[Elecraft] BATTERY SIZING for portable operation

[Alan Bloom]

Another factor to consider when using a vehicle battery is that after an
extended radio operation it may take quite awhile for the vehicle's
charging system to get the battery fully recharged.  With intermittent
vehicle operation it could take weeks for full recovery, risking
sulfation damage.  In that case it would be a good idea to top up the
battery with a charger when you get home.  Say 500 mA until the no-load
voltage gets to 14.5V, which could take 24 hours or more.

Ian Cowan VK1BG had a good article, "The car battery as a low cost power
source" in the Feb 1999 issue of Amateur Radio magazine. (Journal of the
Wireless Institute of Australia http://www.wia.org.au/ )

Al N1AL


On Mon, 2008-02-25 at 15:47, Windy Dankoff wrote:
> This started as reply to Ken Kirkley <no4d at bellsouth.net>
> >
> > Subject: [Elecraft] K3 & Portable Operation
> I got carried away, so I'm making it a new subject.
> It answers your question Ken -- YES, you can operate a 100W model at  
> QRP levels OR at full power, given sufficient battery capacity.
> 
> --------------------------------
> 
> HOW TO ESTIMATE OPERATING TIME FROM A STORAGE BATTERY
> 
> Let's calculate how long a transceiver can operate on a battery, for  
> example a 100AH (amp-hour) unit (a typical RV/marine deep-cycle  
> battery). You can substitute numbers for other battery sizes, and  
> other loads -- QRP, multiple rigs, etc.
> 
> First, you need to know the AVERAGE current draw of the transceiver  
> during "average" operation. It depends on transmit duty cycle so we  
> must make some assumptions. I'll base this example on casual operation  
> of a K3/100.
> 
> K3 draws ~1A on receive.
> On transmit, CW or SSB, I estimate the AVERAGE current is about 10A  
> (using full power).
> 
> Assuming you receive 90% of the time, transmit 10%:
> AVERAGE OPERATING CURRENT = (1A X .90) + (10A X .10) = 1.9A   Round  
> that up to 2A.
> 
> Time available from a battery of 100AH capacity would be:
> 100AH / 2A = 50 hours.
> 
> HOWEVER:
> USABLE battery capacity is less.  If the battery is colder than  
> standard temp. (77°F) and new (not broken in) or well-used, it will  
> have less capacity than its factory rating. It would be safe to de- 
> rate the battery to 75%. So:
> 
> Real time available = 50 hours X .75 = 37.5 hours.
> 
> This will discharge the battery to roughly 11V under moderate load.  
> You don't want to do this habitually to a lead-acid battery -- it's  
> the maximum.
> 
> USING A VEHICLE BATTERY
> Are you car-camping or hill-topping? You can use the vehicle battery  
> if you don't discharge it below about 12.0V. That represents roughly  
> 40% discharge. In cool or warm weather, it should still start the  
> vehicle and is not too abusive to the (non-deep-cycle) battery. It's  
> hard to get true AH ratings for a vehicle battery, but a typical one  
> in a pickup truck is at least 60AH. It may not be in great condition,  
> so let's be conservative and consider only 70% of its capacity.  
> Therefore:
> 
> Usable battery capacity = 60AH X .70 = 42AH
> Using 40% of that (so you can safely start the vehicle):
> Available battery capacity = 42AH X .40 = 16.8AH    round it to 17AH
> 
> Time available from battery capacity of 17AH:
> 17AH / 2A = 8.5 hours
> 
> So, you should be able to operate a 100 watt transceiver all day from  
> your vehicle battery if you aren't too long-winded.
> 
> CAUTION:  Watch battery voltage and don't drop below about 12.0 (under  
> load). Measure battery voltage close to the battery terminals so it  
> ignores voltage drop in your load wiring.
> 
> Next, I'll advise on sizing a solar panel (or array) to extend the  
> running time. I was a solar-electric system designer, supplier, and  
> educator for 25 years.
> 
> Windy Dankoff  KM5Q
> K3 #? due in late March
> 
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