In message , Stefan
writes
In article ,
says...
100AH is its total capacity. Normal rule of thumb is to only
discharge it down to 50%. So, you have 50AH of useful
capacity.

The advice for NiCad batteries for mobiles etc. is that it is good for
them to go through complete discharge-recharge cycles.

Only applies where a critical cut-off voltage is involved for the
equipment. Otherwise "memory" is an urban myth

Anyone able to
explain why lead/acid batteries are different - if indeed they are?

Tend to have steeper discharge curves and be used in non voltage
critical applications

--
Keith Lewis

Power is the time rate of energy use. Asking how much power is in a battery
is like asking how much velocity is in a full can of gasoline. There is no
limit implied by the spec quoted.

The Ah spec is usually given at the 20hr rate which is the time to discharge
the battery to 1.75 V/cell (lead acid battery assumed) with a steady current
draw, 5 amps in this case. For a 12 V battery this equates to 10.5 V. It is
a non-linear relationship, halving the current will not double the discharge
time. Battery manufacturers can provide curves or charts that show the
actual Ah for different current draws. The spec is intended to represent
usable capacity, since a battery can be recharged from 1.75V/cell with
little loss of life. Deeper discharges will shorten life. Not too many real
world loads are constant current. Lights will use less current as voltage
drops (constant resistance), motors will use more (constant power.)


"Nigel" wrote in message
...
Stupid question time...
If I have a 100ah battery how much power can I expect to get out of it...
is
100ah the total battery capacity or it's useable/available capacity. My
understanding is that a normal lead acid battery shouldn't be run too
flat,
so when is flat too flat? If I should only let my battery drain down to
say
a 90% charge, have I only got 10ah before I need to recharge?

"Nigel" wrote in message
...
Stupid question time...
If I have a 100ah battery how much power can I expect to get out of
it... is
100ah the total battery capacity or it's useable/available capacity. My
understanding is that a normal lead acid battery shouldn't be run too
flat,
so when is flat too flat? If I should only let my battery drain down to
say
a 90% charge, have I only got 10ah before I need to recharge?

As a very basic answer, a 100 amp/hour battery should provide 100 amps for
an hour or 1 amp for 100 hours, in reality of course it would probably
melt if required to do the former. Then you need to factor in age, type of
use, deep charge trickle or whatever. At best it is a guide only as to the
possible maximum the battery can provide.

Guy Fawkes wrote:

snip
...so a 2 year old 100 Ah battery that has not been deep
cycled but has been used for engine starting may only have 75 Ah capacity
left...
snip more

Guy, you may be being pessimistic. My last car was scrapped complete
with original (working) battery after 8 years & 120,000 miles. If I had
lost 25% in 2 years, after 8 years I'd only have 30% left. Yet it
cranked fine - even when periodically I forgot to turn the immobiliser
off so it didn't fire...
Then again, I may have been lucky.

Andy.

Ah capacity is only slightly related to cranking ability. A battery can be
significantly down on it's rated capacity yet still able to deliver adequate
voltage for the typical period needed for starting an engine, even with the
immobiliser.
--

"Andy Champ" wrote in message
...
Guy Fawkes wrote:

snip
...so a 2 year old 100 Ah battery that has not been deep
cycled but has been used for engine starting may only have 75 Ah capacity
left...
snip more

Guy, you may be being pessimistic. My last car was scrapped complete with
original (working) battery after 8 years & 120,000 miles. If I had lost
25% in 2 years, after 8 years I'd only have 30% left. Yet it cranked
fine - even when periodically I forgot to turn the immobiliser off so it
didn't fire...
Then again, I may have been lucky.

Andy.

Andy Champ writes:

Guy Fawkes wrote:
...so a 2 year old 100 Ah battery that has not been deep
cycled but has been used for engine starting may only have 75 Ah capacity
left...

Guy, you may be being pessimistic. My last car was scrapped complete
with original (working) battery after 8 years & 120,000 miles. If I had
lost 25% in 2 years, after 8 years I'd only have 30% left. Yet it
cranked fine - even when periodically I forgot to turn the immobiliser
off so it didn't fire...

He was talking about deep-cycle batteries. They are not designed for
extremely high current draw applications (such as engine starting).
Just like starting batteries are not designed to be drawn way down.
The difference is due to the plate design. A plate designed for
providing cranking amps isn't good for deep cycle applications,
and vice versa.

Head to your local library and see if they have a copy of
_Boatowner's Mechanical and Electrical Manual_ by Nigel
Calder. The introduction in chapter one gives an excellent
description of the difference between the two battery types.

In fact, this is a "must own" book for any cruiser. I paid
$80 (Canadian) for mine (second edition hardcover) and it was
worth every penny.

