David:
Complex?
Modern glider instruments (including radios) are designed to operate on
a nominal 12 V supply. My Dittel FSG-71 gives the voltage range as
13.8 V+ 10% -20%, which translates into 11 V to 15 V. My SN-10
specifies 10 V to 15 V.
A fully-charged lead-acid or gel has a voltage somewhere in the 12.5 V
range and drops to around 11.5 V as it is discharged to 50% of its
capacity. A voltage of 11 V or below means the battery is pretty much
dead - either old or over-drained. (Standard data gives final cell
voltage of 1.83 V for a moderately slowly-drained battery at normal
temperatures, but it could be lower).
Many low-voltage indication problems are not just battery issues but
wiring problems - thin wiring, corroded connectors with high-resistance
joints. (being such a smart guy, you're no doubt familiar with Ohm's
Law)
Of course, a 14 V battery will give you a bit more power and enable you
to run the batteries down further, but the tradeoff is usually a
cobbled-together non-standard battery stack that you can't charge with
an off-the-shelf charger.
In my Discus, my added batteries are nothing to do with lack of
performance of the existing system which is flawless, but I am adding a
power-hungry transponder. I am reconfiguring the batteries accordingly
and adding a fin battery as backup for my main panel instruments. I
need 6 pounds of weight in the tail anyway, so it makes more sense to
put a useful lead battery there than just dead weight.
Please keep your comments civil.
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