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Author Topic: How many amp hours actually need for battery operation?  (Read 397 times)

N8TI

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How many amp hours actually need for battery operation?
« on: April 03, 2021, 01:29:25 PM »

Hello everyone. I am putting together a “deck” or “back porch” station. I have a FR-891 on order. It takes 23 amps on maximum draw. I will be working CW. I am ready to buy a Battery, one of the Lithium Iron ones. I plan on running 100 watts. Does anyone have practical experience with using a battery at 100 watts and, if so, what size did you find appropriate?  I would use it for a couple of hours at a time and would like to take it to a park every once in a while. I don’t want to simply overbuy, because I would like to keep the weight down. If a 12ah works, then I would rather not get a 15 ah. Thanks.
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AK5B

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Re: How many amp hours actually need for battery operation?
« Reply #1 on: April 03, 2021, 01:36:33 PM »

I use a 30ah Bionenno for my 50-100 watt portable ssb ops, FWIW.  Love it!

You could get by with 15 or 20ah but probably not much less.

73,

Jeff
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N8AUC

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Re: How many amp hours actually need for battery operation?
« Reply #2 on: April 03, 2021, 02:24:34 PM »

It all really depends on how you're going to operate.

You can figure out how many amp hours you'll need like this...

As an example, let's say on average you're going to transmit for 10 minutes out of every hour.
So for those 10 minutes, you're going to draw 23 amps for 100 watts output.
And for the rest of the time, you'll be receiving. So 50 minutes at a current draw of about 1 amp.
10 minutes is 1/6 of an hour, and 50 minutes is 5/6 of an hour. So for each hour, your current consumption will be:

((1/6) * 23) + ((5/6) * 1) =
     3.83        +     0.83      =
4.66 Amp Hours of battery capacity for 1 hour of operation.

And you can continue doing that until the battery voltage drops to the lowest voltage your radio needs to work properly.
Most rigs require 13.8V +/- 15%. So once the voltage on the battery drops to 11.7V, then you're done, and need to recharge.

Knowing that, you can now select what kind of battery to use.
For the Lithium chemistries, it's safe to go to an 80% depth of discharge (DOD).
For lead-acid chemistries, is safe to go to a 50% DOD. Why do you have to consider DOD?
Because you can NEVER use all the energy stored in a battery without seriously reducing the number of usable charge/discharge cycles you'll get out of it.

Now divide your required amp hours by the DOD to figure how much capacity you really need per hour.
You mentioned Lithium, so 4.6 / 0.8 = 5.7 Amp Hours of capacity for each hour of operation, assuming the 1:6 ratio of transmitting to receiving illustrated above. If you transmit more, you'll need more, if you transmit less, you'll need less.
Want to make your battery last longer? Then reduce your transmitter power. 50W is still a very effective power level for operating CW. When I operate CW portable, like for Field Day, I usually only run 20 watts output.

How many hours do you want to be able to operate at a time? Let's say you want to be able to go for 4 hours.
Then you'd need 4 * 5.7 = 22.8 Amp Hours of battery capacity with a Lithium chemistry to operate for 4 hours.
Want to go for 8 hours? Then you need 8 * 5.7 = 45.6 Amp Hours of capacity.

Hopefully you find this helpful. And I hope you have loads of fun operating battery powered portable!

73 de N8AUC
Eric

« Last Edit: April 03, 2021, 02:28:26 PM by N8AUC »
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AE0Q

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Re: How many amp hours actually need for battery operation?
« Reply #3 on: April 03, 2021, 02:39:26 PM »

I wouldn't get less than a 20AH LiFePO4 battery. 

You could join the Facebook group for Parks On The Air and you will find HUNDREDS of people using the FT-891 at 100w with those batteries for park activations.

BTW, that 23 amp spec is a worst-case number, people using the radio report it is never that much.

You might also get one of these power monitors so you can see what you are really using from the battery:

https://www.amazon.com/Watt-Meter-Power-Analyzer-Measurement/dp/B07JM642NQ/

Glenn AE0Q
« Last Edit: April 03, 2021, 02:57:01 PM by AE0Q »
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WA8NVW

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Re: How many amp hours actually need for battery operation?
« Reply #4 on: April 03, 2021, 02:52:24 PM »

Let's say you're active on CW and running Tx 30% of the time.  If a typical carrier weighting for conversational CW is 40%, then average drain during transmit is 30% of 40% of 23 amps, (that's about 2.8 amp), and receiver drain is 70% of 2.1 amps, or 1.47 amps. You'll consume 2/3 of the battery's capacity during transmit and 1/3 while receiving, and that will be about 4.25 amp-hours for each hour pounding brass at full power.  Decide long you want to operate on that contesting duty cycle before recharging, then pick your battery capacity. 
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KB1NO

