wylbur37 said:
For the typical flashlight battery (AAA, AA, C, D),
all of them are 1.5 volts, but how much current do they offer?
(I'm guessing it's around 100 to 150mA, and it varies according
to the type of load that's on it).
And does it differ according to the battery size?
(does a "D" cell offer more current than an "AA"?)
or is it just that the D lasts longer?
Two seperate ratings involved. The 'current' is limited by the internal
resistance of the battery, the resistance of the load, and by the way the
chemistry behaves. A dead short across even a AA battery, can produce
currents over 10A, for a short time, with some battery designs (NiCad in
particular). Yes, the available current does rise with battery size, with
more volume for the chemistry, and areas/thicknesses for the electrodes.
The second rating, is the mAHr rating. This gives a figure for how much
current can be delivered for how long. This figure is normally based upon
the current that will discharge the battery in 10 hours. So a battery with
a 1000mAHr rating, should be able to deliver 0.1A, for 10 hours.
Now a typical AA Zinc-Cabon battery might offer perhaps 1000mAHr, while a
D cell will perhaps offer 4000mAHr. Note that the latter implies a 'test'
current 4 times as high as that used on the smaller battery.
Some battery chemistries function better at high discharge rates than
others. So (for instance), a NiCad D battery, may well support operation
at 50* it's 'ten hour' current, and still give perhaps 25% of the full
capacity, while designs like zinc-air, will only deliver low currents,
even into a dead short.
When multiple 'capacity' versions of the same battery exist, the extra
capacity is often gained at the cost of other features. So (for instance),
on NiMh batteries, where versions with higher capacities exist, these
often show higher internal resistance (so will work less well at high
loads), and increased 'self discharge' rates (they will run down quicker
when not in use).
A typical zinc-carbon D cell, delivering half an amp, will run for perhaps
4 hours. A high quality alkaline manganese dioxide battery into the same
load, will top 20 hours. The internal resistance of the D cell, will
typically be less than 0.2ohms, while the AA of the same type, may well
have an internal resistance over 0.4ohm.
Generall, AA batteries offer some of the best volumetric, and gravimetric
power output (pack the most 'power' into the least weight, and volume).
Best Wishes