REFLECTOR: Batteries

[Don Smucker]

Here are my observations.

Make sure you are comparing apples with apples.  The chemistry and 
physics of lead acid batteries is well known and the variations between 
similarly constructed batteries are more a matter of small degree, but 
can easily appear to be large differences based on statistical 
advertising, difference in how the capabilities of specific batteries 
are presented, or comparing one kind (or sub-kind) with a different kind 
or sub-kind.  That said, to avoid arguments, I am talking about 
similarly constructed batteries within a specific class and type, as 
having only small differences from each other.

For our purposes, AGMs seem to be pretty ideal - vibration resistant, no 
memory effect, longer life, lighter weight for the power (acheived by 
maximizing case use, lead plate size and spacing and close relationship 
to fiberglass mats impregnated with electrolyte), non-leakable, 
reasonable cost and mountable in any position.  If you really want the 
"ideal" battery for an aircraft (from a physical characteristics 
perspective - light weight, GREAT amp draw capability and good reserve 
power), go Lithium - but also be willing to pay the much higher $$ price 
(sticker shock) and you will also require special changes to your 
charging system to handle the special charging requirements, cell 
balancing and to deal with the fire hazard.  A lot of AGMs (other 
batteries as well) are good for both starting power and reserve power 
(supplying power at a constant discharge rate and being able to supply 
that power for a sustained time period), based on a compromise plate 
thickness. Thickness of the plates determines whether a battery is a 
deep cycle battery (lots of reserve and staying power but not meant for 
starting large engines) or a starting battery (short bursts of heavy 
draw), and compromises can be made here like in every other desired 
feature/aspect, by the manufacture choosing a compromise plate size.   
Keep in mind, however, that you can easily ruin an AGM battery by how 
the battery is charged - you really need a newer smart charger that has 
an AGM setting on the charger or you can kill one in short order.  Also 
keep in mind that "momentary amps" is NOT the same thing as Cold 
Cranking Amps.   And that Amp Hours differs from Reserve Capacity.  You 
really have to compare apples to apples.  As noted by prior discussions 
here, the correct battery for specific applications  also depends to a 
large degree not only on the application (starting vs reserve power, or 
a compromise construction), but delivery of the needed amperage at the 
proper location as well.  Ie, wire size, distance from the place high 
amperage is needed, etc, and that the decision is also impacted by 
weight, weight and balance, cost, wire runs and their costs, physical 
size limitations, etc.

I ended up with this AGM battery.   Mounted in the nose, with an 
aircraft that I did not build, with existing #2 wiring to the engine.  I 
chose this battery by comparing my needs, including physical size (it 
just fit my battery tray), with what I could find.  I wanted a good 
starting battery, but also one that could provide the AH rating that 
would allow prolonged power in the event of an alternator failure.  Cost 
was also a factor, but not the overriding one.  I am also very light 
weight (135 pounds) and enjoy flying solo, so needed nose weight.  This 
battery replaces a relatively new Odyssey costing over 2x the price with 
less capacity that died prematurely.

My 2 cents worth.

http://www.batterymart.com/p-9agm47-agm-starting-battery.html