care what voltage you feed it.

[Marilyn de Queiroz]

Originally posted by Peter Rooke:
Or could could buy a Battery Tester

Yeah. Batteries used to come in a package that included a battery tester. Now they don't. I figured that I could buy a multimeter and get the voltimeter and a few other things besides for the same price.

[Marilyn de Queiroz]

Originally posted by Ernest Friedman-Hill:
But of course, we don't live in an ideal world. As the reaction progresses inside the battery, we get lumps, and bubbles, and little bits of gunk that build up; either expected reaction products, or by-products due to impurities. And the electrons have trouble getting past those bits, and so the resistance (nominally) goes up, and thus the voltage (nominally) goes down.

But the things is that there's no formula that governs this: it's not a simple thing anymore. It's dirt and glop and chaos. Yes, the resistance of a dead battery may go up. No, there's no reliable measure by which you can relate resistance to battery freshness.

Thanks, Ernest. I'll stop trying to put values into a formula that doesn't seem to be working as advertised.

[Burkhard Hassel]

Hi,

does this 1.4 / 1.5 V rule of also work with rechargeable batteries?

Bu.

[David O'Meara]

Rechargeable AA's are usually 1.2V rather than 1.5, which is a whole other kettle.

[David O'Meara]

Just to update:

it appears the nickel varieties (MH or Cd) are 1.2V and are cheaper.
The Lithium rechargeables are more expensive but can store plenty of power at 1.5V and are therefore preferred for todays devices.

[Marilyn de Queiroz]

In that case, I'm going to stick with the Lithium rechargeable batteries, especially for things like my camera because I want to get as many pictures out of a set of batteries as possible before I have to recharge them.