help with math

kc6cnn · October 2013

I am looking to purchase a hf radio amplifier. I do not have one to test with a meter. The manufacture states in stand by it uses 20 watts, at transmit it uses 980 watts.
Using it say 4 hours a day at a 50 percent duty cycle. This is what I came up with but not sure. Could someone check my figures to make sure I am right.

980 watts for 2 hours at 110 volts = 8.9 amps so say 9 amps
Stand-by is 20 watts for two hours at 110 volts = .18 so say .20 amps

Total power consumption for running it 4 hours at a 50 percent duty cycle would be a 9.2 amp draw on my battery bank.

Is that correct, not counting the losses from my inverter.

Thanks
Gerald

kc6cnn · October 2013

Re: help with math

or would it be 980 x 2 1960 wh and 20 x 4 = 80 so 2040 x 5 = 10200 wh at 24 volt system would be 425 amps a day usage.
I am so confussed.

RandomJoe · October 2013

Re: help with math

You're getting closer with the second post.

980W TX x 2hrs = 1960 W-h
20W RX x 2 hrs (not 4) = 40W-h

Total 2000W-h per day, not sure why you multiplied by 5?

2000 W-h / 24V = 83.3 Ah/day.

The bare minimum battery bank (50% DOD each day used) would be 83.3 x 2 = 167AH, if you want several days' autonomy with no solar input then 3 days x 83.3 Ah/day = 250Ah x 2 = 500Ah battery bank to be at 50% DOD after 3 days of no input.

kc6cnn · October 2013

Re: help with math

Okay Thank you RandomJoe.
So it is the power used multiplied by the hours of use to equal total watt hours of use.
Then the what hour per day divided by the voltage of the battery bank or inverter voltage to equal total amp hours per day of use.
Then you took that load times two to equal only 50 percent of battery drain to determine the size of the battery bank to use.
Okay that will help. so to be able to recharge my battery bank with lets say 4 hours of sun I would need a pv array that was ? equal or more than the amount of watt hours per day?
I still have a lot of figuring to do, with all the appliances and lights, etc, but starting with the amp to try to learn how to do this.
Thank you again for the reply. If I become too much of a bother send me a private message. Just trying to figure this out.
I really want to get off the grid in a cabin with solar and generator backup. Well water and enjoy off grid living.
Thank you
Gerald

BB. · October 2013

Re: help with math

Gerald, I think you are stymied by Amps and Amp*Hours vs Watts and Watt*Hours.

If you want, you can post a link to the HF radio you are looking at... AM radios (if I recall) correctly starts at 25% of rated power and >80% when fully modulated. And, a 450 Watt Transmitter would use ~1,000 watts on full transmit.

Next, the question about Amps... Note that Power=Volts*Amps -- So you need to know both the Voltage and Current to run the numbers....

For example your 980 Watt transmitter AC power:

P=V*A; A=P/V
A= 980 Watts / 110 VAC = 8.9 amps (@ 110 VAC)
A=980 Watt / 12 VDC = 81.7 Amps (@ 12 VDC)

So--The Watts and Watt*Hours is a "complete unit"... Amps and Amp*Hours only "make sense" if you know the operating voltage too.

In this case, 120 VAC to 12 VDC is a 10x increase in current.

Your system:

kc6cnn wrote: »

I am looking to purchase a hf radio amplifier. I do not have one to test with a meter. The manufacture states in stand by it uses 20 watts, at transmit it uses 980 watts.
Using it say 4 hours a day at a 50 percent duty cycle. This is what I came up with but not sure. Could someone check my figures to make sure I am right.

So, I like to keep working in Watts and Watt*Hours until it is time to "convert" to the needed operating voltage.

Your transmit wattage is probably a "soft" number... Basically, if you are doing HF AM, you will see between 25% and 100% modulation... And you will only see 100% modulation when keying. And if you were keying for 4 hours straight (no listening) you probably would still be at a 50% duty cycle (assuming full break keying vs just modulating keying).

