At this point I'll preface my regards by saying that I don't claim to be an
expert on batteries, just someone who became quite interested and looke into
it a bit. It's getting abit off topic for photo as well!
I'm not an expert either (nor did I stay at a Holiday Inn), but I
did work on lead acid battery equalizer circuits in a prototype hybrid
electric car for my M.S. in EE.
Not quite that simple. You get more benefit than the simple math
predicts, since batteries are more efficient at lower [dis]charge rates.
I will admit to making asumptions regarding camera use. If the camera is left
on continuosly with the LCD lit the discharge rate (although the same
whatever batteries are used) may well be sufficiently high that it's rate
relative to the batatery size comes in to play. However remember that you
have the opposite and counterbalancing effect as well in that each battery
has a self discharge rate and the bigger the battery the higher the self
discharge rate and hence the more wastage.
As you say - not so simple.
Sure, in absolute terms the discharge rate is higher for a larger
Ah battery... so the relatively small drain of the camera becomes less
significant compared to the self-discharge. At these rates, however, the
self-discharge current is very small... nominally 2-3% per *month* for
lead-acid IIRC. That's about 2-3 years for a full discharge, or
about C/20e3... :)
This is the accepted theory for lead acid batteries. However experiments have
shown that (for a given amount of discharge) if the discharge rate is fast it
is possible to recharge the battery faster than if discharge is slow
demonstrating that a memory effect is there. Granted that memory effect is
the least of your worries with lead acid technology though.
"Memory effect" is often abused terminology. The original
definition of it was that if a cell was discharged repeatedly to exactly
the same non-zero state-of-charge (SOC), it would eventually be incapable
of discharging further than that level. What you are describing is some
sort of rate-dependent charge/discharge efficiency.
Those pulse chargers have been shown to be little more than snake oil.
Wot's wrong with snake oil? In actual fact I once owned one of these and
apart from being generally very satisfied (and amazed) at the capability of a
tiny little box to maintain a bank of assorted large batteries in good health
I have direct evidence that it does something special in the case of totally
discharged and unusable lead acid batteries. My father had a big 500 AH
leisure battery which had been left discharged and was essentially dead and
various large and expensive chargers failed completely to do anything at all
- presumably because there was no lead exposed at all on the plates.
Connecting it to my little matchbox started it up without any trouble
(although charging it completely on that would have taken weeks so the job
was finished on a conventional charger). This was a device that had specific
functionality for this, if it sensed that no charge was taking it pulsed
several hundred volts to start the charge process.
I've heard of these sorts of "last ditch" efforts to revive cells.
In fact, I've done it myself (think: charge a large cap up to a few dozen
volts and discharge into a NiMH cell). It will sometimes slightly
resurrect a dead battery, but it will still be pretty unhealthy. The
snake-oil debunking I read about was whether or not the pulse charging
increased the longevity of a *healthy* battery. A different situation.
There is a lot of conflicting information about, here's some research by
Motorolla into battery cycles and depth of discharge, quote as follows:
'Depth of discharge (DOD) is defined as the level to which battery voltage is
taken during discharge. For instance, 100 percent DOD means that the battery
voltage has been taken down to the lowest level recommended by suppliers.
Twenty percent DOD means that 20 percent of the battery capacity has been
removed. This level of DOD is often referred to as a shallow discharge.
Discharging to less than the recommended voltage is known as overdischarge.
The shallower the discharge, the more cycles the battery will provide. This
is true for all battery chemistries.The relationship between DOD and cycle
life is logarithmic. In other words, the number of cycles yielded by a
battery goes up exponentially the lower the DOD.'
Note the 'exponential' rather than linear i.e it's not a simple state of you
get the same total energy lifetime for both deep and shallow cycles. From
practical experience rather than theory I would also say that although the
council of perfection for a lead acid battery is to generally to keep it at
80% charge, in practice if you can keep it above 50% and ideally don't leave
it for any extended period below 80% you will get really good battery life.
Logarithmic doesn't inherently mean it that much different than
linear for the region of interest, but I will ignore that pedantic
argument. Suffice to say that discharging deeply will make a battery last
less long... thus my distain for the ignorance-induced demand
"discharge/charge" chargers and procedures followed by many. An
occasional discharge helps Ni-based chemistries, but one certainly doesn't
want to do it every time or it needlessly ages the batteries.
Some lead acid batteries are built especially for deep cycling (usually
called marine batteries) and these have a different internal structure for
the plates giving a lower peak current rating but more resistance to the ill
effects of deep discharge (plates have smaller area but stronger structure to
resist buckling, at least for the ones I looked at) but even these perform
better if you don't go below 50%.
Yep... The cells I've seen (sealed, small, brickish type) are
generally designed for the deep-cycle duty rather than "cold cranking."
Roughly same
net energy. Leave it totally discharged for a few days and you'll have
lost the majority of the capacity.
You may get some of it back by reconditioning, but yes, don't do this.
The previous graduate students on the hybrid card I worked on left
the pack flat after driving the car down and destroyed it. They were
high-performance lead-acid and the pack cost a few thousand dollars to
replace. So, yes... don't do this... :)
If you've left the camera on with the LCD going, it draws about 1/4 amp
from what I measured on my -DS awhile back. That's about C/50, so the
battery should last about two days.
That's just cruelty to cameras. You could at least let the poor thing have a
rest overnight.
The camera doesn't care. I'd be more concerned about the
backlight on the LCD aging. I've seen a number of laptops with the
screens on 24/7 have dead backlights.
For just a couple of days it probably doesn't matter.
-Cory
--
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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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