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Author Topic: Calculating reserve time, check my understanding and math for me?  (Read 4617 times)
GRANDKODIAK
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« on: May 12, 2012, 03:29:43 PM »

I have an auxilary battery in my car that is rated for 110 reserve minutes. Online says that that number is derived on how many minutes a battery can provide 25 amps before it dies.

So if the battery ran my radio 12.5v (right?) at max output (50 watts) which is a 4 amp draw...

(110 min x 25 amps)= 2750 amp life

50 watts / 12.5v = 4 amp draw at max power

2750 amps / 4 amps = 687.5

687.5 / 60 minutes = 11.45 hours broadcast till dead battery?

or

2750 / 687.5 = 4 hours broadcast till dead battery?

Sorry, it's been awhile since science class and I hate math Smiley I'm getting lost in the radios current draw, whether that 4amps is per minute or hour?
« Last Edit: May 12, 2012, 03:31:33 PM by GRANDKODIAK » Logged

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KB3HG
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« Reply #1 on: May 13, 2012, 04:02:23 AM »

Q) I have an auxilary battery in my car that is rated for 110 reserve minutes. Online says that that number is derived on how many minutes a battery can provide 25 amps before it dies.

First thing,  is this a deep cycle or SLA battery (regular starting battery)

Q) So if the battery ran my radio 12.5v (right?) at max output (50 watts) which is a 4 amp draw...

Most radios get flaky when supplied voltage drops much below 11 volts. This auxiliary battery it is isolated or tied in parallel to main starting battery? Parallel is not the best idea.

Q) (110 min x 25 amps)= 2750 amp life

Where did 25 amps come from?
for easy numbers; round 110 min up to 120 minutes or 2 hours, 2 hours x 25 amps = 50 ahr (amp hours). A big Battery.

 A wild rule of thumb if you want the battery to have a long life discharge would only be 20% of total capacity.


Q) 50 watts / 12.5v = 4 amp draw at max power

     2750 amps / 4 amps = 687.5   

     687.5 / 60 minutes = 11.45 hours broadcast till dead battery?

      or

     2750 / 687.5 = 4 hours broadcast till dead battery?

The Battery voltage will drop as time increases when in use. In other words,all your initial suppositions for your calculations are based off steady state terminal voltage. A decrease in voltage will manifest an increase in current draw. 
  A battery is not a linear device.

It would be best to come back with battery type and size. from there it would be much easier to figure out.
Type 24, 27 ect, deep discharge
The manufacturer should have specs. Remember as temperature drops so does capacity, same thing if the temperature rises.  Above spec. I  read an article some time ago about how batteries failed in hot climates, don't remember where but thought I'd include that comment.

The next item, if the battery is isolated from the starting battery starting the engine should recharge the battery. The Gas tank should keep it charged many many hours.

Myself, I have a 900w 2 cycle generator a power supply can be attached to the auxiliary battery and the batter charged. Advertised run time in the ball park of 11 hrs +/- 50% load.
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GRANDKODIAK
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« Reply #2 on: May 13, 2012, 06:03:31 AM »

I would have an isolator system with (national luna solenoid and dual charge meter http://www.columbiaoverland.com/catalog/batteries-kits)

The battery in there now is a Autozone Duralast 34 DT-DLG (dual terminal) but they dont advertise the amp hour ratings.
800 cold crank amps, 1000 cranking, 115 minute reserve. I'm guessing its a standard because it doesnt say deep cycle or AGM.

I would consider thier marine deep cycle or dual optima's, but for now I'd probably either get two new ones of the above, or 1 new one if i can do that safely, seeing as they are ~$100ish and the others are double that.

I got the 25 amp while trying to convert the advertised minutes by the desciption that that rating comes from how many minutes a battery can supply a steady 25 amps before it fails. im just guessing here, thats why I asked you fine folks to double check my guestimates Smiley

I have a 2000 Land Rover Discovery, and while the engines in decent shape, I would never idle it if I didn't need to... theyre are many design flaws, and almost all common problems come from overheat stress (factory idle fluctuates 195-220 as the fans kick on) so I'd never trust it in the woods or on the trail just to operate radios etc... thats why I'd love a dual battery system for this particular vehicle.

ps. I'm in NJ, so it never gets artic cold nor desert hot for long, but there are a few days in winter that can peak -5F overnight wind, and 105F freak summer days... neither condition usually lasts more then a week, and is rare. I'd say the average cold overnight would be lowest 15F and daytime hot summer high ave 95F? but the engine bays much hotter in this british/bosche/buick hodgepodge POS cooker! Wink
« Last Edit: May 13, 2012, 06:06:45 AM by GRANDKODIAK » Logged

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K5LXP
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« Reply #3 on: May 13, 2012, 07:24:29 AM »

Your ~12 hour run time at 4Ah is correct, but KB3HG's observation about terminal voltage over the discharge time must be accounted for.  Radios are typically rated for 13.8V +/- 15%.  So that puts your minimum usable voltage about 11.75V, which under load, is about 50% of your battery's capacity.  Suddenly your 50Ah battery only has 25 *available* amp hours.

