12 volt 7amp/h versus10amp/h ??

[Udo Rumpf]

If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
Both batteries are of the same type and starting from
the same voltage with surface charge removed.
Udo

[COLIN LAMB]

"If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
Both batteries are of the same type and starting from
the same voltage with surface charge removed.
Udo"

Something seems mixed up here. Wattage is determined by multiplying the
current times the voltage. Excatly 5 watts would only be dissipated during
a brief period, since the voltage and the current will be changing during
the discharge cycle. I am wondering if the 5 watt rating of the resistor is
misleading you.

What is the resistor value in ohms? The manufacturer should have a curve
showing the voltage at various discharge rates vs. time.

Colin

[01-- Zero One]

Udo, what is the resistance of the resistor?



Larry

"Zero One" USA





"Udo Rumpf" > wrote in message
:

> If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
> with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
> Both batteries are of the same type and starting from
> the same voltage with surface charge removed.
> Udo

[Udo Rumpf]

I used a 12 volt Halogen light rated at 5 watt.
The two batteries were set up at the same time and
each had the identical 12 volt light, the discharge lasted for 51/2 hours
and ended at 12.00 volts for the smaller battery and 12.05 volt with the
bigger one. both started at 12,70 volts no resistor attached.
Udo


"COLIN LAMB" > wrote in message
nk.net...
> "If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
> with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
> Both batteries are of the same type and starting from
> the same voltage with surface charge removed.
> Udo"
>
> Something seems mixed up here. Wattage is determined by multiplying the
> current times the voltage. Excatly 5 watts would only be dissipated
> during a brief period, since the voltage and the current will be changing
> during the discharge cycle. I am wondering if the 5 watt rating of the
> resistor is misleading you.
>
> What is the resistor value in ohms? The manufacturer should have a curve
> showing the voltage at various discharge rates vs. time.
>
> Colin
>

[COLIN LAMB]

The 5 watt rating would be at a specified voltage. Let us assume 12 volts
nominal. That would mean a current draw of .417 amps. I am not looking at
the battery discharge cur es, but a 7 amp hour battery would normally be
rated at 7 amp hours using a total discharge between something like 13.8
volts and 10.5 volts. I am just using those numbers for illustration, but
most manufacturers rate the total amp-hours using an unrealistic low voltage
and often a higher voltage than might be used in a glider (since it might be
removed from a charger for some time).

5.5 hours at .417 amps would yield a total of 2.3 amp hours. That seems
pretty consistent with the voltage drop you measured for the 7 amp hour
battery. The 10 amp hour battery should be proportionally longer lived, so
it should take 10/7 x 5.5 hours or 7.86 hours to arrive at the same 12
volts.

Thus, it appears that the 10 amp hour battery has lost some capacity.

Colin

[Bob Salvo]

Udo, if the light bulb is receiving the same heat dissipation in both cases
(hanging freely in the same ambient air temperature), the time to discharge
to a particular voltage should be proportional to the AH rating of similarly
constructed batteries (batteries with similar discharge curves).

Bob

"Udo Rumpf" > wrote in message
.. .
> I used a 12 volt Halogen light rated at 5 watt.
> The two batteries were set up at the same time and
> each had the identical 12 volt light, the discharge lasted for 51/2 hours
> and ended at 12.00 volts for the smaller battery and 12.05 volt with the
> bigger one. both started at 12,70 volts no resistor attached.
> Udo
>
>
> "COLIN LAMB" > wrote in message
> nk.net...
> > "If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
> > with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
> > Both batteries are of the same type and starting from
> > the same voltage with surface charge removed.
> > Udo"
> >
> > Something seems mixed up here. Wattage is determined by multiplying the
> > current times the voltage. Excatly 5 watts would only be dissipated
> > during a brief period, since the voltage and the current will be
changing
> > during the discharge cycle. I am wondering if the 5 watt rating of the
> > resistor is misleading you.
> >
> > What is the resistor value in ohms? The manufacturer should have a
curve
> > showing the voltage at various discharge rates vs. time.
> >
> > Colin
> >
>

[[email protected]]

Typically, lead acid batteries of the kinds used in our applications
have amp hour ratings based on the load where it will take 20 hours to
discharge the battery. In higher rate applications, like running that
light bulb, amp-hour capacity performance is normally degraded. The
degradation may not scale linearly so comparisons between 2 different
packs may not work out using straight ratios. Capacity curves as well
as voltage curves for the batteries at various discharge rates should
be available on-line at the manufacturer's web site.

Other things to consider...

The charging requirement for the different capacity packs may vary. Are
the batteries each fully charged at the start of the test?

How consistent are the 2 bulbs you are using? Could 1 bulb be drawing
more current than the other?

Capacity does drop off with use. Capacity could also be adversly
affected by abuse such as deep discharge. Could the use (or abuse)
history alter the expected results?

Danny Brotto
LS-8-18 "P6"

[Andy]

The resistance of filament lamps is current dependent. I think you'd
have a better comparison test if used used a fixed value resistor.

Andy

[Andy]

The resistance of filament lamps is current dependent. I think you'd
have a better comparison test if used used a fixed value resistor.

Andy

[COLIN LAMB]

>The resistance of filament lamps is current dependent. I think you'd
>have a better comparison test if used used a fixed value resistor.

Although it is true the resistance of a lamp changes with applied voltage,
it is not an issue since the current flowing through a fixed resistor will
change as the voltage decreases. As the voltage decreases, current
decreases. As a matter of fact, as the voltage on the lamp decreases, the
temperature of the filament will decrease, which will decrease the
resistance, which will increase the current drawn. Thus, the lamp should
result in more constant current flow and improve the accuracy of a constant
current graph.

