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DC clamp meter

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blues750

Joined: 06 Jun 2015
Posts: 56

Posted: Sun May 03, 2020 7:20 am Post subject: DC clamp meter

Looking for recommendations on a suitable DC clamp meter to do capacity checks on my EarthX batteries.  Prices seem to be all over the place.  Am curious what kind others might be having good experience with.  Thanks all!



Dave

nuckolls.bob(at)aeroelect
Guest

Posted: Sun May 03, 2020 6:27 pm Post subject: DC clamp meter

At 10:20 AM 5/3/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "blues750" <den_beaulieu(at)yahoo.com>



 Looking for recommendations on a suitable DC clamp meter to do capacity 

 checks on my EarthX batteries.  Prices seem to be all over the place.

 Am curious what kind others might be having good experience with.  Thanks all!



 Dave 	


  Help us out with your mission. A 'capacity test' is

  a measured depletion and recharge of a battery

  to deduce its contained energy.



  This could be accomplished at a prescribed discharge

  rate commensurate with your anticipated 'endurance

  loads' (useful capacity of a battery varies with

  load).



  In the attached chart you can see how a battery

  rated at 18Ah under a 20Hr discharge rate

  delivers only 12Ah if discharge over 1Hr rate.

  2/3rds of its capacity is tossed off as internal

  heating of the battery.



  To accomplish a capacity test, you need a means

  for establishing a constant current load at

  some selected rate while recording the battery's

  output voltage until it falls below some

  artfully selected cut-off voltage.



  Alternatively, consider a device like

  this:



  https://tinyurl.com/moaoaus



  I have used these on numerous projects

  over the years. I'm thinking of upgrading

  my 20+ year old device. The newer ones

  have some really nice software features.



  I'm wondering if you're interested in the

  battery's cranking (load bearing) characteristics.

  This is a different test that quantifies the

  battery's internal resistance -and- state of

  the chemistry under heavy load.



  For this you need a critter like this



  https://tinyurl.com/yag4529y



  I have a couple of these HF testers . . .

  got a $high$ AutoMeter tester too but

  the el-cheeso works just fine for our

  purposes.



  You hook this to your battery under test.

  Just before you crank up the load such that

  battery voltage falls to 9V, push the 15

  second timer button. Adjust load

  during the 15 second interval

  to maintain 9V reading. Take note of

  the CURRENT delivered by the battery

  at the end of 15 seconds.



  Acquire this benchmark value for a new

  battery. Note the value with a sharpie

  on the battery case along with date.



  At any later time, should the battery

  perform to less than 80% (*) of new,

  consider replacing the battery. Same

  thing goes with capacity checks 80%

  is the 'retirement' value for

  batteries in TC, flight-for-hire aircraft.



  (*) Of course, you CAN establish your own

  benchmarks if some other numbers make

  more sense for how you operate your airplane.

  But pick other numbers based on understanding,

  not because your trying to squeeze more

  service out of the battery.



  EARTHX batteries (and all other LIFePO4

  batteries) are special cases. Your load

  test value will probably be more than

  9V . . . internal resistance of these

  batteries is very low and you probably

  can't load it down to 9v without

  tripping internal current limits.



  If you want to capcheck your EARTHX,

  fire up your airplane's anticipated

  endurance mode accessories and measure

  the time before auto-disconnect. Make

  note of that time for the new battery.

  Think about replacing the battery when

  those minutes fall by 25% or so.



  As for DC clamp-on ammeters, they've

  been around for some time. I have

  a Fluke meter I bought about 20 years

  ago . . . hall-effect device . . . works

  mostly okay for large measurements

  (10+ amps) but flakiness typical of

  contemporary technology at low current

  levels.



  I have one like this



  https://tinyurl.com/y9gb4cgs



  . . . but haven't had much occasion to

  exercise it's capability. It has a

  ZERO read function on DC amps that

  wipes out most of the variability

  in the hall-sensor at low current

  readings. Seems to work well. Will follow

  up when I've learned more . . . but

  the price/performance numbers are

  certainly attractive. 



