On Aug 20, 5:48*pm, Martin Gregorie
wrote:
On Fri, 20 Aug 2010 12:11:21 -0700, Darryl Ramm wrote:
Although it obviously varies widely a typical power consumption number
for a glider avionics is roughly around 0.8 amp (as Evan noted his is)
for what I am guessing is a typical setup of C302 style computer, a PDA,
and VHF radio. Owners should measure and calculate the loads in the
glider and estimate the battery capacity needed or run time available
from the batteries they have. Do not just divide the nominal "Ah
capacity" by amp load, especially at higher loads, you need to use the
discharge curves data from a manufacturer to estimate the available run
time of a battery at a particular load (most good VRLA batteries are
close enough to use another manufacturers spec sheet for a similar sized
battery).

I'd seriously suggest a visit to your local RC model shop to look at
battery chargers. $60 - $100 gets you a cycling charger that can not only
peak charge a partially discharged SLA battery without harming it, but
can measure its capacity. Record the measured capacity every year and bin
the battery when it shows a 30% drop and you may even save money.

Slinging an SLA battery every three years is common wisdom here, yet last
winter my three year old batteries still had 90% of their nominal
capacity. Bin them? I think not!

FWIW these batteries have always been charged with a peak charger.

PS; sorry for hi-jacking the thread, but it seemed appropriate.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

I'm not sure what "peak charge" means in the context of a VRLA
battery. Peak charge/delta-peak/delta-V relies on effects usually seen
in other battery chemistries not in the lead-acid family. A multiple
battery type charger might be labeled delta-whatever but not use that
charge method for lead acid batteries.

The main thing you want to do is pick a charge specifically designed
for VRLA (valve regulated lead acid) aka SLA (sealed lead acid)
battery. The battery charger should be sized so it's bulk charge amp
specs is around C/10 to C/5 (no more) where C is the capacity in amp-
hours of the battery. The charger should support three charge phases
bulk/absorbtion/float but sometimes the documentation does not mention
absorbtion or three stages only to even if the charger really does
this properly.Charge the batteries in a cool environment. If the
charger does not float the batteries at between 13.5 to 13.8V after
the first two phases do not leave the battery connected to it for long
periods (a common cause of death of VRLA batteries by evaporating the
electrolyte out the vent valves) -- some RC model chargers
specifically do *not* do this right. My favorite AC charger is a
Xenotronix HPX series but others are good as well.

I (and I know Eric does as well) recommend the CBA III as a battery
discharge tester (http://www.westmountainradio.com/CBA.htm). It has
the benefit of being able to charge monitor, so you can watch a
charger step though the charge phases on a battery. Handy for checking
out chargers and suspect batteries. A great purchase for a club, FBO
or individual pilot.


Darryl