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Author Topic: Field power with lead-acid battery -- How many Ah?  (Read 5182 times)
WS4T
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Posts: 179




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« on: July 16, 2012, 01:49:02 AM »

We have a field day here in Estonia in early August. It's a VHF/UHF contest. I want to operate the 144 MHz part and the 432 MHz part. That means the following:

- Saturday evening, 6 to 10 PM (4 hours)
- Sunday morning, 6 to 10 AM (4 hours)

Radio: FT-897 (or FT-817 as back-up)

I've searched eham.net a lot and there is plenty of info about lead-acid batteries, but I'm still confused. Can anybody with experience with lead-acid batteries give me a rough idea of how much battery capacity (Ah) I would need to operate these contests? I am assuming I can (partially) recharge the battery over Saturday night.

Another problem: It looks like the FT-897 can behave erratically if the power drops below 11.7 V. In your experience, is that a big problem?

I'm hoping to find a nice location in the countryside, set up my portable yagis and enjoy this contest. I think a generator is not an option in this case, so I am stuck with battery power.

Thanks for any thoughts!

73,
Gary, ES1WST
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KA4POL
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Posts: 1920




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« Reply #1 on: July 16, 2012, 03:29:07 AM »

Lets do a bit math including a safety margin as well:

FT-897, Transmit according to manual 22 A @ 13.8 V +/-15 %, receive 1 A

4 hours of contest, 3 hours transmit is 3*22=66 Ah, 1 hour receive is 1*1=1 Ah. So you'll want to have roughly a 70 Ah battery if you can fully recharge it. I'd take a 100 Ah battery to be on the safe side for both parts including a partial recharge. The 817 is considerably lower in consumption so we can forget it for the worst case calculation.

The 13.8 +/-15% tells you that the minimum voltage is 11.73V. That's what Yaesu guarantees. So 11.7 will be really marginal.

Good luck in the contest.
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WA3SKN
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« Reply #2 on: July 16, 2012, 04:33:47 AM »

Not sure the power levels you are planning to use.  Yaesu has 4.5 ah batteries for that if you are planning 10 watts out.  But don't forget the headphones!  Using these can lower your power drain and extend operations... they can make a BIG difference in operating time!
73s.

-Mike.
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K5LXP
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« Reply #3 on: July 16, 2012, 08:49:06 AM »

What mode(s)?  The duty cycle for SSB is different than CW, and different again for digital/continuous carrier.  What is your TX/RX duty cycle?

My calculation for a rig like this running CW, 75% TX duty cycle would be about 10.5Ah.  Factor in your voltage cutoff of 11.7V and you would need about a 168Ah battery to run 8 hours.  Subtract Ah for what you can restore to the battery overnight.  If you come up with more detailed operating conditions (modes, duty cycles) I can more specifically determine Ah requirements.  If you know what kinds of batteries you will be using, I can factor battery merit as well.

168Ah is a lot of battery to buy and haul around.  What's hurting you is the 11.7V cutoff, as you're leaving half of the battery's capacity unused.  You can remedy that with a boost converter.

Does it have to be pure battery power?  Otherwise just run the rig off a vehicle, and keep the battery topped off by starting the vehicle periodically. 


Mark K5LXP
Albuquerque, NM


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WB6BYU
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Posts: 13028




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« Reply #4 on: July 16, 2012, 04:40:35 PM »

Quote from: K5LXP

...you would need about a 168Ah battery to run 8 hours...



So if you could do a total recharge overnight, a 64Ah battery would be
sufficient.  Reducing output power to 50 watts (or 20W when signals
are strong) would give you longer operating time or a smaller battery.
If you are operating SSB, or spending more time listening, you won't need
as big of a battery either.

The typical deep cycle battery is around 100Ah for one about the same
size as a car battery.  That's probably a good size for this purpose,
assuming that you can recharge it overnight.
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G4AON
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Posts: 518




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« Reply #5 on: July 16, 2012, 11:59:38 PM »

See my web page at: http://www.astromag.co.uk/batteries/

There is a worked example of how much capacity you need, and also information on battery types and charging for portable radio use.

73 Dave
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WS4T
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Posts: 179




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« Reply #6 on: July 17, 2012, 03:42:30 AM »

Hi -- Thanks for the replies. My plan is to operate from a bird-watching tower, and I will need to hike a ways to get there, so I want to limit myself to about 10 kg battery weight at most. I'm lazy.

I looked at the Varta AGM batteries but they seem to weight a minimum of 18 kg (http://www.varta-automotive.com/index.php?id=344).

Could anyone peruse this list from my local source and see if anything looks like a good choice for radio use in the range < 20 AH? I  also have to avoid buying something that is poorly made because I want to store this battery in the house (in the cellar) and I don't want any problems with toxic fumes. So that means I need a good brand.

https://www.elfa.se/elfa3~ee_en/elfa/init.do?filterClause=COL_1%253D12%2520V&toc=19037&pageSize=100

QRP operation is starting to look more tempting!