 

 

   Bob . . .

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blues750

Joined: 06 Jun 2015
Posts: 56

Posted: Mon May 04, 2020 9:12 am Post subject: Re: DC clamp meter

"Help us out with your mission."



Hi Bob, I simply want to put a typical inflight load on the fuly charged battery and perform the below method /test as my benchmark for battery capacity check and/or replacement.  When at 80% of the benchmark, replace battery. I was going to use old style truck headlight bulbs for a resistive load.   Cranking capacity is not a significant concern for my style of flying operation... (famous last words?)  Thanks for input and reply so far!!



From the Earth FAQ's listing...



Test Method #2 (with timer and DC current meter):



a. Fully charge the battery with an appropriate charger.



b. Turn on all electrical loads and start a timer. The same load must be used in subsequent years for accuracy.



c. Measure and record the battery’s discharge amps using a DC clamp-on current meter at the positive terminal of the battery.



d. Using the measured amps in the previous step and the battery’s nameplate rated capacity (in Ah), calculate the time to discharge the battery to 80%.



Time to Discharge 80% (Hours)=Rated Capacity in Ah * .8Measured Discharge Amps



For Example (16 Ah Rated Capacity, 5 amp measured discharge rate)



Time to Discharge 80%=16 Ah * .85= 2.56 hours



e. Terminate the test after the number of hours calculated in the previous step has expired or if the battery is over-discharged (shuts off discharge current). If the battery is still supplying power at the termination of the test, then the battery’s capacity is greater than 80%. If the battery’s capacity is greater than 80% of it rated or capable of supporting the aircraft’s emergency load for the required amount of time, then the battery has passed the test.



f. Fully charge the battery with an appropriate charger.[/i]

nuckolls.bob(at)aeroelect
Guest

Posted: Tue May 05, 2020 8:56 am Post subject: DC clamp meter

At 12:12 PM 5/4/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "blues750" <den_beaulieu(at)yahoo.com>



 "Help us out with your mission."



 Hi Bob, I simply want to put a typical inflight load on the fuly charged battery and perform the below method /test as my benchmark for battery capacity check and/or replacement.  When at 80% of the benchmark, replace battery. I was going to use old style truck headlight bulbs for a resistive load.   Cranking capacity is not a significant concern for my style of flying operation... (famous last words?)

  

 >From the Earth FAQ's listing...



 Test Method #2 (with timer and DC current meter):



 a. Fully charge the battery with an appropriate charger.



 b. Turn on all electrical loads and start a timer. The same load must be used in subsequent years for accuracy.



 c. Measure and record the battery’s discharge amps using a DC clamp-on current meter at the positive terminal of the battery.



 d. Using the measured amps in the previous step and the battery’s nameplate rated capacity (in Ah), calculate the time to discharge the battery to 80%.



 Time to Discharge 80% (Hours)=Rated Capacity in Ah * .8Measured Discharge Amps



 For Example (16 Ah Rated Capacity, 5 amp measured discharge rate)



 Time to Discharge 80%=16 Ah * .85= 2.56 hours 	


    If one is going to test the battery in-situ, i.e.

    on the airplane with plan-b endurance loads running,

    then the exact current demand is somewhat irrelevant

    to the test. Just time the interval until electro-

    whizzies shut down. That time represents the present

    capacity-at-plan-b-load.  You can do that with a

    new battery to benchmark T1. The replacement

    milestone would be at T0.80 of the as-new value.



    You should measure or accurately calculate plan-b

    loads to (1) predict plan-b endurance based on

    battery specifications and (2) verify the battery's

    capabilities at the time of your benchmark

    test on a new battery.



    You can easily measure plan-b loads by inserting

    an ammeter in series with your battery lead and

    turning on the plan-b compliment of electro-

    whizzies.  This is a good thing to do to verify

    your load analysis values.  



