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decreasing voltage of a battery

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samcheetah

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a couple of days ago i noticed that my DMM started giving off wrong readings. it is a DMM in which u have to select the ranges. when i selected the lowest range of the ohm-meter function the LCD display vanished and i guessed that the battery was dead. i bought a new one and the DMM "got back into bussiness".

now for the question. i checked the voltage of the dead battery. it started from 6.50 and then 6.49 then 6.48 and so on. it kept on decreasing. i didnt see where it would end because i saw it go down to 4.77

now i know the DMM is working fine. so the battery is wrong. but why is it behaving like that. what is this behaviour called in electronics terminology
 

Nigel Goodwin

Super Moderator
Most Helpful Member
samcheetah said:
now i know the DMM is working fine. so the battery is wrong. but why is it behaving like that. what is this behaviour called in electronics terminology

It's called a flat battery :lol:

But more seriously, the battery is seriously flat to do that with a DMM (10MOhm) across it. When you remove the load off a flat battery, it will actually recover a little with no load at all on it - if you do this with a TV remote control battery you may be able to get an extra 'press' out of it.

But as soon as you apply any sort of load the battery fades very quickly, which is why your reading kept falling.
 

samcheetah

New Member
thanx

i know that was a dumb question. altho i have played alot with electronics but whenever there was a flat battery i just throwed it. this was the first time i observed this behaviour.

and yes i have noticed the effect with the TV remote control batteries. infact thats what i always do when the batteries are near to run out.
 

stevez

Active Member
Not a dumb question at all - that's how many of us learn.

Some battery testers have a resistor in them to help reveal a battery's true condition - to the extent that you can with a simple tester.

I've found that I can get about 20 more minutes of battery life out of my arm-band radio (in the gym) by putting the batteries under hot water (from the tap, not boiling). The electronics in the radio turn it off when the voltage drops below some point but there is actually some life left in the batteries.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
samcheetah said:
thanx stevez

your observation is interesting. i didnt know hot water can do that. ill try that out too.

Yes, batteries are quite temperature dependent - particularly NICAD's, they really die off in cold conditions - I seem to remember that polar expeditions have to use dry batteries for that reason.
 

stevez

Active Member
Another "discovery" with my armband radio - if the batteries have been in for a while (month) and the radio cuts out I will open the battery compartment, rotate the batteries so as to cause the metal on the battery and the metal on the battery clips to grind together. The radio will be good for another 10 minutes - I presume that is because of a very tiny amount of oxide that might serve to increase contact resistance - that oxide is scraped away by the rotation. It's not much but enough to be worth doing one time.

If the radio (a good Sony) quits and I just leave it off for a minute the battery will recover enough for it to come on another 30 seconds or so. I can repeat that process - most likely a number of times but usually give up in favor of rotating or hot water.

I think it's great when people make "discoveries" by careful observations and measurement -reinforced with good discussion. It's an excellent way to learn. Sharing the experience helps others to learn. Sure, an expert could have told you that but it means so much more this way.

Some time ago I had wondered just how much power was actually left, once a battery got to the point that Sam described. I figured if there was enough left that it might be worth employing some efficient DC/DC convertor to suck every ounce of life out of the battery - all the way down to zero. The modest amount of research I did revealed that once voltage starts to drop off drastically that very little is actually left in terms of amp-hours or milliamp-hours.
 

ChrisP

Member
Nigel Goodwin said:
samcheetah said:
thanx stevez

your observation is interesting. i didnt know hot water can do that. ill try that out too.

Yes, batteries are quite temperature dependent - particularly NICAD's, they really die off in cold conditions - I seem to remember that polar expeditions have to use dry batteries for that reason.

If you think about it, common batteries are producing electricity via a chemical reaction. In many chemical reactions, applied heat will act as a catalyst with the effect of increasing the chemical activity, or speeding up the reaction. It is for this reason that applying some heat will often produce some additional current.

In the same vein, haven't you ever turned the headlights on for a few minutes to warm up the car battery on a cold morning? This will often kick up the activity and thus the output enough to allow starting. :)

BTW -- it's also this condition that has led to the misconception that storing a car battery on the cement floor will kill the battery. In reality, the cement floor acts as a huge heatsink, cooling the battery to the point where the normal chemistry is slowed enough to seriously limit the battery's output.
 

Roff

Well-Known Member
ChrisP wrote:
BTW -- it's also this condition that has led to the misconception that storing a car battery on the cement floor will kill the battery. In reality, the cement floor acts as a huge heatsink, cooling the battery to the point where the normal chemistry is slowed enough to seriously limit the battery's output.
Can't argue with that. I once had the opposite result (many years ago when I was young and foolish - now I'm old and ...). I put a car battery on a concrete floor and connected it to a cheap charger overnight. Killed my concrete floor. :cry:
 
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