2-Batteries

Lew Hartswick wrote:
Gary Emerson wrote:


With 18Amps of forward current capability (each), there isn't really
any "need" for the two diodes in parallel, but for an extra $5 and a
few extra minutes of wiring, if any one diode ever did fail "open" the
other would still provide power. Most likely overkill.

Yes because most often (probably at least 90% of the time) a diode
fails "short"
Therefore you may be reducing the reliability. :-)
...lew...

Fail "short" won't leave you without power on a flight. Fail "open"
might...

Gary,

I developed a circuit card with isolation diodes for two batteries and
a pair of efficient voltage regulators, one for high current devices
(radio and transponder) and the other for the rest. As Dick indicated
the panel has two SPST switches for two batteries. The diodes drop .3
Volts, but the regulators operate down to low voltages. There is a
price to pay for operating the regulators. But, I felt that
considering that I use 12 Ah batteries, I had plenty of capacity and I
was more interested in supplying regulated power to my instruments.

The problem was that the large regulator couldn't handle the start up
load from the transponder. So, my quick fix was to bypass that
regulator with the second switch. I expect to revisit the project some
day and finish it properly.

Jim Hendrix


Gary Emerson wrote:
wrote:
By all means a second battery should be installed in our electrically
driven modern sailplanes. After many years of quickly flipping my
3-position battery switch, and trying not to have my logger to
momentarily dropout, I have concluded that is best to use 2 single-pole
battery switches. That way one can have either or both batteries
connected at the same time.

I saw the light when Jim Hendrix brought his sailplane to Caddo Mills
for Wing Deturbulator flight testing, and it was wired like that. You
will hear much more about that amazing new invention at the coming SSA
Convention.

Thermally,

Dick Johnson

Better yet is to use diodes so that both batteries will always be "on"
in parallel and you're always pulling from the best battery with no
fiddling required from the pilot. Relatively low voltage drop diodes
are available with 18 Amp forward capacity. For redundancy, I used two
in parallel on each battery. The diodes are available in the TO-220
package and it's easy to incorporate a small heat sink, but I have no
reason to believe they ever attained any temperature at all.

With two batteries connected with switches, if one battery does really
"die" then any time spent with both switches in the "on" position causes
the good battery to attempt to charge the "dead" battery to no avail, so
ultimately you're wasting what power you do have during this time. The
diodes eliminate any chance for cross charging...or discharging. Used
this system for several years, never experienced a single power issue.

Being the incurably curious type, I decided to test this theory. I
took two known good 7Ah 12V SLA batteries and discharged one to 8 volts
(resting) with a 12V light bulb. The other battery I topped off with a
charger to 13.6 volts. I connected the two using less than two feet of
18 gauge wire and a ordinary toggle switch. Using a 60 Mhz bandwith
oscilloscope and a hall-effect type current probe I looked at the
resulting waveform when I closed the switch; a nice square edged rise
to about 3 amps, tapering down to 2.5 amps in a few seconds. Because I
didn't know what the frequency response of this current probe was, I
inserted a precision .001 ohm current shunt in line (very high
frequency response) and used the scope to watch the voltage drop across
it. The results were identical; no current spike, no inrush of current
- just a nice square edged waveform rising to about 3 amps. This simply
isn't going to weld contacts, burn out switches or blow (properly
sized) fuses. As for "wasting energy" by dumping from the good battery
into the dead battery when switching over - just do the math. Even if
the two batteries were connected for as much as 5 seconds while
switching from one to the other (two switch or "make before break"
switch arrangement), you will be using less than one thousanth of the
good battery's capacity to charge the "dead" battery.

RF

Doug Haluza wrote:
This is how I finally set up my glider--with two main battery switches.
But if you want to run this way without bouncing the logger, the two
switches should be on for the whole flight. You don't want to run one
battery down like a fuel tank, then switch when it is empty. If you do,
you will get a big current spike when you switch them both on. At best,
this wastes energy from the new battery by dumping it into the dead
one. You can also burn-out your switch, weld the contacts, or blow a
fuse.

What you want to do is run two identical batteries, bought at the same
time from the same production batch. Then always charge and run them
together. They should age together, and share the load properly this
way. You only want to use the two switches in opposite positions to
separate the batteries for testing, or if one battery fails.

wrote:
By all means a second battery should be installed in our electrically
driven modern sailplanes. After many years of quickly flipping my
3-position battery switch, and trying not to have my logger to
momentarily dropout, I have concluded that is best to use 2 single-pole
battery switches. That way one can have either or both batteries
connected at the same time.

