9v battery min voltage

HI

I am building a unit that will have a small 9v battery for power. I
will put a voltage comparator and it will activate a led to warn for a
low voltage battery. Iwould like to know at what voltage dos the
battery need to be change, when is it dead ? is it 7.5 v ? or 8?....

thanks
ken

On 30 Dec 2006 16:52:40 -0800, "lerameur" <(E-Mail Removed)> wrote:

>HI
>
>I am building a unit that will have a small 9v battery for power. I
>will put a voltage comparator and it will activate a led to warn for a
>low voltage battery. Iwould like to know at what voltage dos the
>battery need to be change, when is it dead ? is it 7.5 v ? or 8?....
>
>thanks
>ken

That's up to you to decide - how low a voltage can _your_ device
tolerate?

I think somewhere around 0.9 volts per cell is considered pretty dead
for disposable batteries - that would be 5.4 volts for a 9 volt
battery.



--
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On 2006-12-31, lerameur <(E-Mail Removed)> wrote:
> HI
>
> I am building a unit that will have a small 9v battery for power. I
> will put a voltage comparator and it will activate a led to warn for a
> low voltage battery. Iwould like to know at what voltage dos the
> battery need to be change, when is it dead ? is it 7.5 v ? or 8?....

if it's non-rechargable when it's too weak to use.
if rechargable when the voltage drops 1.2V below the full charge
voltage.

--

Bye.
Jasen

jasen wrote:
> On 2006-12-31, lerameur <(E-Mail Removed)> wrote:
>> HI
>>
>> I am building a unit that will have a small 9v battery for power. I
>> will put a voltage comparator and it will activate a led to warn for a
>> low voltage battery. Iwould like to know at what voltage dos the
>> battery need to be change, when is it dead ? is it 7.5 v ? or 8?....
>
> if it's non-rechargable when it's too weak to use.
> if rechargable when the voltage drops 1.2V below the full charge
> voltage.
>

A bit of a challenge here: if the battery can power the led, then
perhaps it isn't really dead.

An alternative approach is to use the led to indicate a "low" battery,
rather than a dead one. It is much better (usually) to know that
changing the battery is now prudent, than to know that the battery
should have been changed earlier because it is now dead. ;-)

If you take that route, 7.5 or 8 volts might be just enough to operate
your device and indicate time for a battery change. If I recall, battery
voltage drops fairly quickly once the battery begins its decline in earnest.

Chuck

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ok good, and what would be the voltage for a 1.2v NmHI batteries ?
0.9 v ?

ken


chuck wrote:
> jasen wrote:
> > On 2006-12-31, lerameur <(E-Mail Removed)> wrote:
> >> HI
> >>
> >> I am building a unit that will have a small 9v battery for power. I
> >> will put a voltage comparator and it will activate a led to warn for a
> >> low voltage battery. Iwould like to know at what voltage dos the
> >> battery need to be change, when is it dead ? is it 7.5 v ? or 8?....
> >
> > if it's non-rechargable when it's too weak to use.
> > if rechargable when the voltage drops 1.2V below the full charge
> > voltage.
> >
>
> A bit of a challenge here: if the battery can power the led, then
> perhaps it isn't really dead.
>
> An alternative approach is to use the led to indicate a "low" battery,
> rather than a dead one. It is much better (usually) to know that
> changing the battery is now prudent, than to know that the battery
> should have been changed earlier because it is now dead. ;-)
>
> If you take that route, 7.5 or 8 volts might be just enough to operate
> your device and indicate time for a battery change. If I recall, battery
> voltage drops fairly quickly once the battery begins its decline in earnest.
>
> Chuck
>
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Well, you will have to lower the supply voltage to your project by steps
to determine the approximate voltage at which it fails to operate
reliably (or at all) and then set your warning light (comparator) to a
voltage somewhat higher than that.