73,
Gary, ES1WST

PS: I should also mention that I lean towards CW but SSB and FM are also options in this contest.
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WS4T
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Posts: 179




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« Reply #7 on: July 17, 2012, 04:59:09 AM »

Now I found a 24AH AGM battery from Varta available in Tallinn. What do you think about this one (search page for "AGM")? It's marked for marine and caravan use: http://aku.ee/index.php?aid=29&id=7
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W9GB
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Posts: 2600




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« Reply #8 on: July 17, 2012, 05:30:27 AM »

For your battery weight constraint (10 kg or ~ 22 lbs), LOOK at U1 sized batteries.
This battery size is used extensively in medical industry for wheel chair and scooters, etc.  In those devices, they place 2 cells in series for 24 Volts, so you only need one U1 cell (12 Volts).
They are also used with some golf caddies.
http://www.mkbattery.com/

Battery boxes (plastic) and carry straps are also plentiful, due to medical / motorized-cart  usages.

These U1 sized cells come in a variety of types/chemistry formulations (AGM, Gelled Cell, new Lithium formulations) -- depending upon intended usage,
but Lead-Calcium is most common and Ah ratings (up to about 35 Ah).
===
For Estonia region:

Sweden, Norway & Denmark      
Nordic Battery Power AB
Marieholmsgatan 2
Goteborg
41502
Sweden
(Tel): 0046 31212000
(Fax): 0046 31212066

Finland       
Akkuvoima Oy
Höyläämötie 11
00380 Helsinki
Finland
(Tel): +358 207 999 640
(Fax): +358 207 999 641
E-mail: info@akkuvoima.fi
 
Wamtechnik Co. Ltd.
Branch Office & Production Dept.
19 Czajewicza Street
05-500 Piaseczno, Poland.  
(Tel): + 48 22 750 21 42
(Fax): + 48 22 750 21 39
Contact person: Mr. P. Kostrzewa.

« Last Edit: July 17, 2012, 05:54:04 AM by W9GB » Logged
G4AON
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Posts: 518




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« Reply #9 on: July 17, 2012, 06:14:52 AM »

Now I found a 24AH AGM battery from Varta available in Tallinn. What do you think about this one (search page for "AGM")? It's marked for marine and caravan use: http://aku.ee/index.php?aid=29&id=7

The Varta Professional deep cycle series are very good, but is 24 Ah enough? You should only look to use half that capacity before recharging, you cannot use "24 Ah" or the battery will not last long.

73 Dave
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K5LXP
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« Reply #10 on: July 17, 2012, 06:27:55 AM »

I will need to hike a ways to get there, so I want to limit myself to about 10 kg battery weight at most.

That would've been useful to know first time around.

Quote
QRP operation is starting to look more tempting!

As WB6BYU noted, ramping down your power a bit will make a huge difference in power used.  Cutting your power to 25W will double your run time, but cut your signal by only an S-unit.  The better you do with the antenna, the less power you need.

Quote
I want to store this battery in the house (in the cellar) and I don't want any problems with toxic fumes.

Well, the fumes from a flooded battery aren't terribly toxic and it's difficult to get them to vent appreciably in the first place, but I think the most important feature you need is being non-spillable for easy transport.  Storing in a cool place is a good thing, and be sure you use a proper charger/maintainer with it or after a period of months all you'll have is an expensive doorstop.

Quote
Now I found a 24AH AGM battery from Varta

AGM is going to give you the best performance in lead acid batteries so if you're going to go small, having high efficiency is a good thing.

Quote
I should also mention that I lean towards CW but SSB and FM are also options in this contest.

I recalculated based on 25W output (about 50W input) and now you're down to just over 5Ah load.  With an 11.7V cutoff you'd be able to operate about 2.5 hours CW or FM, longer on SSB.  Note that you're still leaving 1/2 your battery capacity unused because of the 11.7V cutoff.  If you had either a boost converter or a radio that could draw the battery down completely, you could double your operating time with the same battery.


Mark K5LXP
Albuquerque, NM


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WX7G
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Posts: 5920




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« Reply #11 on: July 17, 2012, 07:44:33 AM »

With the 897 and field day like use the average current draw will be 4-5 amps. For 8 hours you need 32-40 A-H and a 100 A-H battery do the job given the 11.7 volt limit.
« Last Edit: July 17, 2012, 07:46:10 AM by WX7G » Logged
K3VAT
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Posts: 701




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« Reply #12 on: July 17, 2012, 10:47:32 AM »

...
My calculation for a rig like this running CW, 75% TX duty cycle would be about 10.5Ah.  ... Mark K5LXP
Albuquerque, NM

Mark,
thanks for your multiple postings on this subject.  I recall some info from the ARRL (maybe from the exposure guidelines) indicating that CW has ~ 40% duty cycle.  May change your figures somewhat.

73, Rich
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K2CMH
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Posts: 275




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« Reply #13 on: July 17, 2012, 12:20:44 PM »

Is solar an option?  While solar would not completely power your station, it may help keep you from draining the battery nearly as fast.  If you use the 817 and an adequately sized panel, the panel would have enough output to carry the radio in receive mode so there would be no battery drain.  If you are usign the 897, the panel would still help preserve your battery life.
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K5LXP
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« Reply #14 on: July 18, 2012, 07:14:30 AM »

  I recall some info from the ARRL (maybe from the exposure guidelines) indicating that CW has ~ 40% duty cycle.  May change your figures somewhat.

Normally I would agree the duty cycle should be lower but in this case it's a V/U contest.  I'm guessing that with the few contacts you make during these contests there's going to be a whole lot more transmitting than receiving.  I would say this is worst case, so it can only get better if there's actually anything to hear.


Mark K5LXP
Albuquerque, NM
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