[Asking following just out of curiosity, and on these groups because somebody just might know.]
Wife's handheld Icom transceiver ceased working; dropping in 8 AA new batteries fixed it.
Out of curiosity, checked the 8 Energizer AA batteries that came out of it, using a cheapo cigarette-pack-sized Radio Shack VOM on its lowest (10 VDC) scale:
One of them totally dead
5 of them read between 1.5 and 2 V (scale is compressed in that range, but many of them were right up around 2 V)
2 of them repeatedly read 4 VDC
I know batteries are a complex subject -- but what's going on with those last two?
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There's no way a single "1.5V" alkaline cell should produce more than maybe 1.6V. That's all the chemistry of the cell can produce.
Could you (or someone else) perhaps have accidently connected your VOM while it was set on a current range? If the VOM does not have good overcurrent protection, that might cook some of the internal parts and throw the calibration off. It's easy to do.
Measuring batteries without a load on them gives you basically no information at all... even if the battery is dead it will give you normal voltage, and occasionally spuriously high ones. Use a load resistor or a flashlight bulb in parallel with the meter.
So if the battery's dead it _may_ give a normal reading or low or high... BTW, what's a good load resistor value? Does it depend on battery type or capacity? Ron
The best answer is to get a battery tester, which includes the appropriate load for testing a battery of any given type, selected by a switch. You can get a perfectly good one where you got the multimeter (Radio Shack).
Seriously, this doesn't call for any precision in selecting the value.
I'm going to be deliberately sloppy here in the use of technical language, it's _clearer_ to non-technical types that way. :)
dead/dying batteries can act sort-of like a small capacitor. they'll show a voltage across them, but it doesn't have sufficient power to sustain output at that voltage for very long. Even a moderately high resistance connected across the battery terminals will discharge that capacitor effect in a very short (as in a few seconds *at*most* -- usually a small fraction of a second) time.
You don't want to use a "very high" (e.g. mega-ohms) resistance, because that _will_ make the discharge time "rather lengthy". Typical low-end voltmeters have 'internal resistances' on the order of 50k ohms/volt --really bad ones might be only 20k ohm/v, 'better quality' ones around 500k ohm/volt; The old- fashioned "VTVM" was in the multiple mega-ohm per volt range.
Anything 'small enough' to let the capacitor effect 'leak off' in a few seconds is adequate to see what the battery is actually 'generating', rather than the 'phantom' effect from a 'virtually dead' one.
On the other side of the coin, you don't want the value _so_ small that it appears as a 'short' to the battery. This can lead to a battery over- heating, and possibly (unlikely, but possible) exploding. Not to mention that 'excessive drain' rates are damaging to a _good_ battery.
For anything of conventional "consumer-type" single-cell construction and size -- e.g. "AA","C","D" -- nominal 1.2-2 volt output, something in the 'a few hundred ohms' range is "adequate", being within an order of magnitude (decimal!) in either direction is sufficiently precise.
For units that are designed for "much higher" loads -- e.g. an automotive battery, you'll want a _lower_ resistance load. Something on the order of
10 ohms, maybe. That'll give around a 1A current flow -- capacitor effect can sustain that for probably only a _couple_ of seconds. Again, no precision is called for -- 'order of magnitude' _is_ sufficient (subject to the caveat in the next para.:)
Be cautious using small values, and make sure the unit is rated to dissipate the _heat_ involved. e.g. a 5 ohm resistor across a 12v battery will draw roughly 2.5 A, which equates to about _30_ watts. You might get a way with a "25 watt" rated unit, since it will be a more-or-less 'momentary' use, but something rated for 40, or even 50, watts is a better idea.
For the littler 'consumer' batteries (actually single cells), this is a 'non-issue', because the typical loads are _much_ smaller. "Double-A" cells that will run a portable radio for, say, 100 hrs, are producing
25-40 _milli-amps_. That's a _working_ load of around 40 ohms. A 'test load' that measures at 10% of the 'working' current level has a power consumption of around 5 milliwatts. The smallest 'hand usable' resistors I know of, that one can readily buy, are rated for a full eighth of a watt -- some 25 times higher than the test load. Many places may only stock
1/2 watt units -- that gives a safety factor of around 100x on the heat dissipation. One can be off by an order of magnitude and _still_ have a
10x safety factor. Nothing to worry about, here. :)
Actually, there's a fair bit of misinformation kicking about here.
Measuring a battery voltage without a load *does* give you useful information, it just can't be interpreted the same way as a measurement taken with load.
The no-load voltage of a battery depends mostly on the chemical process in the battery. The original poster reported a voltage reading of 4 volts from a single AA cell. That *has* to be a sign of an inaccurate voltmeter because, for chemical reasons, single alkaline cells don't go above 1.62 volts. After slight use, such a cell levels off at 1.55 (no-load) for much of its life. As the battery runs down, it gets down to about 1.50 volts (no-load) before losing the ability to deliver substantial current.
The no-load voltage decreases slightly as the battery runs down. The voltage under load decreases a lot more as the battery runs down, which is why testing is usually done with the load in place. Before the digital era, voltmeters weren't accurate enough to distinguish (say) 1.50 from 1.62 volts reliably.
As others have noted, there is a wide range of opinion as to what an appropriate load should be. The best load for a test is the same as the load the battery will have to power in its intended use. Acceptable voltage under load is typically 2/3 of the maximum no-load voltage, but this varies depending on the application.
Look, I don't mean to argue here, because if I knew enough about batteries to argue about them I wouldn't have written the post in the first place (though I do know a bit about electricity, having been an active experimentalist and professor of EE for 42 years).
These batteries had been in very light service (used for a few minutes a week) in an Icom handheld transceiver (not mine) for a prolonged period (many months).
When the Icom owner discovered the Icom was barely usable in a weekly net checkin, the batteries got pulled out; and out of curiosity I checked them with an inexpensive hardware-store-type battery-powered VOM, which was fully functional on all its functions and ranges (well, I didn't check the ammeter function, but ACV, DCV and ohmmeter were all OK).
A couple or three of the batts read close to zero volts; a couple or three read around 1.5 to 1.6 volts (this is low down on the 10 VDC scale, which is the lowest range on this model, so I can't be more accurate); and a couple or three, to my surprise, read right around 4 V.
These readings were repeatable on each individual battery, measuring them repeatedly, in random order, and multiple times: the same couple or three batts always read 4 V; and they measured the same when I pulled several of them out of the discard jar several days later.
The batts have gone to the recycle center at this point, so I can't repeat the experiment (though the VOM is still on the garage workbench). I suppose one could argue the VOM must have been "inaccurate" in some fashion in the 4 V readings, but the puzzle seems to be how it could have been apparently accurate on some of the batts and yet "inaccurate" in this way on others -- and consistently and repeatedly so.
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