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Author Topic: battery question  (Read 5808 times)
VK4TJF
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« on: January 06, 2012, 10:13:31 PM »

 ok i intend to do some portable operations I have a icom 7200 that drains like 22 amps on transmit
and I have a 50 amp hour battery how long can I run the rig for on say 100 watts and 10 watts
using cw? keep in mind that the radio likes to see not much lower then 12 volts to operate without problems.
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WB6BYU
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« Reply #1 on: January 06, 2012, 10:28:46 PM »

What is the current draw on receive?  On transmit at 10 watts?  How much time do
you spend transmitting vs. receiving?

Basically you start by calculating the average amps consumed for each hour of
operation.  Let's say you are carrying on a CQ QSO, so you are listening half the
time and transmitting half the time.  When transmitting the carrier is only keyed
about half the time, so we'll say you are receiving 75% and transmitting 25%.
Then your average current draw over one hour is 75% * the receive current for
your rig + 25% * the transmit current for the power level you plan to use.
(That would be 22A at full power, but could be 2 to 5 amps at 10W.)

That gives you the average current per hour, then we have to look at the battery
capacity.  I'd start by cutting the rated capacity in half, because the batteries
don't last very well if you keep draining them all the way.  You may need to back
off a bit more to maintain the 12V level - this will also depend on the voltage drop
in your cabling.  The manufacturer should have datasheets online that show you
the expected capacity vs. terminal voltage for different discharge rates, but in
this case a factor of 1/3 is probably reasonable, perhaps a bit conservative.

So if we assume 1A for receive, 4A for transmit at 10 watts and 20A for transmit
at 100W (because they make it easy), we can run some quick calculations:

At 100W, average current is 0.75 * 1 + 0.25 * 20 = 5.75A.  Battery capacity is
50 / 3 = 17Ah.  You could operate for about 3 hours.

At 10W, average current is 0.75 * 1 + 0.25 * 4 = 1.75A and you could operate
for about 10 hours.

Obviously you have to run these calculations with the numbers for your specific
radio, and the equations will shift if you do a lot more listening than transmitting.
But you should see how the equations work.
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KH6AQ
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« Reply #2 on: January 07, 2012, 12:56:22 AM »

On receive the current draw is about  1 amp. Average draw with
CW will be about 6 amps if you TX half the time. You can take about 25 A-H of charge from your 50 A-H battery giving you 4 hours on the air.

But with real world operating most of the time one is receiving. So you are looking at 12 hours or more of operation.

Running a 100 watts radio at 5 watts during a 24 hour contest I can power the radio off of a single, new 40 amp-hour automobile battery. The power draw at 10 watts should be essentially the same as at 5 watts. The 100 watt rigs I have used are very inefficient when transmitting at QRP levels.
« Last Edit: January 07, 2012, 08:07:46 AM by WX7G » Logged
K8AXW
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Posts: 6505




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« Reply #3 on: January 07, 2012, 09:56:30 AM »

Whenever I'm confronted with a question like that, instead of calculating numbers which seldom represent real life, I would operate the radio from the battery at home and see what happens. 

Do this a couple times and you will have your real life answer!
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KB1LKR
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« Reply #4 on: January 07, 2012, 10:10:01 AM »

It should be noted that for Pb-acid batteries the published A-hr rate is generally based on a 20 hour discharge... the actual capacity for a few hours to fully discharged may be only 70-80% of the twenty hour rate (due to greater I^2*R l losses from the higher current thru the battery's internal impedance [R]), You might, based on an initial estimate of 3 hours, assume 0.75*50 A-hrs (call it 37 A-hrs in round numbers) or just assume 0.75*3 hrs or 2 1/4 hours. Actual trial will give a better real life answer, the calcs. give you an initial estimate though.

For NiCd & NiMH (and Li-ion?) cells the capacities are generally based on a 5 hour cycle, but the de-rating is likely similar, e.g. 1 vs. 5 hrs for NiMH is probably similar to 4 vs. 20 hrs for lead acid.

