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Author Topic: Battery for Field Day  (Read 1467 times)
KD8IZZ
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Posts: 282




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« on: June 16, 2009, 09:22:54 AM »

This will be my first field day and I plan to operate my FT-450 on top of a high ridge in SE Ohio (QTH #2). I will drag my hustler 4BTV up to the ridge top and operate at low power from a marine battery.
I calculated that 25 watts will take 6 amps which means a large battery will last the entire day with fairly frequent QSOs.
I need some advice from those with experience. Would a 115 amp hour battery be sufficient for 12 hours of contesting at 25 watts? Or should I go to lower power?
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N3QE
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« Reply #1 on: June 16, 2009, 06:01:16 PM »

My two cents: your rig will take about an amp on receive, and up to 4 amps on transmit if you limit yourself to 25W out.

120AH will easily get you 40 hours of operation at those levels, even assuming a high transmit duty cycle.

Your rig would draw circa 15-20 Amps doing 100W out, but that's only when you're transmitting. It looks like you'd have enough power to run at 100W for a full 24 hour day if you had a reasonable duty cycle (transmitting less than half the time).

Unless your battery is a ginormous gel-cell or a marine deep-cycle battery, you do not want to run it all the way down.

My station has been completely solar powered for the past two months. I use an old Ten-Tec Triton which only draws 0.5A on receive, have an 80W solar panel, and a 80AH marine deep-cycle battery. I've operated through several contests overnight and never came close to running the battery low. Then again, transmit duty cycle was very reasonable.
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N4OGW
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Posts: 297




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« Reply #2 on: June 16, 2009, 06:10:52 PM »

Note that to be in the "Battery" category for Field Day, you have to run 5 watts or less RF output.

Tor
N4OGW
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K3AN
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Posts: 787




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« Reply #3 on: June 17, 2009, 03:17:09 PM »

Depending on how well the FT-450 handles low DC supply voltages, you may not be able to use the full capacity of your marine battery. Check the specs in the FT-450 manual. It may say something like "Supply Voltage: 13.8 Volts DC +/- 10%." In this example, the minimum would be 13.80 - 1.38 = 12.42 Volts. When a deep cycle battery hits that level it still has more than half of its capacity remaining, but the radio can't use it.

Use whatever numbers are shown in your manual to calculate your radio's minimum voltage. Once the battery voltage drops below that level, you're skating on thin ice. The radio may still operate properly, or it may not.

To be able to use all the "juice" your battery is capable of putting out, you'll need a "battery booster" like MFJ and N8XJK make. The booster can take a battery voltage as low as 10 Volts and boost it up to 13.8.
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KD8IZZ
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Posts: 282




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« Reply #4 on: June 21, 2009, 05:07:04 PM »

Thanks everyone. I will endeavor to operate 5 watts from the ridge top.

Only if no one can hear my will I operate at higher watts. I've created special cards for Field Day so I better get a lot of contacts.
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AB7E
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Posts: 117




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« Reply #5 on: June 26, 2009, 12:47:02 AM »

I usually operate Field Day with just 5 watts from my Elecraft K1, but a few years ago I used a 115 amp-hour deep cycle battery to power a rig similar to an FT-100D (it was a friend's rig and to be honest I don't remember what it was).  I ran 100 watts on CW and called CQ repetitively, and about 16 hours into the contest I had to switch to a second battery.

Limit yourself to 5 watts if you want to score points (because of the higher multiplier), but I believe you could probably run the entire contest at 50 watts if your battery was in good shape and fully charged when you began.  The math bears that out ... an average of 30% to 50% of peak current while transmitting, a duty cycle of maybe 30% transmitting (versus tuning and listening to the other guy's report), and maybe a battery discharge to 40% capacity before the voltage becomes an issue.

73,
Dave   AB7E
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N3OX
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« Reply #6 on: June 27, 2009, 10:21:07 PM »

"maybe a battery discharge to 40% capacity before the voltage becomes an issue.
"

I bought one of those "battery booster" DC-DC converters just for this reason.

Keeps the voltage up at 13.8 so you don't have to stop operating when the battery voltage drops to 11.wherever your radio starts doing strange things.

This year I operated about four hours in the backyard so I didn't really need it, but it's a nice thing to have around.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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