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Re: How many amp hours actually need for battery operation?
« Reply #5 on: April 03, 2021, 03:06:11 PM »

Bioenno has charts on their website to help calculate time based on mode/power level and T/R%.   
In my opinion, their LiFePo4 batteries are excellent.    I've used a 12AH Bioenno for QRP on Field Day in the past. 
I upgraded to a 20AH Bioenno last field day and operated SSB for about 8 hours at 70W still had AH remaining.   
In fact, I'm thinking of listing my 12AH for sale with charger.   If you are interested my email is good on QRZ.
Good luck. 
John, KB1NO
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N4UM

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Re: How many amp hours actually need for battery operation?
« Reply #6 on: April 03, 2021, 03:10:30 PM »

I've done Parks on the Air activations for up to 5 or 6 hours with my Bioenno 40AH battery running 100 watts with my FT-891 on phone and CW using a very heavy duty cycle without running out of juice.
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AE0Q

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Re: How many amp hours actually need for battery operation?
« Reply #7 on: April 03, 2021, 03:12:01 PM »

I would use it for a couple of hours at a time and would like to take it to a park every once in a while.

If you decide to activate parks for POTA (Parks On The Air) and WWFF park hunters, you will probably end up transmitting MORE than receiving.  To activate a park you have to CQ all the time if not working someone so (1) park hunters can find you, and (2) so you can keep your frequency in the SSB bands (not a problem on CW).

Transmitting 50% of the time and MORE is pretty normal during a park activation, you will be calling CQ a LOT !!  So battery use is how long you want to stay at a park for the day :-)

https://parksontheair.com/

https://wwff.co/

You may not be interested in WWFF / POTA right now, but if you go to a park and start giving out the park number on 20 or 40m, be prepared for a PILEUP of park hunters, any day of the week.  It is a blast.  I wouldn't get a battery that is barely big enough, you might find it poops out way too soon once you start having fun :-)

I also use a Bioenno 40AH battery running 100 watts, it lasts much longer than I can stay out at a park for the day.

Glenn AE0Q
« Last Edit: April 03, 2021, 03:14:58 PM by AE0Q »
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WA2EIO

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Re: How many amp hours actually need for battery operation?
« Reply #8 on: April 03, 2021, 04:57:03 PM »

In addition to the charts on the Bioenno site, you can take a look at this one:
http://www.4sqrp.com/Battery_Capacity/index.php

For a 100 Watt station, I would also suggest a minimum of a 20Ah battery.   The Bioenno is about 5 lbs.

73    Ron   WA2EIO
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KB1NO

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Re: How many amp hours actually need for battery operation?
« Reply #9 on: April 03, 2021, 07:02:54 PM »

One other point I forgot to mention.  Depending on your $$ and size / weight budget the 40 AH Bioenno is the lowest cost per AH based on HRO prices.
50 AH= $9.40 / AH, 40 AH=$9/ AH,  30 AH= $9.33 / AH, 20 AH = $9.65/ AH,  15 AH= $10/ AH.
I was not aware of the website Ron WA2EIO suggested, but I tried it out and found it a useful resource.
Great info from everyone.
John, KB1NO
 
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VK3YE

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Re: How many amp hours actually need for battery operation?
« Reply #10 on: April 05, 2021, 03:50:45 PM »

The power budget calcuation from N8AUC above is a really good approach.

As a rough and fairly conservative rule of thumb with QRP gear and active operating I go by a rule of thumb of 1 amp hour per hour operating.

So a 7 AH battery will give 7 hours operating.

In your case though the FT891 is much more a current guzzler on receive than the FT817/8. That's important as most of the time you'll be listening.

Plus your transmit output power will be higher. While you can reduce consumption by cutting tx output power efficiency drops if you do that with a 100w rig turned down to say 5w compared to if you were using a dedicated QRP rig.

After considering these factors you might then end up with a minimum capacity of 20 AH or more, depending on how long your operating stints will be and style of operating (eg as a net participant you'll hardly ever be transmitting versus the SOTA activation mentioned before).

The flip side of this is that if lightness / low weight is important then another approach, eg a QRP righ with a 30 - 50 watt amplifier (used only when required) might work out more economical. That way you get the low power usage on receive as well as low usage on transmit when you don't need the higher power.
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G4AON

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Re: How many amp hours actually need for battery operation?
« Reply #11 on: April 06, 2021, 03:03:27 AM »

Another consideration is if you are logging using a laptop with a 12V converter (I use a 12V to 19V car laptop PSU). This potentially can consume more power than your transceiver! My own portable contest operating consumes around 30Ah for 8 hours with a TS-480SAT and a laptop.

73 Dave
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