At 07:55 PM 8/13/2008 -0500, you wrote:
>I purchased one of the West Mountain units for the same reason that you
>are concerned about, Mike. It's a nifty tool.
>
>I was surprised to find the 11 a.h. battery I had laying around the shop
>was actually only good for 7 a.h.
>Sam
Keep in mind that the nameplate capacity of any battery is based on
some published discharge rate. For most of the SVLA industry, the
published capacity is at a 20 hour rate. See:
http://aeroelectric.com/Mfgr_Data/Batteries/Panasonic/lc-rd1217p.pdf
Note in the box under "Characteristics" we find a RANGE of
capacity values that runs from 17 a.h. at 850 mA discharge
down to 10 a.h. at a 10 Amp discharge.
There is also a plot of battery capacity vs. discharge rate.
Here's an enlarged copy.
http://www.aeroelectric.com/Pictures/Curves/17AH_12V_Capacity_vs_Load.gif
When sizing your battery selection you need to know the current
draw of your alternator-only loads. Then go into the data
for the battery under consideration and extrapolate the battery
capacity AT THAT LOAD. As noted, one can test a battery
under various loads using devices like the West Mountain
cap tester. I have two of those things. Not only are they
useful as programmable load cap testers, you can put them
into a very low current mode (0.01A) and use them as a
data acquisition system.
For example, when studying the performance of the various
battery charger product, the West Mountain cap tester
was used to acquire voltage plots like these:
http://www.aeroelectric.com/Pictures/Curves/Battery_Minder_Recharge.pdf
http://www.aeroelectric.com/Pictures/Curves/Battery_Tender_Recharge.pdf
http://aeroelectric.com/Pictures/Battery_Chargers/Schumacher_Chargers/SC2500-50AH.jpg
The AA alkaline battery studies I've published over the
past several years use this same piece of test equipment
and associated software to gather a publish families of
performance curves on various battery products. See:
http://www.aeroelectric.com/Pictures/Curves/AA_Battery_Tests_80622.jpg
Like yourself, I've found the West Mountain Radio battery
tester to be very useful.
http://westmountainradio.com/CBA_ham.htm
Keep in mind also that your battery has a service-life.
The industry benchmark for battery sizing is to pick
a device with about 125% more capacity-at-load than
you need for meeting design goals. Then replace the battery
when it falls to 80% of as-new capacity so that those
design goals are maintained.
Obviously, these considerations will drive you to
install a battery with a name-plate rating that is
substantially larger than the service rating that
meets your design goals.
Bob . . .
