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  #1   Report Post  
Timothy Murphy
 
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Default Some questions on rechargeable batteries

(1) Does it really harm batteries to over-charge them?

(2) If so, where can one get a charger
that switches off when the battery is charged?

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?

(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?

--
Timothy Murphy
e-mail (80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland
  #2   Report Post  
Gordon Henderson
 
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Default

In article ,
Timothy Murphy wrote:
(1) Does it really harm batteries to over-charge them?


What type of batteries? NiMH, NiCd, Sealed Lead Acid, Vented Lead Acid?

The answer is generaly yes, though.

(2) If so, where can one get a charger
that switches off when the battery is charged?


You need the right type of chrger for the battery type you are
charging. Usually you get them from the same place you getthe batteries,
or try Maplin, Rs, of TLC...

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


You can't really tell with just a multimeter. A battery right off the
charger might well read above full voltage, but drain a little bit of
current from it and it might appear flat again. (Although lead acid
types are more tolerant of this test)

(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?


The critical voltage is the voltage at which your device stops
operating.

Different types of battery will behave in different ways - Old Zinc cells
and Lead Acid types will degrade gently through their life, alkaline have
a flatter discharge curve and Lithium even flatter, going from almost full
voltage to next to nothing in a matter of minutes (or less!) depending
on the application.

Sorry if this answer doesn't help much.

Battery technology is forever changing though, so who knows what will
come next year...

Gordon
  #3   Report Post  
Alex
 
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Default

"Gordon Henderson" wrote in message
...
In article ,
Timothy Murphy wrote:
(1) Does it really harm batteries to over-charge them?


What type of batteries? NiMH, NiCd, Sealed Lead Acid, Vented Lead Acid?

The answer is generaly yes, though.

(2) If so, where can one get a charger
that switches off when the battery is charged?


You need the right type of chrger for the battery type you are
charging. Usually you get them from the same place you getthe batteries,
or try Maplin, Rs, of TLC...

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


You can't really tell with just a multimeter. A battery right off the
charger might well read above full voltage, but drain a little bit of
current from it and it might appear flat again. (Although lead acid
types are more tolerant of this test)

(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?


The critical voltage is the voltage at which your device stops
operating.

Different types of battery will behave in different ways - Old Zinc cells
and Lead Acid types will degrade gently through their life, alkaline have
a flatter discharge curve and Lithium even flatter, going from almost full
voltage to next to nothing in a matter of minutes (or less!) depending
on the application.

Sorry if this answer doesn't help much.

Battery technology is forever changing though, so who knows what will
come next year...

Gordon


Here's one for you - why do batteries on quality tools (nicad and nimh) seem
to go from almost full power to nothing very quickly, whereas on cheaper
power tools they gradually get slower and slower as their charge runs out?

Alex


  #4   Report Post  
Gordon Henderson
 
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Default

In article ,
Alex wrote:

Here's one for you - why do batteries on quality tools (nicad and nimh) seem
to go from almost full power to nothing very quickly, whereas on cheaper
power tools they gradually get slower and slower as their charge runs out?


I'd guess that the cheaper ones might use NiCd and the expensive ones
use NiMH these days. NiCd have more of a discharge curge than NiMH. And
I'm sure that within the battery types there are expansive types and
cheaper ones which don't perform so well.

They might also have better internal circuitry - eg. a 12V battery,
but only run the motor at 9V with a power regulator inbetween that will
cope with a volt or 2 drop then shut off before draining the battery
completely. Hard to tell though without knowing exactly whats inside.

In the past few years I've seen NiMH AA capacity go from barely 1AH
capacity to over 2, so as I said, who knows! It wouldn't surprise me if
NiCD development had all but stopped though.

Gordon
  #5   Report Post  
 
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Timothy Murphy wrote:
(1) Does it really harm batteries to over-charge them?

It depends on the battery type and the *rate* at which you overcharge
them but all rechargeables will suffer eventually. NiCd will tolerate
a low rate of overcharge without too much damage (most will anyway, it
does depend on the design). A 'low rate' of overcharge means something
less than the 10 hour rate, preferably something like the 20 hour rate.
Many 'overnight' NiCd chargers run at this sort of rate and won't do
*much* damage but it's still better not to overcharge.


(2) If so, where can one get a charger
that switches off when the battery is charged?

Lots of places, but they need to be designed specifically for the
battery type. It's quite difficult to tell whether the 'automatic'
chargers you see advertised are really intelligent ones or whether
they are just timed, it's a bit of a minefield.


(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?

Not easy from the voltage.


(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,


Yes, probably, one of my multimeters even has markings for 'good',
'marginal' and 'bad' 1.5 volt and 9 volt batteries. However it's not
perfect and, as others have said, it does depend very much on the
application. You need to apply a load to a bettery when measuring
the voltage to see if it's any good too.

and if so, what is the critical voltage for 1.5v batteries?

Probably somewhere in the 1.0 to 1.2 volts area, but sobject to so
many caveats that doesn't really help you very much.

