# 9v battery min voltage

Discussion in 'Electronic Basics' started by lerameur, Dec 31, 2006.

1. ### lerameurGuest

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

2. ### Peter BennettGuest

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.

--
Peter Bennett, VE7CEI
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3. ### jasenGuest

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.

4. ### chuckGuest

A bit of a challenge here: if the battery can power the led, then

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

5. ### lerameurGuest

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

ken

6. ### chuckGuest

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

7. ### lerameurGuest

yes thats good, thanks

8. ### ehsjrGuest

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

9. ### chuckGuest

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

10. ### ehsjrGuest

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/nickelmetalhydride_appman.pdf

Ed

11. ### chuckGuest

Thanks for the interesting reference, Ed.

I see that you were referring to a potential situation involving
series-connected cells in which one or more cells caused a voltage
reversal on one or more of the other cells. This could cause damage to
the reversed voltage cell.

Good point, and thanks for the correction.

I still would argue, of course, that such a consideration is one of many
that would enter into a full-blown project design that involves numerous
tradeoffs. While the "cure" is to ensure that the total voltage remains
above 0.9 volts/cell x number of cells, there is a real cost to doing
so. And what is the expected benefit from this precaution?

Unfortunately, that information is not given in the reference. "May"
cause damage is not sufficient information to evaluate the
cost-effectiveness of a fix. "May" could well range from a probability
of 0.01 to 0.99. Even if the probability is near 1.0, the extent or
nature of the "damage" is unstated. Does the damage mean catastrophic
component failure? Does it mean a 10% reduction in capacity (truly a
form of damage) or a 90% reduction?

Charge and discharge rates are further examples of tradeoffs involving
possible damage and costs.

I would repeat that the purpose of this project is surely not the
preservation of the batteries. It is to accomplish some other purpose.
Whether the inconvenience of more frequent battery charges would offset
the benefit of reduced (but unknown) risk of battery damage (of unknown
consequence) is a design decision. Good engineering practice REQUIRES
that such tradeoffs be considered, but it does not require that
batteries be made free of risk of damage, although that is always an
option.

Having said all of that with much huffing and puffing, I must admit that
a possibly more conservative "battery low" criterion is probably a
pretty costless step and a good suggestion, Ed.

Chuck