I have a little flashlight that uses 2 AAA batteries. I have 6 old AAA batteries, and using a multimeter (I grew up calling it a "volt meter", they each report a charge of 1.5 or better. To me, that would mean they should be fully capable of lighting the lightbulb in the flashlight. None of them in any combination of two batteries will.

I have two Energizer rechargeables, and after charging them, put them in the flashlight, and it lights up just fine. When I just tested *them*, they only show about 1.3 on the voltmeter, yet they light up the flashlight just fine.


I am setting the voltmeter to "20" on the little dial.

Thanks for any explanation you can share.

That sounds like a normal voltage reading for either type according to my Fluke meter.
Why it will not light with the first batteries I have no idea.

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Grateful11

Multimeters differ from "battery testers" in that they don't load the battery. When the weak batteries are in the flashlight the voltage drops to close to zero.
That's the reason to check a vehicle batter when the engine is cranking, weak batteries voltage drops significantly under load.

So, Catzilla, are you saying that the batteries that won't light the bulb may show a full charge, but when under load, their "true" charge shines through (or in this case, doesn't shine)?

Exactly.

Also, NiMH batteries react differently than Alkaline batteries. When fully charged, NiMH batteries are about 1.3 volts and when discharged they're about 1.1 volts under load. Alkaline batteries start around 1.7 volts and you'll see the battery voltage under load dropping almost immediately until they're exhausted.

my Craftsman Multimeter ($10 on sale, $20 regular price from Sears) has a battery testing function.

There are 2 scales: 1.5V and 9V

On each of these scales, the device applies a small load to the battery. I measured the current (with another multimeter) and I think it draws around 75mA DC.

So when I put a fresh battery and measure the voltage (using the 2000 mV scale), is shows 1.65V or so.

When I put the same battery on the battery test scale (1.5V), it shows 1.55 or so (IIRC).

With an older battery, you may read 1.4-1.5V on the voltage scale, but when you test it on the battery test function, you can see how the voltage drops, let's say it start with 1.3V and if you keep testing it for a minute or so (all this time the device consumes 75mA), you see the voltage dropping to 1.1V or even lower.

Now a flashlight may draw as much as 500 mA (or maybe even more), and those batteries will collapse under such load.

With HIMH batteries, their nominal voltage is 1.2V, but that us under normal load; without load, they measure 1.3V, or even 1.45 when coming out of the charger. But if you do the test on the battery test scale, you read 1.3V and it stays there, it does not drop any further since they have enough "juice" to deliver.

I realize the explanation is not very academic, but it's the best I can do on a New Year's Day after a night of little sleep, not because I was out partying, but because the baby was sick and we didn't get much sleep.

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Catzilla
Multimeters differ from "battery testers" in that they don't load the battery. When the weak batteries are in the flashlight the voltage drops to close to zero.
That's the reason to check a vehicle batter when the engine is cranking, weak batteries voltage drops significantly under load.

absolutely correct.
it is the current that does the work, and that can only be measured under load.
I=ExR
I = current
E = voltage
R = resistance (load)

Does anyone know a source (link) that has recommended loads to test various batteries, i.e. 75mA for, say a AAA, but perhaps 100mA for a AA, and the resistive load (ohms) to accomplish the load?

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yeoman
Does anyone know a source (link) that has recommended loads to test various batteries, i.e. 75mA for, say a AAA, but perhaps 100mA for a AA, and the resistive load (ohms) to accomplish the load?

I think the exact value of the load is not that important, my DMM has only 1.5V battery test function for all AAA/AA/C/D. As long as there is some load, the test should be good. The problem is that with no load you get meaningless reading.

So you could in fact put a resistor in parallel to the voltmeter and test batteries that way.

for a 1.5V cell, if you want to draw 75mA, the resistor would be 20 Ohms and rated power 0.11W, so anything higher than that (pretty much any resistor) would do.

Thanks, all!

I love this kind of explanation.

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H1N1

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Catzilla
Multimeters differ from "battery testers" in that they don't load the battery. When the weak batteries are in the flashlight the voltage drops to close to zero.
That's the reason to check a vehicle batter when the engine is cranking, weak batteries voltage drops significantly under load.

absolutely correct.
it is the current that does the work, and that can only be measured under load.
I=ExR
I = current
E = voltage
R = resistance (load)

you know you got the math wrong, don't you? replace "x" with "/"

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space-time

Quote
H1N1

Quote
Catzilla
Multimeters differ from "battery testers" in that they don't load the battery. When the weak batteries are in the flashlight the voltage drops to close to zero.
That's the reason to check a vehicle batter when the engine is cranking, weak batteries voltage drops significantly under load.

absolutely correct.
it is the current that does the work, and that can only be measured under load.
I=ExR
I = current
E = voltage
R = resistance (load)

you know you got the math wrong, don't you? replace "x" with "/"

Or swap E and I. Same difference.



Edited 1 time(s). Last edit at 01/01/2010 03:44PM by tenders.

Each type of battery has a different fully charged no load open voltage, load voltage, and peak current capability.

I modified my Mom's old Radio Shack volt meter with a scale for each type because a fully charged NiCD would read lower than a dead Alkaline. These are approximate open scale readings (IRRC):
1.33/1.38 V - NiCD/NiMH
1.60 V - Carbon Zinc (old standard)
1.70 V - Alkaline

NiCD and expecially NiMH have much higher peak current capabilities than Carbon Zinc.

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in tha 510.

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Filliam H. Muffman

NiCD and expecially NiMH have much higher peak current capabilities than Carbon Zinc.

in fact I think NiCD has higher peak current than NiMH, which why you still see them these days in Power Tools. Otherwise NiCd would be dead.

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tenders

Quote
space-time

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H1N1

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Catzilla
Multimeters differ from "battery testers" in that they don't load the battery. When the weak batteries are in the flashlight the voltage drops to close to zero.
That's the reason to check a vehicle batter when the engine is cranking, weak batteries voltage drops significantly under load.

absolutely correct.
it is the current that does the work, and that can only be measured under load.
I=ExR
I = current
E = voltage
R = resistance (load)

you know you got the math wrong, don't you? replace "x" with "/"

Or swap E and I. Same difference.

early in the morning.

E
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IxR

Cover what you want, then do the math.