If you start at a certain voltage and discharge to a defined ending voltage,
it should not matter whether your resistance is slightly variableor fixed.
In neither case will you have a constant discharge, which is often what the
hypothetical battery discharge graph imagines.

I have used lamps successfully for years to check out batteries. The secret
is to select a lamp with the same current draw as the equipment you are
using. You can glance over at the lamp and see that all is well.

Colin

[Birdbones]

One other thing to note with lead acid batteries is that the voltage
will stabilize over about two hours once you let it rest. Resting means
no discharge or charge. If you wait a couple hours after disconnecting
the load you may see the battery voltage creep back up. The larger
battery may gain more than the smaller one. The opposite holds true
after charging. The voltage will drop over two hours once you
disconnect the charger.
Randy
ps. My experience is with large battery banks in PV systems.

[Doug Haluza]

The 10AH (20 hour) size DryFit battery is also available as a 13AH
battery rated at a 10 hour discharge, BB HR 15-12 (around $30 from
internet resellers). I find the 10 hour rated battery is a much better
match for glider applications, as I often fly 8-10 hours, but seldom
more than 12, and never close to 20. The high discharge version has a
little more lead inside, but has the same dimensions. I carry two, and
can run a VHF radio, transponder, encoder, SN10, GPS, two PDA's and a
logger for around 8 hours without a low battery warning. For longer
flights, I limit the transponder/encoder use when below 200' AGL.

wrote:
> Typically, lead acid batteries of the kinds used in our applications
> have amp hour ratings based on the load where it will take 20 hours to
> discharge the battery. In higher rate applications, like running that
> light bulb, amp-hour capacity performance is normally degraded. The
> degradation may not scale linearly so comparisons between 2 different
> packs may not work out using straight ratios. Capacity curves as well
> as voltage curves for the batteries at various discharge rates should
> be available on-line at the manufacturer's web site.
>
> Other things to consider...
>
> The charging requirement for the different capacity packs may vary. Are
> the batteries each fully charged at the start of the test?
>
> How consistent are the 2 bulbs you are using? Could 1 bulb be drawing
> more current than the other?
>
> Capacity does drop off with use. Capacity could also be adversly
> affected by abuse such as deep discharge. Could the use (or abuse)
> history alter the expected results?
>
> Danny Brotto
> LS-8-18 "P6"

[Udo Rumpf]

Doug,
what are the dimensions of your batteries? I am trying to fit them
in a 6"X 2,5" opening. Height is not as critical, but should not be
more then 4.5"?
Udo


> The 10AH (20 hour) size DryFit battery is also available as a 13AH
> battery rated at a 10 hour discharge, BB HR 15-12 (around $30 from
> internet resellers). I find the 10 hour rated battery is a much better
> match for glider applications, as I often fly 8-10 hours, but seldom
> more than 12, and never close to 20. The high discharge version has a
> little more lead inside, but has the same dimensions. I carry two, and
> can run a VHF radio, transponder, encoder, SN10, GPS, two PDA's and a
> logger for around 8 hours without a low battery warning. For longer
> flights, I limit the transponder/encoder use when below 200' AGL.

[Andy]

Don't disagree with anything you said. However Udo's test was to
measure voltage after a fixed discharge time, not to measure time to a
fixed end voltage. The slower voltage decay of the larger battery will
keep the lamp at a higher resistance than the lower capacity battery
and I think that will skew the result wont it? Doesn't it make the
larger capacity battery look better in comparison than it really is?
Perhaps the difference is negligible for the voltage range in question.

Andy

[COLIN LAMB]

"Don't disagree with anything you said. However Udo's test was to
measure voltage after a fixed discharge time, not to measure time to a
fixed end voltage. The slower voltage decay of the larger battery will
keep the lamp at a higher resistance than the lower capacity battery
and I think that will skew the result wont it? Doesn't it make the
larger capacity battery look better in comparison than it really is?
Perhaps the difference is negligible for the voltage range in question"

Well, we are picking at nits - but that is what soaring pilots do until the
"sun's up".

The larger battery is supposed to have a slower decay. That slower decay
will mean the voltage is higher. Although the bulb will burn brighter,
causing the resistance to be higher, the higher voltage will cause more
current to flow. The end result will be that the actual current flow with a
bulb may be more constant with a lamp than with a resistor. So, it means
that we can compare two different size batteries easily.

The bottom line is that batteries with greater capacity will cause the bulb
to lamp longer. If you compare a 10 A/H with a 7 A/H, you will find that a
good 10 A/H battery will cause the lamp to burn approximately 30 to 45%
longer. If the current drawn by the lamp is close to what the equipment
draws, then you can approximate and compare the effectiveness of the two
batteries.

Close is good enough. In my experience, people start complaining about
their battery when the capacity is down to about 50% of new. Probably a
good idea to make a test when the battery is new, then test it maybe every
winter when you get bored. Just remember how you made the test and where
you recorded the results. Batteries are usually rated for so many
charge/discharge cycles. If they are discharged below a minimum voltage,
such as 11 volts, they often will not achieve their rated discharge cycles.
If they are left in a discharged state, they may not last long. If they are
placed on an ordinary charger and left, they will not last as long.

A good charger that monitors the voltage and does not overcharge, is a good
investment. Usually, you can judge the charger quality by checking the
temperature of the battery after the charge cycle is completed.

Colin

[Andy]

Then I think we both agree that Udo should measure time to an end
voltage, rather than voltage after a fixed time, no matter what the
load device is.

I used to use a computer A/D input to record and plot battery discharge
voltage and current. I gathered lots of data. All I found was that in
flight battery failure was usually caused by failure of one cell and
that could not be predicted, only confirmed after the event, from the
discharge plots.

Andy