My intention is to charge the big
battery every time the camera LCD tells
me its depleted. I'll go on doing this
until the battery dies then I'll get
another for the princely sum of 8Euro --
in about 2012. The price may have gone
up (or down) by then of course. Outside
in the forest I'll use NiMH charged with
a La Crosse charger. When they die I'll
get more.
Don
Rob Smith wrote:
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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See feature: The Cement Company from Hell
Updated: Photomicro Link -- 18 05 2005