 

  	  Quote: 	
		  e. Terminate the test after the number of hours calculated in the previous step has expired or if the battery is over-discharged (shuts off discharge current). If the battery is still supplying power at the termination of the test, then the battery’s capacity is greater than 80%. If the battery’s capacity is greater than 80% of it rated or capable of supporting the aircraft’s emergency load for the required amount of time, then the battery has passed the test.



 f. Fully charge the battery with an appropriate charger.[/i] 	


   The Earth-X testing is okay but I think 

   overly complicated. YOUR primary interest

   as the guy sitting in the left seat is

   "how long will the panel stay lit up"?



   Doing the in-situ test tells you what the

   battery will do right now. The 80% value

   is a somewhat arbitrary number established

   by convention and experience with rechargeable

   batteries (when a battery drops to 80% of 

   as new capacity, it's on the down-hill-slide

   toward useless).



   Your multimeter probably has a 10 amp DC

   current measurement capability that can be

   used to quantify your real, plan-b loads.

   You don't need to buy a special instrument

   for this measurement.



 

   Bob . . .

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blues750

Joined: 06 Jun 2015
Posts: 56

Posted: Wed May 06, 2020 5:17 pm Post subject: Re: DC clamp meter

Bob, I cannot imagine how to go about doing the voltmeter method with an electrical dependent engine running...hence my plan...



Once batteries are charged my normal inflight electrical load is approx 20 amps when not using pitot heat. This is consistent with my predicted load analysis. I have a dual ECU electrically dependent engine which requires approx 13 amps to keep me flying (derived from the engine manufacturer's chart).  By shutting down one ECU and a few other items I can safely continue flight and recover to an airfield or suitable airstrip (day VMC) with about the same electrical load of 13 amps.

My plan was to test each battery separately with a 15 amp load (hence the need for a DC clamp meter) and see how long I got energy from the battery before the voltages dropped off below 10V (ECU/engine quitting time) or the EarthX BMS shuts down due to over discharge.  As I've already got a few flying seasons on the batteries, I can only compare my discharge amp-hrs to the battery specs and then use the 80% factor for battery changeout.



Does this sound like a reasonable way to do a meaningful capacity check? It does to me, but what the heck may I be missing?  Seems like the hardest thing to do will be rigging up the resistive dummy load (open for ideas) so I can consistently do the load test per battery year to year.



Thanks again for the insight and ideas!!

gilles(at)elixir-aircraft
Guest

Posted: Wed May 06, 2020 5:41 pm Post subject: DC clamp meter

Le 07/05/2020 à 03:17, blues750 a écrit :

 	  Quote: 	
		   



 Bob, I cannot imagine how to go about doing the voltmeter method with an electrical dependent engine running...hence my plan...



 Once batteries are charged my normal inflight electrical load is approx 20 amps when not using pitot heat. This is consistent with my predicted load analysis. I have a dual ECU electrically dependent engine which requires approx 13 amps to keep me flying (derived from the engine manufacturer's chart).


	


Performing actual voltage and current measurements at cruise power would 

be interesting.



It can safely be done on the ground.



FWIW,



-- 

Best regards,

Gilles

http://contrails.free.fr

nuckolls.bob(at)aeroelect
Guest

Posted: Fri May 08, 2020 6:04 pm Post subject: DC clamp meter

 	  Quote: 	
		  Does this sound like a reasonable way to do a meaningful capacity check? It does to me, but what the heck may I be missing?  Seems like the hardest thing to do will be rigging up the resistive dummy load (open for ideas) so I can consistently do the load test per battery year to year.\ 	


   Okay, got a tighter box around your goal. Probably

   doesn't need to be that complicated. You know your

   endurance loads to a reasonable degree of accuracy.