Chuck

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chuck wrote:
> Well, you will have to lower the supply voltage to your project by steps
> to determine the approximate voltage at which it fails to operate
> reliably (or at all) and then set your warning light (comparator) to a
> voltage somewhat higher than that.
>
> Chuck

yes thats good, thanks

lerameur wrote:
> chuck wrote:
>
>>Well, you will have to lower the supply voltage to your project by steps
>>to determine the approximate voltage at which it fails to operate
>>reliably (or at all) and then set your warning light (comparator) to a
>>voltage somewhat higher than that.
>>
>>Chuck
>
>
> yes thats good, thanks
>

No - not good. The project may keep working, but
discharging the cells too far risks damaging them.
When the cells discharge to 1.0 volts per cell,
it is time to recharge, regardless of whether your
project is working.

Watch out for "9 volt" rechargeable batteries.
They are really 7.2 volts. There may be
exceptions to that - I don't know.

Ed

ehsjr wrote:
> lerameur wrote:
>> chuck wrote:
>>
>>> Well, you will have to lower the supply voltage to your project by steps
>>> to determine the approximate voltage at which it fails to operate
>>> reliably (or at all) and then set your warning light (comparator) to a
>>> voltage somewhat higher than that.
>>>
>>> Chuck
>>
>>
>> yes thats good, thanks
>>
>
> No - not good. The project may keep working, but
> discharging the cells too far risks damaging them.
> When the cells discharge to 1.0 volts per cell,
> it is time to recharge, regardless of whether your
> project is working.
>
> Watch out for "9 volt" rechargeable batteries.
> They are really 7.2 volts. There may be
> exceptions to that - I don't know.
>
> Ed

Ed has pointed out yet another design parameter. The problem is not that
the batteries would be damaged: every charge/discharge cycle decreases
battery capacity somewhat. Discharging below 1.0 volts causes greater
decreases.

One's purpose in life, however, is usually not the preservation of
battery capacity.

As the designer, you get to choose whether you want to keep the
batteries for as long as possible, at the price of frequent recharges;
or you want to recharge less frequently because it's a pain, with the
knowledge that your batteries might not last for as many recharges? as
many years? etc., etc.

Electronic design is full of tradeoffs. Haven't even mentioned
alternative battery chemistries, charging techniques, voltage
regulation, etc., all of which will affect battery life; and the
appropriate cost-benefit analysis. ;-)

Chuck

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chuck wrote:
> ehsjr wrote:
>
>> lerameur wrote:
>>
>>> chuck wrote:
>>>
>>>> Well, you will have to lower the supply voltage to your project by
>>>> steps
>>>> to determine the approximate voltage at which it fails to operate
>>>> reliably (or at all) and then set your warning light (comparator) to a
>>>> voltage somewhat higher than that.
>>>>
>>>> Chuck
>>>
>>>
>>>
>>> yes thats good, thanks
>>>
>>
>> No - not good. The project may keep working, but
>> discharging the cells too far risks damaging them.
>> When the cells discharge to 1.0 volts per cell,
>> it is time to recharge, regardless of whether your
>> project is working.
>>
>> Watch out for "9 volt" rechargeable batteries.
>> They are really 7.2 volts. There may be
>> exceptions to that - I don't know.
>>
>> Ed
>
>
> Ed has pointed out yet another design parameter. The problem is not that
> the batteries would be damaged:

The problem most certainly *is* that cells may be damaged.

See "Discharge Termination" in the Energizer
Battery Application Manual, pages 14 - 16.
http://data.energizer.com/PDFs/nicke...ide_appman.pdf

Ed



> every charge/discharge cycle decreases
> battery capacity somewhat.

> Discharging below 1.0 volts causes greater
> decreases.
>
> One's purpose in life, however, is usually not the preservation of
> battery capacity.
>
> As the designer, you get to choose whether you want to keep the
> batteries for as long as possible, at the price of frequent recharges;
> or you want to recharge less frequently because it's a pain, with the
> knowledge that your batteries might not last for as many recharges? as
> many years? etc., etc.
>
> Electronic design is full of tradeoffs. Haven't even mentioned
> alternative battery chemistries, charging techniques, voltage
> regulation, etc., all of which will affect battery life; and the
> appropriate cost-benefit analysis. ;-)
>
> Chuck
>
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