I saw the light when Jim Hendrix brought his sailplane to Caddo Mills
for Wing Deturbulator flight testing, and it was wired like that. You
will hear much more about that amazing new invention at the coming SSA
Convention.

Thermally,

Dick Johnson

On Dec 29, 6:25 pm, "Tinwings" wrote:
Being the incurably curious type, I decided to test this theory.
-- snip --
...you will be using less than one thousanth of the good battery's
capacity to charge the "dead" battery.

Holy cow!! Blasphemy!!!!

We can't have people posting empirical evidence complete with a
complete description of the experiment messing up our continuing
propagation of "old wives' tales" in this group.

Seriously, thanks for doing the experiment and reporting the results
here. Perhaps now we can close out the power switch discussion once
and for all. Naaaah, I'm sure it will come back.

-Tom

Excellent! Beat to the punch... I was reading this thead and thinking
I'd just go measure this to quiet concerns about inrush currents.
I've.been operating my glider with 2 x 12 Ah batteries with seperate
spst master switches on each battery. I'll run one battery at a time
but both will be on for a short while when switching batteries. I've
never had a problem with switches and currents and never expected that
I would.

This really should not need an experment to prove it... (but its fun
and who would believe anything on ras without it?). While AGM batteries
have very low internal resistance when fully charged, like all lead
acid batteries the internal resistance increases as they discharge due
to the reduction in electrolyte conductivity (the sulfuric acid is
turning into water as the sulfate ions form lead sulfate on the
plates). Internal resistance might go from ~0.002 ohm fully charged to
maybe an 0.1 to an ohm or so if you really discharge things (your
mileage will vary widely). You won't find the internal resistance specs
anywhere except at full charge, but you can infer them from the
standard manufacturers discharge curves (usually voltage vs. log time
at various discharge currents). That fall off in voltage measured in
those discharge curves is actually telling a lot about the internal
resistance increasing as the battery discharges. The internal
resistance increasing inside the battery causes the external measured
voltage under load to decrease - this dominates the voltage measured
under load much more than the cell chemical potential decreasing due to
the weaker electrolyte concentration. The battery voltage is not
getting lower as much as it is getting harder to pull whatever is in
the battery out. And for our switch story, it is luckilly also harder
to push charge back into the flat battery.

So while a fully charged lead acid battery and especially (for their
size) AGM batteries can sink huge currents into a short circuit the
other battery just does not look like a short circuit if it is
discharged, and if it is not very discharged then the small voltage
differences (fractions of a volt) don't generate a large current even
with fairly small total internal resistance of both batteries.

I run my glider with two 12 Ah batteries and two seperate master
switches because I like to be able to see what is going on and I like
the redundancy and to be able to control things. I don't like the diode
idea since in normal operation you can't see if you have a weak battery
(like when you make a mistake and don't fully charge one battery - I'm
more worried about operator error (me) like that than by one of the
batteries actually being sick. I just think I'm more likely to spot a
mistake like that with the batteries not wired in parallel with diodes.

BTW in the test described below the current starting at 3 amps and
dropping to 2.5 amps is casued by surface charge -- surface chemsiry
effects of the electrolyte in the very porous surface of the plates. It
it also the reason why a "12 volt" lead acid battery measure up over 13
volts, if you burn of the surface charge you'll find the true open
circuit cell voltage is around 12.5 volts (depends slightly on battery
chemsitry and temperature). Surface charge is why a damaged old battery
can sometimes charge up over 12 volts but rapidly fall and why just
pulling a battery off charge and measuring it's open circuit voltage
without either waiting (many hours) or deliberately drawing current to
burn off the surface charge is next to useless. But it is always
amusing watching people do this at the gliderport... (see the surface
charge effect slides in the link below, they show the surface charge
being discharged and the battery voltage allowed to relax back again
prior to a proper open circuit voltage measurement to estimate the
state of charge - a measurement that even if properly made tells you
*nothing* about the actual battery capacity).

I was asked to give a talk at one of our PASCO seminars on glider
batteries. I'm not sure how useful the slides will be by themselves but
they are available here
http://www.pacificsoaring.org/articl...SCO%202006.pdf

