a volt is a volt is a volt

[1eyedjack]

Not sure where to ask this, so what the hey I'll ask it here.

 

I have a garden watering system that includes an electronic timer powered by battery. The instructions in the manual require that (1) you do NOT use rechargeable batteries and (2) that they MUST be alkaline. The instructions are highly specific, warning that the widget may not otherwise function correctly.

 

Now, I am a simple soul. Just as a pound, dollar or whatever loses its source identity once it hits my wallet, I do not expect an item of electronic equipment to be overly "concerned" whether an ampere here or there comes from an alkaline battery, NI-Mh, lithium-ion, polymer or what the heck. If the potential diffence between plus and minus fits the spec, then great. Same goes for whether the battery has been run down, charged up, run down again however many times.

 

For sure, I can well believe that one battery may maintain its charge longer than another, or that its long term life may be limited by recharge frequency etc. But that is not the point being expressed in the manual. I am enjoined to use alkaline, non-rechargeable batteries, and abjured from using else on pain of .... pain.

 

What's that all about?

[Al_U_Card]

Often it has to do with the battery's depletion curve and time-keeping. Alkaline batteries have the longest "constant" voltage supply over lifetime and they then fade quickly at the end of their charge holding capacity.

 

Like 1.57 V for the first 90%

1.57 to 1.29 for the next 9%

Less than 1.25 in the last 1%

 

Thus, they "keep" time well by not slowing down the timer with insufficient voltage for the clock to run accurately.

[jtfanclub]

It may be that the connectors they use to the battery tend to corrode when in contact with something acidic. I've seen that. Whether it's to make the connections work while wet or because they're too cheap to use copper, I dunno.

 

Or, they could just be bastards.

[hotShot]

While you use a battery the Voltage goes down a little.

 

Rechargeable batteries are a little lower in the voltage then specified.

[1eyedjack]

Rechargeable batteries are a little lower in the voltage then specified.

Yep I have come across this before. Typically a new, fully charged AA rechargeable battery delivers (I am told) 1.2 Volts despite a 1.5 Volt spec.

 

I cannot believe that it is beyond the wit of the manufacture to construct an AA rechargeable battery which, when fully charged, delivers 1.5 Volts if that is what it says on the tin.

 

So, what's that all about? Or is it simply that case that each time you recharge, the maximum achievable potential difference drops compared with the last recharge?

[1eyedjack]

Often it has to do with the battery's depletion curve and time-keeping.  Alkaline batteries have the longest "constant" voltage supply over lifetime and they then fade quickly at the end of their charge holding capacity.

This explanation has a ring of credibility, for me. But it suggests a poor design. It was my undersanding that electronic clocks in this day and age ran along the following principles: provided that the supplied potential difference falls somewhere between two limits, then the timer keeps good constant time. Once the PD drops below the lower limit, the timer does not just slow down like a clockwork device .. it simply ceases to function, period. The mechanism that keeps the time is powered by some vibrating crystal or whatever whose frequency is constant provided that there is enough power supplied to make it vibrate.

 

This is all a bit beyond my ken, but you can buy a wristwatch in Soho market for two squid that will keep nigh on perfect time for 3 years, and then, pfut.

[NickRW]

Rechargeable batteries are a little lower in the voltage then specified.

Yep I have come across this before. Typically a new, fully charged AA rechargeable battery delivers (I am told) 1.2 Volts despite a 1.5 Volt spec.

 

I cannot believe that it is beyond the wit of the manufacture to construct an AA rechargeable battery which, when fully charged, delivers 1.5 Volts if that is what it says on the tin.

 

So, what's that all about? Or is it simply that case that each time you recharge, the maximum achievable potential difference drops compared with the last recharge?

Different types of batteries produce different voltages. But "1.5" is a sort of standard - similar to people saying they play SAYC and then have haven't heard of Jacoby 2nt.

[1eyedjack]

Different types of batteries produce different voltages.  But "1.5" is a sort of standard - similar to people saying they play SAYC and then have haven't heard of Jacoby 2nt.

I am unconvinced by this analogy, but am open to persuasion. If two batteries of different type which purport to conform to "AA" standard produce different voltages then I can think of only two explanations that might account for the difference:

1) The respective types are capable of producing a battery to an agreed, common, voltage, but manufacturers choose not to do so, preferring instead to attempt to dominate the market with their own standard, or

2) One of the types is incapable of producing a battery to the voltage of the other.

 

Now I fully agree and accept that the process of settling on a common standard is a tortuous one, and possibly a process of evolutionary stages. Had we been in the early days of AA batteries I might have been persuaded that this is the explanation for the apparent discrepancy. I cannot see that it is in the financial interests of any manufacturer to encourage divergent standards depending on battery type; quite the reverse, in fact, and markets tend to follow their financial interests.

 

I am no technician, but instinctively I find it hard to swallow that there are physical or engineering constraints on the manufacture of a rechargeable battery to be contained in an AA casing which render it incapable of generating 1.5 volts or that there are such constraints on a non-rechargeable battery that render it incapable of generating 1.2 volts.

 

So, if there are no physical constraints that force this separation, and there are financial incentives to adopt a common standard, and there are no conflicting financial incentives to adopt divergent standards, and sufficient time has elapsed for that common standard to evolve and achieve market dominance, there has to be a third explanation for the discrepancy that I have yet to imagine.

