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Author Topic: Type of battery for Field Day  (Read 316 times)

AD6AD

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Type of battery for Field Day
« on: February 13, 2021, 10:34:11 AM »

I plan on using a solar cell for power on Field Day.

I've heard a Gel Cell battery is good for deep depletion and recharge,
but some hams seem to favor the portable car battery "Jump Starters."

Any recommendations on which battery to choose?

Thanks in advance.
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W7CXC

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Re: Type of battery for Field Day
« Reply #1 on: February 13, 2021, 11:05:07 AM »

Depending how big you plan to go....A marine deep cycle battery such as NAPA 8240 or 8270 will work just fine. Gel cell is not really any better than flooded, it is more of a mater of internal construction. Deep cycle have fewer but thicker plates. Less peak amp draw but more capacity at lower amp draw There are regular and deep cycle gel cell batteries as well but lots of $. Absolute best are deep cycle golf cart batteries.  A deep cycle group 31 battery (NAPA 8301) is also a good choice. 73's David
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W1VT

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Re: Type of battery for Field Day
« Reply #2 on: February 13, 2021, 11:30:44 AM »

Scooters and other mobility devices use deep cycle batteries.  A used one may have enough life left for your needs.

Zak W1VT
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KF5LJW

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Re: Type of battery for Field Day
« Reply #3 on: February 13, 2021, 12:25:50 PM »

Do not use gel, especially with solar. You want to use a true deep cycle battery, either AGM or FLA. Golf cart batteries work great. Get a pair of 6-volt 225 AH golf cart batteries and you will not need solar to run a 100-wat rig all weekend. Only thing solar will do for you is make RFI.
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WB6BYU

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Re: Type of battery for Field Day
« Reply #4 on: February 13, 2021, 01:13:56 PM »

How much power are you planning to run?  For how long?

That, along with the capacity of the solar panel, determines the
required battery capacity.  Different types of batteries may be
better choices at various power levels.

I've run QRP Field Day on a single 35 Ah gel cell without
any recharging, and still had plenty of energy left.
But you need a lot more capacity at 100W, especially
if you are operating all night (when the solar charging
won't help you a lot).  While a lithium battery might
be a good choice for the former, large flooded batteries
may be needed for the latter, and often in sizes that
require multiple people to move if they aren't broken
down into smaller units.

Even with solar charging, you still need to calculate
how much storage you need for your planned operation
when solar charging isn't available.

W9IQ

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Re: Type of battery for Field Day
« Reply #5 on: February 13, 2021, 01:18:02 PM »

Deciding what type and what size of battery starts with determining your energy requirements. The factors that go into this calculation include mode of operation, output power and duty cycle (transmit vs. receive time). It is also affected by the hours of continuous operation. The solar panel specifications are also important.

If you can fill in the blanks, a more specific and realistic recommendation can be made. Otherwise everyone is just guessing.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

AD6AD

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Re: Type of battery for Field Day
« Reply #6 on: February 14, 2021, 09:36:40 AM »

Thanks, Glenn  (W9IQ). Nice call, by the way.

At my home base I have just a simple 20M dipole, the bottom being just 6 feet off the ground,
running ~80 watts and and I get one contact about every 15 minutes, so receive/transmit ratio
is about 10:1, considering I'm responding to calls, but often not heard.

In a field setting, again with a lame, rickety antenna, I expect the ratio would be about
the same, maybe closer to 5:1 for a pile-up day like Field Day.
At most, I would be in operating mode for up to 4 hours in the field.
It IS easier to compute power then, with probably 150 watts on transmit and receive.

I estimate that would compute to about 800 watts over 4 hours, resulting in a drain
of about 70 amp-hours.
A 100-amp-hour battery might be sufficient, considering I would have at least some
charging ability from the solar cell. My estimate anyway.
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WB6BYU

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Re: Type of battery for Field Day
« Reply #7 on: February 14, 2021, 10:14:47 AM »

We ran Field Day one year using 110 Ah batteries and 100W transceivers.
Generally the big batteries dropped too low after 3-4 hours or so,
depending on the mode, condition of the battery, and style of the
operator. 