--
Chris Green


  #6   Report Post  
Dave Plowman (News)
 
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Default

In article ,
Alex wrote:
Here's one for you - why do batteries on quality tools (nicad and nimh)
seem to go from almost full power to nothing very quickly, whereas on
cheaper power tools they gradually get slower and slower as their
charge runs out?


The cheaper tool might well have one or more faulty cells.

--
*Okay, who stopped the payment on my reality check? *

Dave Plowman London SW
To e-mail, change noise into sound.
  #7   Report Post  
Timothy Murphy
 
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Default

Gordon Henderson wrote:

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


You can't really tell with just a multimeter. A battery right off the
charger might well read above full voltage, but drain a little bit of
current from it and it might appear flat again. (Although lead acid
types are more tolerant of this test)


Sorry to be persistent, but if you can't tell with a multimeter
how do the intelligent chargers know when the battery is charged?

--
Timothy Murphy
e-mail (80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland
  #8   Report Post  
Alistair Riddell
 
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Default

On Tue, 31 Aug 2004, Timothy Murphy wrote:

Sorry to be persistent, but if you can't tell with a multimeter
how do the intelligent chargers know when the battery is charged?


varies with battery type but the main technique used is measuring
the current flowing through the battery with temperature sensing often
used as a backup.

--
Alistair Riddell - BOFH
IT Manager, George Watson's College, Edinburgh
Tel: +44 131 446 6070 Fax: +44 131 446 6090
Microsoft - because god hates us
  #9   Report Post  
Dave Liquorice
 
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Default

On Tue, 31 Aug 2004 18:47:31 +0100, Alistair Riddell wrote:

varies with battery type but the main technique used is measuring
the current flowing through the battery with temperature sensing
often used as a backup.


NiCd and NiMH chargers are constant current, indeed the only battery
technology that I can think of that uses constant voltage charging is
Lead Acid...

Detection of the rate of change of the terminal voltage is normally
used. Once the rate starts to fall the battery/cell is reaching fully
charged, the temperature will also start to rise more rapidly as the
supplied energy is no longer stored but dumped (for want of a better
expression).

--
Cheers
Dave. pam is missing e-mail



  #10   Report Post  
The Natural Philosopher
 
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Default

Timothy Murphy wrote:

(1) Does it really harm batteries to over-charge them?


Defnitely it does some.

Lithoum polymer and lead acid can both explode...

(2) If so, where can one get a charger
that switches off when the battery is charged?


What battery?

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?


Yes fopr lead acid and lithium polymer, yes, but its not that simple
with NiMh and NiCd.

If so, at what voltage would one say the battery needed charging?


That depebnds entirely on teh chmistry

(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?


About 0.1v. After that they won't even power the meter needle.




  #11   Report Post  
The Natural Philosopher
 
Posts: n/a
Default

Alex wrote:

"Gordon Henderson" wrote in message
...

In article ,
Timothy Murphy wrote:

(1) Does it really harm batteries to over-charge them?


What type of batteries? NiMH, NiCd, Sealed Lead Acid, Vented Lead Acid?

The answer is generaly yes, though.


(2) If so, where can one get a charger
that switches off when the battery is charged?


You need the right type of chrger for the battery type you are
charging. Usually you get them from the same place you getthe batteries,
or try Maplin, Rs, of TLC...


(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


You can't really tell with just a multimeter. A battery right off the
charger might well read above full voltage, but drain a little bit of
current from it and it might appear flat again. (Although lead acid
types are more tolerant of this test)


(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?


The critical voltage is the voltage at which your device stops
operating.

Different types of battery will behave in different ways - Old Zinc cells
and Lead Acid types will degrade gently through their life, alkaline have
a flatter discharge curve and Lithium even flatter, going from almost full
voltage to next to nothing in a matter of minutes (or less!) depending
on the application.

Sorry if this answer doesn't help much.

Battery technology is forever changing though, so who knows what will
come next year...

Gordon



Here's one for you - why do batteries on quality tools (nicad and nimh) seem
to go from almost full power to nothing very quickly, whereas on cheaper
power tools they gradually get slower and slower as their charge runs out?


NiMh. It runs out like a dog. Also cheaper batteries maintain voltage
less well under discharge.


Alex



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The Natural Philosopher
 
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Dave Liquorice wrote:

On Tue, 31 Aug 2004 18:47:31 +0100, Alistair Riddell wrote:


varies with battery type but the main technique used is measuring
the current flowing through the battery with temperature sensing
often used as a backup.



NiCd and NiMH chargers are constant current, indeed the only battery
technology that I can think of that uses constant voltage charging is
Lead Acid...


Umm. Lithium uses conatnt curent/ voltage limited, the same as lead acid...

Detection of the rate of change of the terminal voltage is normally
used. Once the rate starts to fall the battery/cell is reaching fully
charged, the temperature will also start to rise more rapidly as the
supplied energy is no longer stored but dumped (for want of a better
expression).


Nickel chemistry uses peak detection. At some point in th1e charge the
voltage starts to FALL - that's time to stop charging or at least switch
to trickle charge.



