12 volt 7amp/h versus10amp/h ??

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.

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"

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.

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

"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

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