"Doug Dotson" wrote in message
...
100AH is its total capacity. Normal rule of thumb is to only
discharge it down to 50%. So, you have 50AH of useful
capacity.

Doug
s/v Callista


Not quiet. Lets assume lead acid batteries and only engine charging.

Yes the 50% of discharge is a generally acceptable figure, but you are
unlikley to get them much above 80% of fully charged because we tend to use
automotive alternator regulator designs that are built down to a price and
not up to a duty (they do the job very well on a car).

This means that you can only reliably assume 30% of usable capacity. The
problem is that over the winter that 20% between 80% and 100% of fully
charged will sulphate, which further reduces the capacity of the battery.

To overcome this either charge with a mains charger that can charge to 100%
or fit an advanced regulator that may reach 98% of fully charged.

Tony Brooks


"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?

Hi Nigel,

it depends on the type of battery, you are using:

automotive batteries are made for short high current during starting, they
should not been discharged more than 60...80%. In your case 80 AH
discharging will be critical.

marine batteries are made for slow and long discharging and they should not
been discharged too much. Gel batteries should not been discharged more than
60 %, wet batteries not mote than 80 %.

Take a look at the voltage, if you have a digital meter: battery voltage
lower than 10.8 V will damage the battery (deep discharging).

regards

Juergen

"Nigel" schrieb im Newsbeitrag
...
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:

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?

batteries are USUALLY rated on a 10 hour cycle, so a brand new and fully
charged 100Ah battery with put out 10 A for 10 hours, a 60 Ah battery will
put out 6 A for 10 hours, and so on.

The vast majority of batteries, eg car / domestic type, eg NOT deep cycle
type as fitted to fork lifts and milk floats, will die VERY quickly if you
deep cycle them, losing as much as 5% of their total capacity every time
they are deep cycled, 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, given this I personally rate a 100 Ah battery as a 50 Ah battery to
take this into account.

Others are sure to disagree.
--
Liquid Cooled PC? -- http://www.surfbaud.co.uk/

E-mail (rot-13) qnirahyy NG oyhrlbaqre QBG pb QBG hx
EoF

On Wed, 16 Jun 2004 11:09:23 -0400, Guy Fawkes wrote:

Nigel wrote:

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?

batteries are USUALLY rated on a 10 hour cycle, so a brand new and fully
charged 100Ah battery with put out 10 A for 10 hours, a 60 Ah battery will
put out 6 A for 10 hours, and so on.

snip

Also worth mentioning that discharge at a higher current (ie over
shorter time) will produce less amphours - doubling current drain
might take 10percent off amphours. For this reason, batteries are
often quoted at 20 hour rate (C20).

Ian

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

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.

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.

As I normally daysail I do not need a battery in my boat. But when I
occasionally cruise for a few days, I would like one, but only for the
lights. I do not have other power consuming devices in my boat.

So now the question is this: could I simply move my car's battery to
the boat? Let's say for a four days cruise during summer time. Will it
still start the car afterwards? I guess there will be the pleasant
side-effect that the car will be less likely to be stolen.