DMM input impedance

[trem]

Pardon my ignorance, but i have to ask.
Say i am measuring the voltage of a 1.5 Volt cell (AA battery for example), and the meter says 1.607 volts.
If the input impedance of my meter is 10 M Ohms, does that mean i am putting a 10 Meg Ohm load across the battery when i measure voltage with my meter.?
I guess what i am asking is.....what exactly does the input impedance of the DMM mean.?
Thank You

[Jazz P Bass]

"If the input impedance of my meter is 10 M Ohms, does that mean i am putting a 10 Meg Ohm load across the battery when i measure voltage with my meter.?"

Yes.

[trem]

Thank You.
You have made me one answer smarter.

[Jazz P Bass]

In actual fact, the 10m will drop the voltage from what it was before you measured it.

Here is a good read: Fluke expert solves a DMM mystery

[trem]

10-4 Thanks for the read.
I have not picked up a meter (not that you guys should care) in about 8 months. We have been "re-doing" our house, and moving my amp room to a fairly decent shed in the back yard.
It is almost ready for me to move into, and i am having to relearn some of this stuff.
I look forward to working on guitar amps (and asking questions again) in another month or two.
Thanks Again

[Jazz P Bass]

Oh, to add: it is fairly common for a '1.5volt' battery to read 1.6volts when fresh & new.

[trem]

Right, I figure a battery (of the AA-AAA type) is just about gone when it reads its rated voltage, especially a Alkaline.

[Tom Phillips]

Right, I figure a battery (of the AA-AAA type) is just about gone when it reads its rated voltage, especially a Alkaline.

Actually end of life voltage for an alkaline cell is specified to be as low as 0.9V. However, I have found that some equipment starts to malfunction when the voltage drops below 1.2V per cell.

[trem]

You certainly are correct, it does depend on the equipment.
The light meter in a camera would probably be pretty far off before the voltage got that low.
We still lament the loss of mercury batteries.
Silver Oxide are OK...Alkaline are the bane of the industry.
A television remote might work OK at 1.2.?

[J M Fahey]

Not at 1.2 but probably at 2.4V
I use a tougher (and more realistic) test of battery capability: voltage does not tell us much under a 10M (or even 1M) load, and in fact the important parameter is battery internal resistance, a direct indicator of internal chemistry, so I set multimeter to the 10A scale and momentarily (a couple seconds) short the battery with meter leads (measure current across it).
A fresh battery will give you 2 Amperes or better, a usable one at least 0.5A (which means internal resistance is around 3 ohms) , anything less and junk it, I couldnīt care less about voltage.
This applies to carbon zinc, alkaline and NiCad AA and AAA batteries ONLY, a fresh NiMh one can give you more than that and not be advisable, and a LiPo or similar one (modern cellphone batteries, usually 3.7V ) can explode or burn your meter.
But for classic batteries as used in radios, hand torches, remote controls, cassete recorders, toys, battery powered hair trimmers, etc. itīs THE best test, hands down.

For those who find this test way too extreme, that is exactly what self testing batteries did , remember those where you pressed a wart on the side and a scale showed % charge remaining?
It did the same, shorted the battery with a very low ohm resistor which overheated and a thermal sensitive strip darkened depending on power dissipated.
WAY less accurate than using a meter by the way

[trem]

Wow, i never would have thought that.
I understand you are just doing a short test (in terms of time as well as current) .....that doesn't unwind the battery in a bad way.?
I have no idea how long, with it set to current, you would have to hold a meter on a battery to Drain/Damage it.
I guess a second or two does not hurt anything.?

BTW...on a side note...Thomas Edison (of all people) was quoted as saying, "There is something about rechargeable batteries that makes a liar out of a person".

[Mike Sulzer]

But for Alkaline batteries, the open circuit voltage is a pretty good indication of the charge in the battery. Not so much for most other batteries.

Not at 1.2 but probably at 2.4V
I use a tougher (and more realistic) test of battery capability: voltage does not tell us much under a 10M (or even 1M) load, and in fact the important parameter is battery internal resistance, a direct indicator of internal chemistry, so I set multimeter to the 10A scale and momentarily (a couple seconds) short the battery with meter leads (measure current across it).
A fresh battery will give you 2 Amperes or better, a usable one at least 0.5A (which means internal resistance is around 3 ohms) , anything less and junk it, I couldnīt care less about voltage.
This applies to carbon zinc, alkaline and NiCad AA and AAA batteries ONLY, a fresh NiMh one can give you more than that and not be advisable, and a LiPo or similar one (modern cellphone batteries, usually 3.7V ) can explode or burn your meter.
But for classic batteries as used in radios, hand torches, remote controls, cassete recorders, toys, battery powered hair trimmers, etc. itīs THE best test, hands down.

For those who find this test way too extreme, that is exactly what self testing batteries did , remember those where you pressed a wart on the side and a scale showed % charge remaining?
It did the same, shorted the battery with a very low ohm resistor which overheated and a thermal sensitive strip darkened depending on power dissipated.
WAY less accurate than using a meter by the way

[potatofarmer]

In addition to LiPo, I'd suggest not doing this test with any lithium battery, be it Li-ion, IMR, Li-Mn, whatever. Those have specified maximum discharge currents which it's preferable to avoid exceeding 50% of for long battery life (in terms of charge cycles).

They will happily deliver more current than they are rated for when presented with a dead short, at which point hopefully your meter's fuse will open before you have a very nasty explosion / H2 fire on your hands.

Aah, I didn't read Mr Fahey's post closely enough; he covered this already.