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Battery Choices - what is too small AH capacity?

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nuckolls.bob(at)cox.net
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Posted: Thu Aug 14, 2008 5:13 am Post subject: Battery Choices - what is too small AH capacity?

At 07:55 PM 8/13/2008 -0500, you wrote:

 	  Quote: 	
		  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 . . .

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nuckolls.bob(at)cox.net
Guest

Posted: Fri Aug 15, 2008 7:33 am Post subject: Battery Choices - what is too small AH capacity?

At 01:15 PM 8/13/2008 -0700, you wrote:

 	  Quote: 	
		  



Ken,



You wrote: 8 AH Dekka AGM batteries



Can you describe the battery terminals on these Dekka batteries? Even a 

digital photo would be helpful. I have #2 welding cable with #2 lug 

terminals from the main contactor to the main battery terminals. I'm a bit 

concerned my existing battery terminals are too small for long term use & 

vibration resistance.


	


    The driving considerations for battery selection are

    (1) capacity at endurance loads, (2) internal impedance

    at cranking loads and (3) ability of the terminals to

    carry cranking currents.



    (1) Capacity is another discussion.



    (2) For a battery to be a successful engine cranking

    source, 10 milliohms new is a pretty fair benchmark

    for maximum internal impedance. I mention this

    because there are very compact technologies with

    limited capacity (1 a.h. or so) that will dump over

    1000 amps of short circuit current! Smoke from a

    flash-in-the-pan blew through the marketplace

    about 10 years ago when Bolder Technologies

    attempted to launch a thin metal film battery about

    the size of a c-cell. A host of products sprang

    up using this device. "Start Stick" was shown around

    OSH several years. I think Sears marketed a very

    compact car cranking accessory for a time. These

    cells didn't make it for reasons of process limitations.

    They couldn't keep a good electrical grip on the

    edges of the films . . . and had some sealing problems

    too. They're gone now, but they (or something similar)

    will be back.



    For the moment, our practical choices for cranking

    batteries are probably not smaller than 10 a.h.

    or so. B&C has a little 12 a.h. battery that will

    crank engines:



http://www.bandc.biz/BC103-1.pdf



    Concorde used to have a 10 a.h. device that would

    crank but I don't see it listed any more.



    (3) Terminals. You can get a 17 a.h. battery

    with .25" faston tabs, and fat-wire connection

    posts. Both batteries are capable of cranking

    engines with respect to internal construction

    but fast-on tabs are obviously not capable of

    delivering that kind of current.



    IF you find a battery with a very low internal

    impedance, then terminals should be on the order

    of 1/4" bolts through hefty metal tabs -OR-

    10-32 (4mm) minimum screws into brass inserts.



    To prevent damage to your battery's connections

    due to vibration and stress . . . make all

    connections to batteries with 4AWG welding

    cable between battery(-) and ground; battery(+)

    and battery contactor. This is a good thing

    to do even if the rest of your airplane is wired

    with 00 fat-wires.



    After (2) and (3) are satisfied, then capacity

    is a function of meeting design goals for weight,

    cost of ownership, and battery-only endurance.



    Bob . . .

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