See manufacturer's detailed/OEM spec sheets for capacity vs. rate graphs.
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W8JX
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« Reply #5 on: January 07, 2012, 10:43:45 AM »

It should be noted that for Pb-acid batteries the published A-hr rate is generally based on a 20 hour discharge... the actual capacity for a few hours to fully discharged may be only 70-80% of the twenty hour rate (due to greater I^2*R l losses from the higher current thru the battery's internal impedance [R]), You might, based on an initial estimate of 3 hours, assume 0.75*50 A-hrs (call it 37 A-hrs in round numbers) or just assume 0.75*3 hrs or 2 1/4 hours. Actual trial will give a better real life answer, the calcs. give you an initial estimate though.

For NiCd & NiMH (and Li-ion?) cells the capacities are generally based on a 5 hour cycle, but the de-rating is likely similar, e.g. 1 vs. 5 hrs for NiMH is probably similar to 4 vs. 20 hrs for lead acid.

See manufacturer's detailed/OEM spec sheets for capacity vs. rate graphs.

Very good points and few realize this. Many look at a battery the same way they look at a glass of water in that you get same amount out no mater how fast you drain it. That is not the case though.
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KE3WD
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« Reply #6 on: January 07, 2012, 12:20:28 PM »

Excellent online resource for all things Battery: 

http://batteryuniversity.com/learn/



73
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K5LXP
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« Reply #7 on: January 08, 2012, 07:11:13 PM »

The challenge in predicting run time depends a lot on how well the load is characterized (actual currents during operation, and time spent at each current level).  If these are known accurately, then the run time can be pretty accurately determined as batteries are well characterized and understood.

The major factors to understand are the load, battery capacity with Peukert effect factored in, and at what voltage the radio will no longer perform correctly.  Nearly all HF transceivers today are spec'd at 13.8V +/- 15%, which on  the low side puts you right at the 50% capacity point of lead acid batteries.  That means the radio will poop out when the battery is only half empty.  Put another way, you need to haul around twice as much battery as you can effectively use.  Or use an inverter which allows you to use all of the available amp hours with a minor hit on efficiency.


Mark K5LXP
Albuquerque, NM
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KQ6Q
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« Reply #8 on: January 08, 2012, 07:55:02 PM »

If any of your portable operation will be in daylight, pick up some portable solar panels that will put out a couple of amps, and put those on the battery and rig during the day, you'll mainly drain the battery after the sun goes down.
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G4AON
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« Reply #9 on: January 09, 2012, 12:31:30 PM »

I am guessing your battery is a lead acid type, they don't last long when you discharge them significantly. I have been using a deep discharge 110 Ah battery for 4 years of occasional portable summer time use and it's now past it's best. I run a 100 Watt radio (Kenwood TS-480SAT) for 7 to 8 hours at a time on 6m at full power TX using CW and SSB modes.

Using a battery of half that capacity is going to give the battery a hard time. The manufacturers like to publish amazing graphs showing depth of discharge and number of cycles, etc. Unfortunately few of us get anything like that performance in reality.

73 Dave
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K5LXP
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« Reply #10 on: January 10, 2012, 05:39:21 AM »

The manufacturers like to publish amazing graphs showing depth of discharge and number of cycles, etc. Unfortunately few of us get anything like that performance in reality.

It works out that most lead acid batteries are rated for at least 500 cycles to 80% depth of discharge.  It also works out that you can reliably get about 5 years of calendar life.  That translates to two very deep cycles every week, for 5 years.  I suspect very few hams would ever cycle their batteries that deeply, and that often.  Therefore the primary failure mode for any ham radio standby battery is going to be improper charging or storage, or calendar life.  Certainly not the depth of discharge or frequency of use.