There may also be issues with additional losses if there is, for example, a linear amplifier (tube filament heat) and/or DC supply for radio (or HV supply for amplifier, etc.).

Anyway, assuming your "worst case" numbers:

980 Watt transmit * 0.50 transmit cycle * 4 hours = 1,960 Watt*Hours for transmit (4 hours and 50% duty cycle)
20 watts * 0.50 duty cycle * 4 hours = 40 Watt*Hours for receive
Total WH = 2,000 WH for 4 hour session (at 110 VAC)

To size your battery bank, need to know the bank voltage and inverter efficiency. Assuming 12 volt battery bank and 85% efficient 120 VAC Inverter:

2,000 Watt*Hours * 1/0.85 inverter eff * 1/12 volts battery bank = 196 Amp*Hours @ 12 volt battery bus
980 Watt Transmit * 1/0.85 inverter eff * 1/12 volt battery bank = 96 Amps during transmit @ 12 volts
20 Watt Receive * 1/0.85 inverter eff * 1/12 volt battery bank = 1.96 Amps during receive @ 12 volts

Now a 4 hour session and 50% maximum discharge for your battery bank means you need a battery bank with a C/8 (8 hour rate):

196 AH / 0.50 maximum discharge = 392 AH @ 12 volts minimum

Using the Trojan brochure for T105 Golf Cart batteries, we see that they list a C/5 and C/20 capacity of 185 AH and 225 AH... C/8 is in between. Using 185 AH (at C/8 rate):

392 AH / 185 AH per battery = 2.1 strings of batteries

To size the wiring/fuses/breakers for your DC bus:

980 Watts * 1/0.85 inverter eff * 1/10.5 inverter cutoff * 1.25 NEC derating = 137 Amp minimum branch circuit (12 volt bus)

Round up to the next standard breaker/fuse/wiring to pick your components.

For now, close enough to see that you should be looking at 2x 6 volt T105 batteries in series for 12 volts, and two parallel string for 370 AH @ 12 volt bank.

Is this making sense so far? Notice that when I talk about Amp*Hours and Amps--I am very clear at what voltage level I am working at.

-Bill

kc6cnn · October 2013

Re: help with math

The amplifier I am using to figure things out is here http://www.ad5x.com/images/Presentations/KPA500%20Review.pdf
It will be used to communicate with my family as well as play a little on single side band.
I have a 900 amp hr battery bank now that runs my hf radio, and inverter for the refrigerator and lighting. all 12 volt leds.
I am looking to get another system that will run the refrigerator 24/7 as well as a 325 watt small window unit during the summer.
The cabin is 16X40. I have a Honda EU3000IS generator / inverter that will run everything, but Gas is expensive, propane is getter there, and symgas is cheap but have to go out several times a day to refill the wood box. So I have been thinking about a new 24 volt system or maybe a 48 volt system.
I am looking at the outback flex one with the module for the auto generator start. The cabin is being built where one of the long sides will face south. so panels can be roof mounted so the animals don't get to the wiring or panels.

PNjunction · October 2013

Re: help with math

Man, that is one expensive single S-unit.

I understand the desire, but the difference between 100 watts and 500 watts of RF power is just a little over 6db - about 1 S-unit increase. Just want to make sure you know that up front because it may be far cheaper to change antennas. Hard-core dx'ing I can understand or perhaps mobile, but for general purpose use powering an amp relying on solar while it can be done, may be better spent powering up your other gear first.

niel · October 2013

Re: help with math

PNjunction wrote: »

Man, that is one expensive single S-unit.

