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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 3rd Oct 2008, 10:13 am   #1
John12868
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Default 9 Volt PP3 voltage output

I know this may sound a silly question, but what is the voltage output of an average new PP3 battery? I know it isn't going to be "exactly" 9 volts, but would it be higher or lower than 9 volts?

I've measured the output of a couple of "Poundland" Kodak zinc chloride batteries (2 for a quid) and they read something around 9.5 volts.
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Old 3rd Oct 2008, 10:16 am   #2
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Default Re: 9 Volt PP3 voltage output

Quote:
Originally Posted by John12868 View Post
I've measured the output of a couple of "Poundland" Kodak zinc chloride batteries (2 for a quid) and they read something around 9.5 volts.
That sounds about right.

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Old 3rd Oct 2008, 10:27 am   #3
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Default Re: 9 Volt PP3 voltage output

You should also measure the voltage of any battery under normal load conditions. I would expect a new PP3 to read about 9.5v and certainly around 9.3 under normal load. As a battery ages, the resistance becomes higher so you could for instance find the off-load voltage of an old PP3 at near 9v but as soon as normal load is applied, it could drop appreciably lower.


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Old 3rd Oct 2008, 3:09 pm   #4
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Default Re: 9 Volt PP3 voltage output

If you really want to know what a battery is expected to do I suggest you go on something like the Farnell web site, choose the battery you want and download the manufacturers data sheet.

The detail varies from type to type but you will often find performance data and discharge graphs.

Here is an example: http://www.farnell.com/datasheets/50019.pdf

It is interesting to note the end-point voltage being suggested as little as 5V!
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Old 3rd Oct 2008, 4:24 pm   #5
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Default Re: 9 Volt PP3 voltage output

I usually bin batteries when they reach 7.5V on-load. Certainly my bedside radio audio gets very distorted when the battery has reached this level.
WRT new batteries, I've just tested some new ones (Hiwatt brand) that I've got and they have an off-load voltage of 10.1 volts across them.

HTH,
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Old 3rd Oct 2008, 5:49 pm   #6
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Default Re: 9 Volt PP3 voltage output

It is surprising (to me) how much the open-circuit voltage of batteries does vary.

I have yet to see a satisfactory explanation of this perhaps intuitively obvious observation. Chemical reactions are associated with quite precise energy changes and so the theoretical expectation is for the potential of the battery to depend only on temperature and what it is made from, with internal resistance perhaps increasing with age.

Can anyone say what the detailed mechanism is that causes the potential to vary? It seems to happen to most batteries.
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Old 4th Oct 2008, 9:30 am   #7
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Default Re: 9 Volt PP3 voltage output

I can't say for certain, but it's reasonable to expect that the initial internal resistance will vary according to the type of construction, electrode area etc., which will, of course, alter the terminal voltage even when new.
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Old 5th Oct 2008, 7:32 pm   #8
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Default Re: 9 Volt PP3 voltage output

I usually test disposable cells/batteries by connecting a DMM set to the 10A range across them for literally half a second. Comparing the "peak" reading with that obtained with a similar new one is a very good indicator of whether they're as-new, still usable for less demanding applications, or fit only for the bin.

This is a far more meaningful test than using a voltmeter off-load.

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Old 5th Oct 2008, 11:04 pm   #9
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Default Re: 9 Volt PP3 voltage output

I do the same, but using an analogue meter on the 12A range. This works for NiCds and NiMH cells too. A new AA alkaline cell delivers a steady current (for no more than a second or two) of around 5 Amps, zinc-carbon cells around 2A, NiCds and NiMHs about 4A. A used battery or discharged rechargeable cell will produce virtually no meter deflection, or an initial deflection that rapidly falls.

I don't think this is harmful to the individual cells - the internal resistance of the meter will limit the current. Don't try it on a series battery of eight cells, though!

Back to the topic... a PP3, even the cheapest 'three for £1 specials' will give several hundred mA or more.
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Old 6th Oct 2008, 4:39 pm   #10
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Default Re: 9 Volt PP3 voltage output

Nick, Phil, that will work on manu batteries.
most the ones Phil list wil manage ok (but not unharmed) BUT modern lithium cells will be damaged or destroyed with that technique, some lithium polymer cells simply die.
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Old 7th Oct 2008, 6:19 pm   #11
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Default Re: 9 Volt PP3 voltage output

Temperature affects battery output voltage a bit too. This is why smoke alarms with failing batteries usually start bleeping late at night - it's colder so the voltage has dropped a bit and gone below the threshold.
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