  #13   Report Post  
 
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Timothy Murphy wrote:
Gordon Henderson wrote:

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


You can't really tell with just a multimeter. A battery right off the
charger might well read above full voltage, but drain a little bit of
current from it and it might appear flat again. (Although lead acid
types are more tolerant of this test)


Sorry to be persistent, but if you can't tell with a multimeter
how do the intelligent chargers know when the battery is charged?

When a NiCd or NiMh battery reaches full charge there's a little blip
in the voltage, it drops slightly by a few tens of millivolts. The
intelligent chargers detect this and turn off. (Old fast chargers
used to have temperature sensors which stopped charging when the
battery got hot, really good chargers use this method as a sort of
backstop)

--
Chris Green
  #14   Report Post  
Dave Liquorice
 
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On Wed, 01 Sep 2004 00:59:25 +0100, The Natural Philosopher wrote:

Umm. Lithium uses conatnt curent/ voltage limited, the same as lead
acid...


Lithium I don't know. Lead acid is constant voltage, with the voltage
effectively setting the "fast", "standy" etc, any current limit would
be just a limit from the chargers power supply. The current will fall
as cell voltage increases but the applied voltage is constant. "Smart"
lead acid chargers will automaticlly switch the voltage output in
response to the detected charge state.

--
Cheers
Dave. pam is missing e-mail



  #15   Report Post  
John Rumm
 
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Default

Alex wrote:

Here's one for you - why do batteries on quality tools (nicad and nimh) seem
to go from almost full power to nothing very quickly, whereas on cheaper
power tools they gradually get slower and slower as their charge runs out?


It is down to the quality of the cells, and how well matched they are.

In addition to the better cells having lower internal resistance (hence
able to deliver more current) and more capacity, their discharge
characteristics will be far more consistent between different examples
of the same type of cell.

This is quite significant in a battery pack since ideally you want all
the cells that make up the pack to run out at the same time. If they
don't you get into the situation where the performance drops off in
steps with each cell that goes flat. (there is also the possibility that
you will start to damage the flat cell if you keep drawing current from
the battery in this state. Hence you should try not to wring the last
few drops of power out of it as it goes flat).

The "fast electric" radio control enthusiasts will often go as far as
computer matching cells, so as to make up packs from cells with as near
identical drain characteristics as each other.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


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Dave Plowman (News)
 
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In article ,
John Rumm wrote:
The "fast electric" radio control enthusiasts will often go as far as
computer matching cells, so as to make up packs from cells with as near
identical drain characteristics as each other.


It makes a significant difference to the overall performance of a cheap
cordless drill too. I re-celled a pack on my PP 18 volt, and it made a
huge difference to it even compared to a new standard pack. Trouble is it
cost nearly the price of a new drill complete...

--
*If vegetable oil comes from vegetables, where does baby oil come from? *

Dave Plowman London SW
To e-mail, change noise into sound.
  #17   Report Post  
The Natural Philosopher
 
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Dave Plowman (News) wrote:

In article ,
John Rumm wrote:

The "fast electric" radio control enthusiasts will often go as far as
computer matching cells, so as to make up packs from cells with as near
identical drain characteristics as each other.



It makes a significant difference to the overall performance of a cheap
cordless drill too. I re-celled a pack on my PP 18 volt, and it made a
huge difference to it even compared to a new standard pack. Trouble is it
cost nearly the price of a new drill complete...

Well if you really want to fall over, look at the price of a Lithium
pack that will do a kilowatt for 15 minutes.

At about $2.80 for a watt hour, thats arond $1000.

Mind you, you have the equivalant of a moped engine and totally verical
performance on a fair sized plane

I priced up a 'town car' on rechargeable lithium. Target was about 250
bhp peak, 300 mile range and ability to recarge fully in 10 hours on 13A
supply (domestic) on night rates.

Last year it was $150,000 for the batteries. This year its more like
$50,000.

Trouble is the safety circuits and crash resistance are horrendous, and
the likely lifetime of the battery is about 3 years.

Mind you against 1.20 per 300 mile tank of electricity, it starts to
look better..

A 300 mile tank of petrol costs about 30 these days, so at 10,000 miles
per year, thats 3000, and perhaps another 500 on maintenance that the
electric shouldn't need. So...ways to go yet, but its not so far off.

  #18   Report Post  
N. Thornton
 
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Timothy Murphy wrote in message ...

(2) If so, where can one get a charger
that switches off when the battery is charged?


places that sell battery chargers? Look for a multistage charging one.

(3) How can one tell with a multimeter if a battery is charged?
Can one tell from the voltage?
If so, at what voltage would one say the battery needed charging?


yes, though it does depend. For small household cells and carbon zincs
(which can indeed be recharged), an analogue meter on 10A range, touch
the tips on the battery /very/ momentarily, and speed of needle
movement indicates charge. Fast = full, sluggish = empty.

Also a 1.5v cell delivering 1.2v off load is a bit of a giveaway.

Forlead acids you need more than a mulitmeter.


(4) Similarly, for non-rechargeable batteries,
can one tell from the voltage on a multimeter
if the battery should be thrown out,
and if so, what is the critical voltage for 1.5v batteries?


whatever voltage the appliance drops out of function at. If a carbon
cell is down to 1.4v off load, its not got much life left.


NT
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