9v battery min voltage

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.

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 ?

ken

chuck wrote:

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

See "Discharge Termination" in the Energizer Battery Application Manual, pages 14 - 16.

Ed

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

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