14 Volt Gel Cell?

At 17:18 05 May 2004, Brian Penfold wrote:

Brian - some great data in your posting.
What about the scenario for us pilots who often cloud
fly with 'power hungry' artificial horizons? They require
considerable 'umpf' to get going and maintain their
equilibrium. I changed to 14V on ONE of my two cells
(trying to match capacity as you suggest) and have
found it to work marvellously, particularly with erecting
the AH in cold weather. Any comments for improvement
here - would solar cells help?
Cheers
Pete Harvey



I’ll pick up on a point that Derrick raised if I may.
He is right but I’d just though I’d expand on the
issue somewhat further, with an extract from an article
I wrote recently.

The ‘nominal’ terminal voltage of a 2 v cell is just
that – a nominal voltage. It can actually range from
about 1.8v when fully discharged, to 2.2v when fully
charged, and can be as high as 2.45 – 2.5v (14.7-15v
for 12v battery) when the cell is actually on charge.
Add 6 together to get a ‘nominal’ 12v add seven together
to get a ‘nominal’ 14v.

As you don’t get something for nothing, you have to
replace the charge from the battery when the battery
discharged. In order to do this you have to apply
a potential greater than that of the cell. In normal
aviation and automotive applications, this charge is
provided by an alternator or generator connected to
the prime mover or engine. Because engine speed fluctuate,
and because the output of a generator or alternator
varies with input speeds, it is necessary to ‘cap’
or regulate the output voltage of the generator to
prevent both over voltage conditions and to provide
a voltage reference or standard, so that any electrical
equipment connected into the aircraft or vehicle operates
within the correct supply conditions. The, nominal’
12v electrical system and it’s associated components
and instruments consequently has to be capable of operating
across a range of voltages, in particular the generator
output voltages required to maintain the battery in
a fully charged stated. The output voltages of generators
normally is 13.5v ‘nominal’ but can be as high as 14.7-15v
under very low load conditions.

With me so far?

Gliders do not have the luxury of having a prime mover
capable of turning a generator and thereby maintaining
the battery in a fully charged condition. Most avionic
components and instruments are however designed for
General Aviation use (powered), and their use in sailplanes
and gliders is just seen a different branch of aviation.
There is little point in developing a new technical
standard specifically for gliders, when the current
systems works fine. Most of the instruments and components
in general aviation use (including gliding) are designed
to meet the normal voltage range input requirements
of general aviation. That is to say a ‘nominal’ 13.5v
but are generally tolerant to 14.5-15v at the upper
end, and may still continue to work at 8v at the lower
end. You will find many instruments and components
rated at 12v, 14v, 8-16v – but they are generally designed
to work on a nominal 12v system.

What has this got to do with batteries and the 12v
– 14v issue I hear you say….. Well let me continue!

The lack of ability for sailplanes and gliders to recharge
batteries in flight means that with a load being drawn
from the batteries, that eventually they will become
discharged and at the extreme your glider electrical
system will stop working! The need to minimise the
weight of gliders means that practically you need to
balance the weight of the battery with the ability
(capacity) of the battery to supply all your electrical
loads for about a days flying. No point in having a
heavy car battery in the fuselage – no point in your
battery going flat after 2 hours! A good balance
between the two is a battery that will just about last
a day –recharge overnight and then provide enough power
for the next day…etc. As batteries get older they
loose their ability to maintain their charge fully
and many owners find after a period of time that the
battery doesn’t quite last a day. One fairly typical
approach to overcome this has been the somewhat misunderstood
practice of adding another (2v) cell in series with
the 12v battery to try to give it a bit more power
– hence the variety of 12v/14v systems we see around.
Does it work?…….read on.

Battery capacity is measured by a battery’s ability
to provide a relatively constant current over a fixed
period of time at a ‘reasonably’ linear rate of decay.
Capacity is measured in Amperes per Hour (AH) and
will normally be quoted at either a 10 or 20 hour rate.
What this means is that battery rated at 7AH (20 hours
rate) is capable of providing 7A for one hour, or 350mA
for 20 hours (7/20) before becoming fully discharged
(and all calculations in between!). Still with me?
Adding batteries in parallel general increases overall
capacity; adding batteries in series does not increase
capacity (and can actually reduce capacity in some
instances). So what has this to do with the 12v/14v
issue….

Take the following example:

A glider has a 7 AH 12v battery fitted and the total
load impedance is 400ohms. When a fully charged battery
is fitted (assuming a nominal 12v) the battery should
be capable of providing a current of about 30mA for
23.3 hours. (V=IxR). If a 2v cell is added in series
to the 12v battery to give a nominal 14v, then the
total current drawn will be 35mA. The load (impedance)
does not increase but the current drawn is greater,
nevertheless the capacity of the 12v battery (7AH)
is such that it is capable of supplying 35mA for 20
hours, however…and here’s the bit most people don’t
understand….if the capacity of the additional 2v cell
is not at least 7AH (and very few are – most are around
350mA/H)), the 2v cell will discharge at a far faster
rate than the 12v battery. Once it is fully discharged
the system not only operates as a nominal 12v system
anyway, but the discharged cell acts as an in line
resistor to the supply current from the battery. This
causes a voltage drop across it reducing the overall
system voltage in some extremes to less than 12v.
So depending if the right capacity 2v cell is not
used , then you can actually degrade your electrical
system performance. If you use common capacity cells
to create 14v – ie 7 identical cells then unless you
have equipment that is extremely sensitive to supply
voltage, you may see a very tiny increase in the length
of time you can run your sailplane electric’s before
the system goes flat. In practice however, the mismatch
of 2v/12v cell capacities and the age of your battery
will have far more influence on the ability of your
battery to maintain your instruments longer.

Me, I’ve been an electrical, avionic and systems engineer
for the past 25 years and I use 2 x 12v 9AH batteries
(switchable between main and standby)

QED.