Mark K5LXP
Albuquerque, NM
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KH6AQ
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Posts: 7718




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« Reply #11 on: January 10, 2012, 07:48:26 AM »

As I pointed out from many instances of real world portable contest operation you can get about 12 hours of operation at full TX power and 24 hours at 10 watts.
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W8JX
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« Reply #12 on: January 10, 2012, 07:54:36 AM »


It works out that most lead acid batteries are rated for at least 500 cycles to 80% depth of discharge.  It also works out that you can reliably get about 5 years of calendar life.  That translates to two very deep cycles every week, for 5 years.  I suspect very few hams would ever cycle their batteries that deeply, and that often.  Therefore the primary failure mode for any ham radio standby battery is going to be improper charging or storage, or calendar life.  Certainly not the depth of discharge or frequency of use.


And what do you base this on as most "deep cycle" batteries will be dead or with very limited capacity after 500 cycles of 80% discharge. 200 to 300 cycles is more realistic limit for a deep cycle battery but even it will have reduced capacity after 300 cycles of 80% discharge. A car battery will be shot after 10% of that amount or less of 80% discharge cycles and a few deep discharges to take it flat can kill it. The less you discharge in terms of total capacity, the longer it will last. 50 or 60% tops is better for longer life.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K5LXP
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« Reply #13 on: January 10, 2012, 11:28:19 AM »

And what do you base this on

Manufacturer data sheets.  Trojan, Yuasa, Optima, et al.


Quote
most "deep cycle" batteries will be dead or with very limited capacity after 500 cycles of 80% discharge.

Depends on the brand and type.  The venerable 7Ah gels are good for about 350 cycles, some AGM's and floodeds are good for 700 or more down to 80% DOD.  A bargain basement flooded marine battery, maybe 250.  I would offer that even 250 cycles is way more than a typical ham would come close to using.


Quote
A car battery will be shot after 10% of that amount or less of 80% discharge cycles and a few deep discharges to take it flat can kill it.

Agreed, an SLI battery is totally misapplied in cyclic service.


Quote
The less you discharge in terms of total capacity, the longer it will last. 50 or 60% tops is better for longer life.

Absolutely correct, but there's a cost per Ah that needs to be factored in.  Does it make sense to "baby" a battery by buying one twice as big, or only drawing a max of say 50% out of it, only to leave cycles behind when it expires due to age?  The ideal scenario is to size the battery to the application such that it has zero cycle life left at the same time it's expiring from age.  If the battery is only being cycled a few hundred times or less there's little economy to "conserve" it, because it will expire from age no matter how many cycles it has left or how gently it was used.

So, on the conservative side lets say all you're going to get is 300 cycles.  Over 5 years that's still more than one cycle per week.  You're just not going to see that frequency of use in a standby/casual ham application.  For most hams it may be a few ARES exercises, Field Days and occasionally when the power goes out.  Might as well run it into the ground, you'll never use it up that rate.

It works out that most storage batteries expire from improper charging and storage.  Namely, it sits out in the garage or the shed between uses, gets a quick boost before taking it out, then it sits idle again.  You may not get two dozen cycles out of battery treating it like that and certainly ensuring a max of 50% DOD won't change that outcome. 

There are some applications that are very DOD sensitive, namely RE installations where there is a lot of money riding on battery performance and longevity.  A guy running a rig off of a deep cycle battery once in a while isn't one of them.


Mark K5LXP
Albuquerque, NM
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WB6BYU
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« Reply #14 on: January 10, 2012, 12:27:04 PM »

The main reason for limiting the Depth of Discharge in ham use is the minimum terminal
voltage that the radio can operate on.  This is a function of the internal resistance and
the load current, and hence the output power, plus the voltage drops in the wiring.
I had one mobile rig that would lock in transmit mode when operated at more than 20
watts when the car engine was off.  After I improved the connections in the power circuit
it only acted up at 50 watts or more.

But a radio that sensitive to battery voltage isn't likely to work well at 80% discharge.
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