[helene_t]

I suspect Al's explanation is most to the point but here is another issue:

 

The current through an Ohm'ic resistor (the device) with resistance R[external] connected to a battery with internal resistance R[internal] and electromotoric force E is

 

I=E/(R[internal]+R[external])

 

In other words, if the internal resistance of the battery is considerable the current will not simply be inversely proportional to the external resistance, as the effective voltage (I believe "polar voltage" is the right word) U which satisfies

I=U/R[external]

is not a constant as it depends on the external resistance. If I recall correctly, standardization institutes define a 1.5 V battery as a battery providing 1.5 V at a particular external resistance. It is possible that different companies and/or different standardization institutes use different definitions.

 

Since R[external] is probably very large in this case this is unlikely to be an issue for this particular device. Maybe, though, it could be an issue that if R[external] fluctuates quickly, non-ohmic resistance could play a role. I can imagine some batteries have internal capacitance or inductance which would make the equation for the current more complex in that case.

 

All this said, the most likely explanation is that they are just covering their arses and that any battery with approximately the right voltage would work.

[Wackojack]

1. 1.2 Volts is a physical constant with a rechargeable. You cannot change it.

 

2. Unlike alkaline they self discharge. Thus over time even with equal ampere-hour capacity, they will stop working sooner.

 

3. Contrary to Helene's theory. Recharables have less internal resistance than non. This makes them more liable to explode on short circuit.

 

4. Timer accuracy does not depend on voltage. They run within their accuracy when the voltage is Ok or they stop when it is too low.

 

5. Cheapo design. Just add a voltage regulator chip and you could run with all types of battery.

[vuroth]

I cannot believe that it is beyond the wit of the manufacture to construct an AA rechargeable battery which, when fully charged, delivers 1.5 Volts if that is what it says on the tin.

I strongly suspect this is a basic chemistry problem. Use 2 different metals in a voltaic pile, you'll get a battery at a set voltage. There's really nothing you can do to change the voltage of the pile.

 

V

[Trinidad]

I cannot believe that it is beyond the wit of the manufacture to construct an AA rechargeable battery which, when fully charged, delivers 1.5 Volts if that is what it says on the tin.

I strongly suspect this is a basic chemistry problem. Use 2 different metals in a voltaic pile, you'll get a battery at a set voltage. There's really nothing you can do to change the voltage of the pile.

 

V

Just my two cents:

There is very little "basic" in chemistry problems. The actual output voltage of the battery doesn't only depend on the metals used, but also on the concentrations of the reactants in the electrolyte and in the solid state (which is why the delivered voltage drops over time). In its simplest form this concentration dependence is given by the so called Nernst equation. (See http://en.wikipedia.org/wiki/Nernst_equation) I guess that most bridge players consider this equation already pretty difficult.

 

Now, the Nernst equation describes what voltages can be reached in a state of equilibrium. This means that the Nernst equation is only meaningful when the power that is delivered by the battery is pretty small. When a battery needs to deliver a large amount of energy in a short time (e.g. in flash photography) the reaction kinetics come into play. For some battery types the electrochemical reaction is slow. As a result, it may take quite some time before they will be able to deliver any power again, while for other types this will be relatively easy.

 

Another important property for a battery is the amount of energy it can store or (for rechargable baterries) how many recharging cycles they will survive.

 

Obviously there are trade offs between these properties. One battery may deliver a higher voltage, but can store less energy while another battery may be fast but cannot be recharged.

 

So in short: A Volt (capital 'V') is a Volt, but depending on the type of battery or the application it will remain a Volt for a different time.

 

I agree, BTW, that for a simple, low power application like a timer, any special requirement for batteries is due to poor electronics design or excessive disclaiming.

 

Rik

[helene_t]

Now, the Nernst equation describes what voltages can be reached in a state of equilibrium.

Yes, that's what's called electromotoric force.

 

When a battery needs to deliver a large amount of energy in a short time (e.g. in flash photography) the reaction kinetics come into play.

Doesn't flash photography rely on a condensator as an intermediate electricity storage?

[Wackojack]

Quote"Doesn't flash photography rely on a condensator as an intermediate electricity storage?"

 

Yes. Its actually called a capacitor and it is discharged through a xenon tube to create the flash.

[Trinidad]

Now, the Nernst equation describes what voltages can be reached in a state of equilibrium.

Yes, that's what's called electromotoric force.

Well, that's what used to be called electromotoric force. That term is rather obsolete. You just gave every chemist in the forum a key clue about your age :).

When a battery needs to deliver a large amount of energy in a short time (e.g. in flash photography) the reaction kinetics come into play.

Doesn't flash photography rely on a condensator as an intermediate electricity storage?

Yes, they do use condensers (capacitors) in flash photography. But when you want to take two or more pictures at short intervals (or one picture really quickly) it doesn't really matter. You will have to wait for the capacitor to recharge before you can take the next picture. The time needed to recharge the capacitor is determined by the reaction kinetics inside the battery (seconds) rather than the RC time of the capacitor circuit (I guess about 0.1 s). The RC time of the capacitor circuit is short to make it possible to deliver all the energy quickly (literally 'in a flash').

 

Given that the time between pictures is determined by the reaction (and diffusion and a lot of other complicated) kinetics inside the battery, with a better suited battery in your camera you can take more pictures per minute. And in this case, 'better suited' doesn't equal 'more Volts'.

 

(I thought my earlier post was already complicated enough without the capacitor, so for simplicity's sake I left that part out.)

 

Rik