Anyhow again thanks for the measurements


Darryl Ramm
DG-303 6DX


Tinwings wrote:
Being the incurably curious type, I decided to test this theory. I
took two known good 7Ah 12V SLA batteries and discharged one to 8 volts
(resting) with a 12V light bulb. The other battery I topped off with a
charger to 13.6 volts. I connected the two using less than two feet of
18 gauge wire and a ordinary toggle switch. Using a 60 Mhz bandwith
oscilloscope and a hall-effect type current probe I looked at the
resulting waveform when I closed the switch; a nice square edged rise
to about 3 amps, tapering down to 2.5 amps in a few seconds. Because I
didn't know what the frequency response of this current probe was, I
inserted a precision .001 ohm current shunt in line (very high
frequency response) and used the scope to watch the voltage drop across
it. The results were identical; no current spike, no inrush of current
- just a nice square edged waveform rising to about 3 amps. This simply
isn't going to weld contacts, burn out switches or blow (properly
sized) fuses. As for "wasting energy" by dumping from the good battery
into the dead battery when switching over - just do the math. Even if
the two batteries were connected for as much as 5 seconds while
switching from one to the other (two switch or "make before break"
switch arrangement), you will be using less than one thousanth of the
good battery's capacity to charge the "dead" battery.

RF

Doug Haluza wrote:
This is how I finally set up my glider--with two main battery switches.
But if you want to run this way without bouncing the logger, the two
switches should be on for the whole flight. You don't want to run one
battery down like a fuel tank, then switch when it is empty. If you do,
you will get a big current spike when you switch them both on. At best,
this wastes energy from the new battery by dumping it into the dead
one. You can also burn-out your switch, weld the contacts, or blow a
fuse.

What you want to do is run two identical batteries, bought at the same
time from the same production batch. Then always charge and run them
together. They should age together, and share the load properly this
way. You only want to use the two switches in opposite positions to
separate the batteries for testing, or if one battery fails.

wrote:
By all means a second battery should be installed in our electrically
driven modern sailplanes. After many years of quickly flipping my
3-position battery switch, and trying not to have my logger to
momentarily dropout, I have concluded that is best to use 2 single-pole
battery switches. That way one can have either or both batteries
connected at the same time.

I saw the light when Jim Hendrix brought his sailplane to Caddo Mills
for Wing Deturbulator flight testing, and it was wired like that. You
will hear much more about that amazing new invention at the coming SSA
Convention.

Thermally,

Dick Johnson

Two significant things missing from the comments below are

1. Measured battery capacity depends significantly on discharge current

2. Measured battery capacity dependssignificantly on temperature (esp.
for lead acid batteries)

These two things cause most of the confusion I see glider pilots having
understanding battery run time

For #1. A 12 Ah battery is not 12 amps for one hour, far from it. For
lead acid batteries the standard is to measure the discharge over a 20
hour period. That is at a constant discharge current of 0.05 x C (where
C is the capacity in Ah).

If you draw higher current from a battery you get less total energy
out, the extra energy has gone into ohms law heating in the internal
resistance (I squared R). The internal resistance is only the simplest
model of what is going on, there are other effects that increase losses
as the current increases significantly.

Understanding the dependency of the measured battery capacity on the
discharge current/capacity ratio also explains things like why a single
12 Ah battery can provide more capacity than 2 x 7Ah batteries
discharged one after the other. A higher current/capacity ratio is
being drawn from the 7Ah batteries.

If you want to measure how many amps x hours you really will get our of
a battery then you need to measure it near the actual discharge current
it will have (and near the actual temperature it will be at). The
reason that (as mentioned in the posting) that discharge curves measure
down to different voltages depending on the current/capacity ratio the
measuremnt is made at is due to internal resistance. Higher discharge
currents cause more of an internal voltage drop across the internal
resistance so the external voltage you measure the discharge down to
has to be lower. The easiest way to get the correct cut off voltage is
to look at a discharge graph on a manufactueres spec sheet.

While the measuremnt cut-off voltage (usually around 10 to 10.5 volts
for typical glider battery measurements) may be too low for some older
electronics it is likely to be high enough to power modern
avionics/toys found in gliders (I know some of the older transponders
and radios that really prefer to run at ~14 volt alternator voltrages
may have issues).

For those more interested in capacity measurements and discharge curves
should Google Peukert's equation that describves a useful emperical
relationship between discharge currents and battery capacity.The
"Peukert number" for a battery describes how immune its capacity is to
changes in the discharge current. You have to calcualte this number
yourself from meaurements, manufacters won't usually quote it.

Battery capacity measuremnts usually start with a fully charged
battery. Using a trusted charger and leaving the thing on charge for a
long time is a good way to do this in practice. The open circuit
voltate when you take the battery off charge will measure above 12.5
volts or so but that is an aberation caused by surface charge. You will
see the voltage rapidly drop of (usually in seconds to tens of seconds
depending on the measurement current) as you measure voltage under
load. In practice this surface charge represents no significant battery
capacity. In the measurement mentioned below there should really be no
need to let batteries sit for a day or so before a discharge test.