Another factor to consider that as an SLI battery, it won't have great cycle life.  A couple dozen deep discharges will render this battery toast.  You'd be better off with a deep cycle battery for a cyclic applications.

You need to characterize your load to represent your true duty cycles.  Determine your exact receive and transmit currents, and transmit/receive times.  You can much more accurately gauge your battery run time that way.

Lastly, research Puekert Constant.  By applying this Ah correction factor to your known current draws, you can very accurately determine your run times.

Same question, different forum:  http://www.eham.net/ehamforum/smf/index.php/topic,72368.0.html

Mark K5LXP
Albuquerque, NM
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KB3HG
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« Reply #4 on: May 13, 2012, 07:50:11 AM »

http://live-n-learn.hubpages.com/hub/Choosing-a-Car-Battery---Guide

3 Battery Manufacturers and their Battery Brands:

Delphi - AC Delco and some EverStart;
Exide - Champion, Exide, Napa, and some EverStart; and
Johnson Controls Industries - Diehard (Sears), Duralast (AutoZone), Interstate, Kirkland (Costco), Motorcraft (Ford), and some EverStart.


I buzzed through an article about a sears 34 dg didn't copy it but it said 68 amp hours.

7900r 13.8v +/-15%   about +/-2.07V 11.73v low side.
9 amps 45 watts uhf tx
68ahr/9a = 7.55 hours if battery could do that until ) energy
about 2 hours use 100% transmit to utilize 20% battery capacity. If that helps. Now reality is you won't be 100% transmit.

Now here is a site about mobile operation that has more answers than most people have questions. Al Has done a outstanding job on his website.
  WWW.K0BG.COM

73,
Tom Kb3hg
      
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N6AJR
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« Reply #5 on: May 13, 2012, 12:18:21 PM »

First,  If it is not a deep cycle battery, then if you run it down to  10 or 10.5 volts a couple of times, like 3 or 4 , then it may kill the battery.  the starting battery is made for a large draw ( 200 amps) for a short time ( 10 or 15 seconds) and then having the charged replaced  right away.

 A deep cycle battery  will draw down to 10 or 10.5 volts  perhaps 500 times before dying. most batteries are no good below 10.5 volts for radio gear. 

figuring out draw is a bit more than  50 amps divided by 12 volts. most rigs draw  1 or 2 amps  in receive mode, and in transmit they draw the obvious  current figured by your  tx power divided by volts, but you need to add in another couple of amps for the radio operation itself.  a lot of this goes up as heat from the components, so look at the spec sheet to be sure.

Another  trick is to put a small solar panel on the  battery to keep it topped up all the time, perhaps one of the small 175 milliamp ones you throw in the windshield  when parked  ( ICP makes some ) . This will keep the battery up nicely when  not in use.  I use one on my riding mower which I only use 3 or 4 times a year and the battery lasts for 3 or 4 years and always starts .  Do a google on home brew solar and care and feeding of  batteries used in  off grid  storage.
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GRANDKODIAK
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« Reply #6 on: May 13, 2012, 02:33:01 PM »

I actually looked at that idea briefly, I see some guys take out thier 5w mobiles on extended hikes with those fold out backpack solar units and run almost completly off the panels in good light! now that would be the ticket, I have lots of room on my roof for solar Smiley but i see that they give out od voltage numbers like 9v or 15v or come with 120v transformers etc.. haevnt come across one with a simple 12v out, plus, what could you use to stop overcharging? i killed a motorcycle battery with a charger that decided one day not to go into trickle mode Cry
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AA4PB
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« Reply #7 on: May 13, 2012, 03:21:18 PM »

Reserve capacity is the number of minutes that you can draw 25A before the terminal voltage (on a 12V battery) reaches 10.5V when the battery temperature is maintained at 80 deg F.

Change any of those parameters (load current, terminal voltage, or temperature) and the AH capacity of the battery will change. To be safe, figure that your going to get about 1/2 the operating hours that the mfgs numbers indicate.

