12 volt 7amp/h versus10amp/h ??

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

"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

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

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

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

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

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"

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

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

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