One critical parameter is the minimum voltage your radio needs to
operate properly:  this often is higher than the minimum voltage
used to determine battery capacity, so you don't get the rated
Ah without some sort of booster.  Some radios might operate down
to 10.8V, while others really don't like going below 12.5V.  (The
latter is typical of sets rated for "13.8V +/- 10%".)

Average current draw is less on SSB than on CW, although the
limiting factor may be the peak current draw, which
causes more voltage drop in the battery and cables.

Make sure your cables (and fuseholders) are clean and have
good solid connections:  your limiting factor is voltage reaching
the radio, so you want minimum resistance in the path.

W9IQ

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Re: Type of battery for Field Day
« Reply #8 on: February 14, 2021, 11:22:12 AM »

Here is how I would approach the calculation. I will use some example numbers but you will have to look up your specific numbers in your radio manual under specifications.

Let's use your 5:1 example. That is ~83% listening and 27% transmitting. Let's say your radio draws 3 amps during receive and 20 amps peak during transmit. The actual transmit current will vary according to the mode. So we will use a multiplier to adjust for the mode. For SSB the multiplier is 0.25, for CW 0.5 and for FT8, RTTY, etc. it is 1.0.

So for each hour you are on the air, the average receive current is:

     3 amps * 83% = 2.5 amps

For each hour you are on the air, the average transmit current is:

     20 amps * 27% * multiplier (0.25 for SSB) = 1.4 amps

The energy consumption for your operating time is therefore:

     (2.5 amps + 1.4 amps) * 4 hours = 15.6 amp hours

The amp hour capacity of a deep discharge, lead acid battery is based on a 20 hour discharge time. So a 100 amp hour battery is really only rated for a 5 amp draw over 20 hours. If you draw more current at any time, the total amp hour capacity will be less. This adjustment follows a formula that is described as Peukert’s law.

When you think about the transmitter, it will at times draw as much as 20 amps in this example even though it will only do it for short periods during the 27% transmit time.  If it drew 20 amps for the full 4 hours, it would require an AGM battery with a 100 amp hour rating to barely last for the full four hours of transmit time. So this is the absolutely largest size battery required and a 50 amp hour battery would probably do just fine. The larger the amp hour capacity, the more charge/discharge cycles you will realize from the battery.

Take note that an AGM battery is considered fully discharged when it reaches a voltage of ~11.4 volts. Check your radio manual to see if your radio will work at this voltage level. Typically the receiver will continue to function but often the transmitter will have reduced power output or the radio will power cycle when you try to transmit. If this is a concern, bump up the amp hour rating for the battery to minimize this possibility.

Your solar panel will need to supply more than 15.6 amp hours during the time you are operating if you wish to keep the battery at full charge. It is generally better to think of the solar panel as supplemental energy rather than counting on it to fully replenish the battery, even on cloudy days, when you are operating.

You can redo any of these calculations using your actual values. I didn't include the Peukert’s law calculation details so let me know if you need these or you need another Peukert’s law scenario calculated.

- Glenn W9IQ
« Last Edit: February 14, 2021, 11:33:34 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

G4AON

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Re: Type of battery for Field Day
« Reply #9 on: February 15, 2021, 10:29:32 AM »

As per the reply by Glenn, you can work out the likely capacity of battery needed with reasonable accuracy... especially if you measure the current drawn by your radio, although the transmit will vary a bit depending on the band.

I operate portable quite often, some possibly useful info is on my web site at: https://www.qsl.net/g4aon/batteries/

73 Dave
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W9IQ

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Re: Type of battery for Field Day
« Reply #10 on: February 16, 2021, 12:52:19 PM »

In looking back at my post, I see I was loose in terminology after I did a couple of edits. I stated that amp hours was energy when in fact I should have said electric charge. You would need to multiply the respective amp hour values by the voltage to yield an energy value with units of watt hours. Sorry for any confusion.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
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