2-Batteries

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