From: "Cory Papenfuss" <[EMAIL PROTECTED]>
To: <[email protected]>
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!
Is that 6v 12AH ? If so it should last about the same as five or six
sets of reasonable NiMh AAs. Why not get a nice 400AH battery & charge
it up once a year :-)
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.
As a caution Lead Acid batteries tend to be ideal/designed for high
discharge rates whilst essentially remaining fully charged all the time
(think of car batteries here). When used for what I imagine will be your
long very slow discharge rate usage they can tend to develope a memory
effect that makes them both harder to charge properly, and also their
ability to supply large currents diminishes.
Not really... they may be designed to *tolerate* high current levels, and
that may diminish their deep-cycle capacity a bit, but they are still the
same battery chemistry. Lead acid batteries (nor NiMH, nor NiCd except in
extremely specially-constructed circumstances) do NOT have a "memory
effect."
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.
The death of a lead acid battery is from leaving it
discharged for any length of time.
One of those digital battery conditioner thingies that
pulse charges and continually maintains battery condition might be ideal
for a permanent bench setup - there again it will probably cost more than
a small battery :-)
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.
The deeper you discharge the battery the shorter will be it's
life, ideally you would charge it up when it reaches 80% charge but this
would mean charging it almost as often as you would a set of AAs.
If you look at life cycles of lead-acid batts from a manufacturer's
datasheet, you'll see that's not the case. The total *energy* moved in
and out of the battery is fairly constant... discharge 25% and it will
last 2x as many cycles as a 50% discharge.
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.
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%.
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.
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.
-Cory
--
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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