   

   The really cool thing about Li batteries is their

   very FLAT discharge curve as a consequence of low

   internal resistance. This means that energy delivered

   to the terminals is relatively constant irrespective

   of load. So consider getting some of these.



  https://tinyurl.com/ydg62u2r



   an order of 5 pcs is about 8 bux delivered to your

   door. Mount 4 of these to a chunk of aluminum say

   6 x 6 inches or so. .06 or more thick. Space them

   out over the surface. Hook all four in parallel

   to make a robust, 1.0 ohm resistor.



   This resistor connected to one of your batteries

   will discharge right at 12.5 amps. Time the interval

   to discharge limit with this load.



   Since you have quite a bit of time on these

   batteries, you don't have an as-new benchmark

   but you now have as-is benchmark.



   Maybe the a.h. number isn't really that

   useful. Rather, it's time to trip

   the discharge limiter. That's pretty close

   to your as-is endurance value. When

   that number is below design goals, change

   the battery.

   



 

 

   Bob . . .

racerjerry

Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY

Posted: Sat May 09, 2020 3:24 am Post subject: Re: DC clamp meter

20 BUCKS     WHADDAYA THINK?



https://www.harborfreight.com/automotive/battery-tools-accessories/100a-612v-battery-load-tester-61747.html



If discharge rate is too high for your application, my low budget method for current limiting has always been either an ignition coil resistor from an old car or a group of incandescent light bulbs in parallel (adjustable).



Jerry King

_________________
Jerry King

blues750

Joined: 06 Jun 2015
Posts: 56

Posted: Mon May 11, 2020 4:54 pm Post subject: Re: DC clamp meter

Bob, finally able to do the calculation on how you get a 1 OHM equivalent resistor out of the four resistors rated at 4 ohms in parallel along with power dissipation through the resistors...what a blast from the past! Thank you so much for the help and interest.  I can't wait to get some data!



Dave (most appreciative novice experimentalist)

nuckolls.bob(at)aeroelect
Guest

Posted: Mon May 11, 2020 7:20 pm Post subject: DC clamp meter

At 07:54 PM 5/11/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "blues750" <den_beaulieu(at)yahoo.com>



 Bob, finally able to do the calculation on how you get a 1 OHM equivalent resistor out of the four resistors rated at 4 ohms in parallel. What a blast from the past!  I'll take your word on the Power rating being suitable!! Thank you so much for the help and interest.  I can't wait to get some data!

 	


    Yup . .. 



                        1

    RT = ====================================

            1         1        1        1

          ____      _____    _____    ______

           R1        R2       R3        Rx



    Rtotal equals the reciprocal of the SUM

    of reciprocals of all resistors in the

    array . . .  no matter how many resistors.



    It just so happened that your target current

    worked out close to a 1-ohm load . . . so

    the array of 4 x 4 ohms works nicely.



    The total wattaged needed is much less than

    to total ratings of thsoe four resistors.

    They're offered as 100W devices . . . but

    in practice, they're closer to 50W when

    adequately heat-sinked. So the sum total

    of the 4 is comfortably good for 200W.



    You only need 13A x 13v is 169W.



    This bucket-o-resistors technique is 

    very versatile. For example, a couple

    years ago I crafted a portable, 750

    watt, variable load bank for the purpose

    of airborne cooling tests on alternators.



    An array of 21 resistors not unlike the

    ones I suggested plus a handful of components

    and a fan made for a device that would

    offer loads from 0 to approx 50A fan

    cooled.



    I've used this technique to craft load banks

    up to 2800 watts (28v, 100A) . . . with

    a lot more resistors and a bigger fan . . .

    but knob-adjusted at the control panel.



 

 

   Bob . . .

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blues750

Joined: 06 Jun 2015
Posts: 56

Posted: Wed May 13, 2020 10:02 am Post subject: Re: DC clamp meter

Bob, that's pretty clever.  In my case  how "fancy" should I go? For connections to the load from the battery,  I was thinking of simply using two leads of #10 wire with alligator clips on both ends and having an inline fuseholder on one lead with a 15A fuse.  Or...do I fabricate a switched/fused platform, with ring terminals for connections, etc?  User preference?

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