The standard way to work with battery capacity is AH (Amps/Hour). The battery can supply 25A for 110 min or 1.83 hours. 1.83 Hrs x 25A = 45.75 AH (the AH capacity of the battery under the prescribed conditions). Now divide the AH capacity by the average current draw of the radio to get operating time down to 10.5V. So, 45.75 / 4A = 11.44 hours. Figure that you'll get about half that time if the radio dies at 11.5 volts.

The average radio current draw depends on the receive current, the average transmit current for the mode being used, and the % of transmit time vs receive time. For SSB and CW figure average transmit current is 1/2 of the key down current. For RTTY and FM, figure it at 100% of the key down current.

« Last Edit: May 13, 2012, 03:42:55 PM by AA4PB » Logged
W8JX
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« Reply #8 on: May 14, 2012, 06:44:46 AM »

The standard way to work with battery capacity is AH (Amps/Hour). The battery can supply 25A for 110 min or 1.83 hours. 1.83 Hrs x 25A = 45.75 AH (the AH capacity of the battery under the prescribed conditions). Now divide the AH capacity by the average current draw of the radio to get operating time down to 10.5V. So, 45.75 / 4A = 11.44 hours. Figure that you'll get about half that time if the radio dies at 11.5 volts.

The "problem" with this formula is that a battery is a not a glass of water with same yield regardless of the rate you empty it at. At a 25 amp reserve capacity test it will yield considerably less total capacity than at a lower rate because of efficiency losses frpm internal resistance and gassing of plates. I do agree that I would not draw down to a 10.5 voltage as I believe I would  also draw to 11.5 or a bit lower. One thing I strongly recommend is using a few 100K electrolytic capacitor in parallel with rig. This will greatly steady voltage on rig especially as voltage drops running SSB or CW and extend life of battery some too because it will improve efficiency by reducing peak current draws. You can do same with a mobile rig and you not modulate you lights at times.
« Last Edit: May 14, 2012, 09:05:10 AM by W8JX » Logged

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K5LXP
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« Reply #9 on: May 14, 2012, 11:38:30 AM »

At a 25 amp reserve capacity test it will yield considerably less total capacity than at a lower rate because of efficiency losses frpm internal resistance and gassing of plates.

Correct, and with the Peukert formula you can derive the correction factor based on current draws other than those empirically measured or as stated on a data sheet.  This is what I came up with:

For a 1A receive, 4A transmit radio operating a 20% TX, 80% RX duty cycle it will require 3.4Ah to operate.

Using the Ah ratings supplied in this thread for this battery you get a Peukert constant of 1.125, which is within reason for a flooded SLI battery.

Running the presumed 20/80 duty cycle at those currents, and putting the capacity cutoff at 11.75V, you get an available battery capacity of 32.9 Ah, or 9.7 hours of operating time.

Using even these basic load and capacity calculations you can predict very close to real world results.  Close enough that you know you'll have plenty to spare, or are cutting it too close. 


Mark K5LXP
Albuquerque, NM
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N6AJR
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« Reply #10 on: May 14, 2012, 12:00:51 PM »

a typical solar panel will put out 16 to 17 volts  with "no load" and 12 or 13 volts under load, so they will put out what the battery calls for , more or less. a small panel ( 175 milliamps or so) wwiill keep the  battery topped up.
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W8JX
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« Reply #11 on: May 15, 2012, 05:25:49 AM »


Using the Ah ratings supplied in this thread for this battery you get a Peukert constant of 1.125, which is within reason for a flooded SLI battery.


Given the variables of battery size/capacity and plate to acid capacity the load relative to that, you cannot accurately apply this constant. While it is possible is some scenarios it might be fairly accurate in other it can be the opposite. Only way you can know with reasonable accuracy is to load test battery.
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K5LXP
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« Reply #12 on: May 15, 2012, 06:55:54 AM »

Given the variables of battery size/capacity and plate to acid capacity the load relative to that, you cannot accurately apply this constant.

I was clear that I was using data provided in this forum (I did not independently verify it).  I agree that empirically testing at the target load points is the only way to know exactly what you're going to get with any one battery.  It works out though that if you have a "good" battery (not too old, used up, abused, half charged, etc), using published specs will get you very close and if anything will err on the conservative side.  Example, 7Ah gel cells.  I've used and tested so many of these that as long as I know it's "good", any difference I'll see between them is very small.  The battery specifications take into account all of the variables you mention, and amp hours are amp hours, so the formula will apply if the battery matches the data sheet as long as the load is not extremely different from the points the constant is derived.  For most ham radio applications, the "reserve capacity" rating is close to the typical requirement, so the published specs are usually accurate enough.


Mark K5LXP
Albuqerque, NM






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GRANDKODIAK
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« Reply #13 on: May 17, 2012, 10:41:54 AM »

thanks all! some good info there Smiley
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