BTW as mentioned below even a partial discharge cycle can show up
problems. Again it is internal resistance that is likely dominating the
measurements. Cells that are sick or damanged or just old often have
higher internal resistance than a healthy cell and will cause the
voltage to drop quickly. For those reasons discharge tests at currents
much higher than you will operate the batteries can be useful since the
higher currents will show up internal restance problems quickly
(resistive power losses are proportional to the square of the current)
-- even if the measuremnts don't mean much for calculating expected
battery life under any particular load.

For #2 Temperature effects, that is a longer topic, but be the standard
is to measure lead acid batteries at 20 C. AGM battery capacities
approximately halve down to around -20 to -25C where the electrolyte
can start to freeze. And yet again internal resistance (decreasing
conductivity of the electrolyte vs temperature) explains a lot of this
behavior. I show a few capacity vs. temp curves in the slide talks I
linked to in another reply in this thread. One of the curves shows the
rapid voltage drop of a cell freezing as it discharges (some thing to
worry about on that next really long and cold wave flight :-)

Cheers


Darryl Ramm
DG-303 6DX


COLIN LAMB wrote:
Testing AH of batteries

Battery capacity is measured using a defined minimum voltage. That voltage
may or may not correspond to the minimum voltage your glider equipment will
operate on. If you use diodes, that minimum voltage will move slightly.
Capacity also depends upon a starting voltage - which is significantly
higher than 12 volts.

I often need to test batterys used in our search and rescue radios and have
found that I can test battery capacity quickly without a full discharge. I
charge the batteries, then let them sit for a day. Then, I put a load on
them and simply watch the decay of voltage over time. In a very short time,
you can make a graph that will indicate the trend of the battery and compare
with a new battery. A battery with reduced capacity will drop voltage much
more quickly.

You can determine the capacity of the battery during charge, too. Capacity
is the ability of the battery to resist change. That applies either way.
it means the battery will drop in voltage or charge more slowly. So, you
can simply time the charge of two batteries and learn the comparative
capacity of each.

Colin

wrote:
Nice presentation snipped
... One of the curves shows the
rapid voltage drop of a cell freezing as it discharges (some thing to
worry about on that next really long and cold wave flight :-)

If we had properly designed heater elements surrounding the battery,
powered by the battery itself, could we expect more "useful capacity"
from the battery on those cold flights? Useful capacity being defined as
the amount of current delivered to the avionics.

Tony V.

A question for the group from someone who admits to knowing little
about batteries:

My glider is setup with two 12v9ah (the new "same size as the 7.5ah"
ones) batteries, with each normally powering half my electronics; i.e
one batt is powering the radio and SN10/GPS, while the other runs the
logger/GPS, PDA, and backup vario. Each is connected via a 3-way
master switch, so either battery can power either or both "busses".

This setup does not allow both batteries to be connected to the same
"bus" at the same time. On the rare time I've had to switch due to old
or insufficiently charged batteries, I've had no trouble with the
traces from the loggers.

My question is, is this better, equal, or worse (from the standpoint of
battery usage/efficiency) than the more common "use one batt until it
dies, then switch" 2-battery setup.

TIA,

Kirk
66

One reason to run the batteries in parallel is that the slower the rate
at which you discharge a battery, the more total power you can get from
it. I.e. if you discharge a battery at 1 amp it will not deliver as
many amp hours than if you discarge it at 0.5 amps. This is somewhat
offset by the need ( or good idea) to isolate the batteries with low
loss zener diodes. The 0.25 volt drop of the zeners means that in
effect you can only discharge you batteries to 11.25 rather than to 11
volts - which sacrifices some capacity. You have to get out the data
sheets for the diodes and batteries to see if this is a good tradeoff.

I believe that using one battery switch rather than multiple spst
switches is a bad idea because the single switch becomes a single point
of failure. Having 15 batteries is of no value if the switch breaks.

Tony Verhulst wrote:
wrote:
Nice presentation snipped
... One of the curves shows the
rapid voltage drop of a cell freezing as it discharges (some thing to
worry about on that next really long and cold wave flight :-)

If we had properly designed heater elements surrounding the battery,
powered by the battery itself, could we expect more "useful capacity"
from the battery on those cold flights? Useful capacity being defined as
the amount of current delivered to the avionics.

Yes, if the battery is well insulated so the heating doesn't require
much current, and if it is relatively warm before you launch (usually
the case), so current isn't required to warm it initially. Installing a
warm battery just before flying is one way, or an obsessed pilot could
connect external power to the heater to warm the battery (to, for
example, 90 deg F) before the flight, so no power is required for
heating during the flight.


--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org