sudden death batteries

[dpopiz]

obviously all portable devices would be a lot better if their batteries could provide full voltage for a certain amount of time until they used up ALL of their charge (assuming there would still be a way to measure the amount of charge left).

as opposed to what really happens which is that the voltage gradually decreases as the charge is used up until the voltage is just not enough to runthe device reliably and it shuts it self off, yet there's still a lot of charge left in the battery.


has there been any research done on this sort of thing though? if so, what if anything has been figured out?
could there be a way to do this - to make a battery that keeps providing the same potential until all of its reactants are reacted?

 

[FrankSchwab]

I'd suggest taking a look at the discharge characteristics of NiCd or NiMH cells before spending too many R&D dollars.

www.energizer.com would probably be a good place to start.

/frank

 

[Matthias99]

Originally posted by: dpopiz
obviously all portable devices would be a lot better if their batteries could provide full voltage for a certain amount of time until they used up ALL of their charge (assuming there would still be a way to measure the amount of charge left).

as opposed to what really happens which is that the voltage gradually decreases as the charge is used up until the voltage is just not enough to runthe device reliably and it shuts it self off, yet there's still a lot of charge left in the battery.


has there been any research done on this sort of thing though? if so, what if anything has been figured out?
could there be a way to do this - to make a battery that keeps providing the same potential until all of its reactants are reacted?

A little more specific than the last post: basically, with the kind of sealed chemical batteries we use today, no. Decreasing voltage over time is a byproduct of the physical laws governing the electrochemical reactions going on in the cells.

However, something like a really small generator or fuel cell might be able to do this -- they consume fuel at a set rate, and produce output power at a set rate as long as the fuel supply holds up. However, these are not closed systems like battery cells. Generators need fuel and (usually) oxygen, plus an initial power input to get them going, and they generally produce exhaust of some sort. Fuel cells generally require fuel of some sort, and produce some sort of byproduct (one common type consumes hydrogen and oxygen, and produces water).

 

[Mark R]

Actually with modern power regulation electronics, it is getting increasingly common for devices to use virtually every last bit of energy in their batteries, when the voltage starts to fade, a step-up circuit boosts it to the level required for reliable operation. When the battery is completely exhausted, the step-up circuit is unable to coax any more power from the battery and the circuit shuts down. This is not actually the main aim of the step-up circuit, just a nice side-effect: the main aim is to allow a 3.3 or 5V circuit to run off a single 1.5V battery - some can keep going until the battery voltage drops down to 0.6V or less.

This works fine for non-rechargeable batteries, but is not ideal for rechargeable batteries. Deep discharging is in general bad for rechargeables - they tend to last much longer if only partially drained before being topped up. In fact, lithium ion batteries have to have a fail-safe protection circuit built in, because if they are ever discharged too far they will be permanently damaged.

 

[FrankSchwab]

Actually, I was specifically referring to the discharge curves that you might find there for
NiMH cells
or
NiCD cells
or
Primary Lithium Cells

which show a very stable voltage through over 90% of their stored energy, before dropping swiftly to zero as the stored energy is exhausted.

This appears to be exactly the behavior the original question said doesn't happen. He must have been thinking only of
Alkaline cells
which show a more-or-less linear relationship between voltage and energy storage.

/frank