--lyndon

On Wed, 16 Jun 2004 10:34:33 -0400, Glenn Ashmore
wrote:

NiCads develop a memory over time and can't be fully recharged unless
they are fully discharged occasionally.

This is (almost) a myth. The Ni-Cad memory phenomenon
does exist, but I can almost promise that no-one
reading this has ever experienced it. It requires
that the partial discharge/charge cycle be repeated
*****EXACTLY***** many, many times. It was discovered
in a satellite (and eventually duplicated) where the
battery discharged for a precise length of time into
a precise load, and then was recharged for a precise
length of time at a precise rate. When the program
of the satellite was changed later, it was found that
the battery could not continue providing power after
passing the previous discharge point.

It simply won't happen just because you don't run
a Ni-Cad down to zero before recharging it. In
fact if you DO run it to zero, there is a very good
chance that (because of slight differences in cell
capacity) that you will reverse-charge one or more
of the cells, which WILL ruin that cell quickly.
Better to get it onto the recharger as soon as
you experience a significant drop in output
under load. At the point where your battery drill
stalls at no load or very light load, you may well
be pumping up one of the cells backwards as the
other cells push the last few electrons through
the load.

The commonest way to damage Ni-cads, however, is to
leave them on the charger too long, or to charge
them too fast at some point in the recharge cycle.
Every time this happens, the battery will lose
some of it's capacity.

"Peter W. Meek" wrote in message
...
On Wed, 16 Jun 2004 10:34:33 -0400, Glenn Ashmore
wrote:

NiCads develop a memory over time and can't be fully recharged unless
they are fully discharged occasionally.

This is (almost) a myth. The Ni-Cad memory phenomenon
does exist, but I can almost promise that no-one
reading this has ever experienced it. It requires
that the partial discharge/charge cycle be repeated
*****EXACTLY***** many, many times. It was discovered
in a satellite (and eventually duplicated) where the
battery discharged for a precise length of time into
a precise load, and then was recharged for a precise


I Beg to differ with you.
I have on occasions, and know friends who have damaged
Ni-Cad batteries. They were damaged by partial depletion
then recharging. They were in sealed waterproof torches
and over a period of time the batteries were damaged by
that method of charging. I now always run my batteries from
full to empty no mater what type.
taz.

I have also seen it happen...running wet cell NiCad's for a back up source
of power for cesium beam frequency standards. These units we periodically
discharged and recharged as a preventative maintenance procedure, and they
did develop memory. The fix was to vary the preventative maintenance
discharge time/amount. H/P factory support helped us solve this problem.
Doug K7ABX
"taz" wrote in message
...

"Peter W. Meek" wrote in message
...
On Wed, 16 Jun 2004 10:34:33 -0400, Glenn Ashmore
wrote:

NiCads develop a memory over time and can't be fully recharged unless
they are fully discharged occasionally.

This is (almost) a myth. The Ni-Cad memory phenomenon
does exist, but I can almost promise that no-one
reading this has ever experienced it. It requires
that the partial discharge/charge cycle be repeated
*****EXACTLY***** many, many times. It was discovered
in a satellite (and eventually duplicated) where the
battery discharged for a precise length of time into
a precise load, and then was recharged for a precise


I Beg to differ with you.
I have on occasions, and know friends who have damaged
Ni-Cad batteries. They were damaged by partial depletion
then recharging. They were in sealed waterproof torches
and over a period of time the batteries were damaged by
that method of charging. I now always run my batteries from
full to empty no mater what type.
taz.

In message , taz
writes

"Peter W. Meek" wrote in message
.. .
On Wed, 16 Jun 2004 10:34:33 -0400, Glenn Ashmore
wrote:

NiCads develop a memory over time and can't be fully recharged unless
they are fully discharged occasionally.

This is (almost) a myth. The Ni-Cad memory phenomenon
does exist, but I can almost promise that no-one
reading this has ever experienced it. It requires
that the partial discharge/charge cycle be repeated
*****EXACTLY***** many, many times. It was discovered
in a satellite (and eventually duplicated) where the
battery discharged for a precise length of time into
a precise load, and then was recharged for a precise


I Beg to differ with you.
I have on occasions, and know friends who have damaged
Ni-Cad batteries. They were damaged by partial depletion
then recharging. They were in sealed waterproof torches
and over a period of time the batteries were damaged by
that method of charging. I now always run my batteries from
full to empty no mater what type.
taz.


Top-up charging is the best regime for NiCd's used in a torch. Full
discharge every time simply wears out the cells unnecessarily.

Of course cells vary enormously in quality and it is virtually
impossible to buy decent ones from retail outlets.

--
Keith Lewis