I understand the desire, but the difference between 100 watts and 500 watts of RF power is just a little over 6db - about 1 S-unit increase. Just want to make sure you know that up front because it may be far cheaper to change antennas. Hard-core dx'ing I can understand or perhaps mobile, but for general purpose use powering an amp relying on solar while it can be done, may be better spent powering up your other gear first.

pnj,
you are essentially correct, but there are times when that 6db or so is warranted depending on where it is you wish to reach, the band used, the atmospheric conditions encountered, and even the time of day it is attempted. that means at times that little bit of difference can mean whether or not contact is made or not. it would not be desirable to run this all of the time if it's not necessary to and the aspects you brought up about the antenna is a good point. often times you could just raise that antenna higher to get better results, but this can get complicated when talking hf. sometimes a different type of antenna is more desirable too. if this amp is necessary then it will be a mighty power hungry necessity.

edit to add-it makes me wonder where his family is in relation to him to need the extra power. it also makes me wonder if they too have licenses to operate and if so why it is nobody in the family can figure out the math.

kc6cnn · October 2013

Re: help with math

niel wrote: »

pnj,
edit to add-it makes me wonder where his family is in relation to him to need the extra power. it also makes me wonder if they too have licenses to operate and if so why it is nobody in the family can figure out the math.

Niel
Look at my user name. That is my FCC Call sign.
My family is also licensed to operate, my Daughter is upgrading to general class.
The amplifier will be used only when needed to insure contact with my Daugher.
I take trips so I can be in Alaska one month and all over the usa, and Costa Rica.
I have been a ham for over 27 years. The math is not hard, but figuring out how to get the formulas right is what I am trying to do. I was using the amp only to get the formulas down so that I could do this on my cabin. The cabin has a 45 foot tower with a beam antenna on it. 10 15 and 20 are not hard to make good contacts, but at night 40 and 80 are a different story. that is why I am looking to add a little power to the cabin station. I run a resonant dipole cut to freq and also a ZS6BKW dipole for 40 and 80 meters. Before you start wondering the worse about someone take the time to check them out first and not smear them on a great forum like this.
Thank you
Gerald

niel · October 2013

Re: help with math

that was not meant in a bad way as i truly was wondering why nobody else had the math down right being hams also. i did assume at least one other to be licensed and i looked you up, not your family and i guess i am dismayed at someone with an extra class license having this kind of a dilemma with the math.

maybe we got off on the wrong foot here, but i did take your side on the issue of the amp and i was not criticizing your possible need for it at times so just who is doing the wrong judging here? it will, however way you shake it, be a big item to run from solar and any efforts made to be able to not need it is a plus.

kc6cnn · October 2013

Re: help with math

I have been trying to learn the difference between all the available systems, 12, 24, 48 volt systems and losses, efficiencies, etc. I have to agree with you after looking at the usage with a kill-a-watt meter, that amp draws more than almost all of my other appliances together. lol heck my small window ac only pulls 425 watts. I think that I might have to have a small generator as a back up for those times that I have to be QRO. Most the time I will be QRP or 100 watts. I enjoy this forum a lot. I have gotten most of my information about systems from here. I appreciate all the willingness to help and to teach. I am slowly getting all the information together and will hopefully reach my goal of living off grid soon.
maybe we did get off on the wrong foot Niel, we will put it behind us and carry on. I appreciate all the help sir.
Thank you
Gerald

jcheil · October 2013

Re: help with math

kc6cnn wrote: »

lol heck my small window ac only pulls 425 watts.

What make/model is that AC unit?
Most small 5000btu units pull 550ish when the compressor is on and 90ish when it is off.
I have been looking for a smaller (smaller than 5000btu) unit for a long time.
I would really be interested in buying the model you have IF it REALLY is that low.

kc6cnn · October 2013

Re: help with math

it is a Frigidaire 5000 btu window unit that I took to Central Heating and Air. The worked on it and put a hard start kit and some other stuff, unknown to me.
When it was first purchased it pulled 575 watts, after they worked on it it pulls 425 watts when compressor is on and about 70 when compressor is off.
Not exactly sure what all was done to it, but sure helped with powering it off grid.
I will try to go by there and ask them what they did if you would like.
Thanks
Gerald

niel · October 2013

Re: help with math

using a generator is a good solution for those times when power usage is out of hand for whatever reason. unfortunately, there will be times when no matter what you do the ability to communicate will not always be there. if ssb is not successful then consider other modes such as digital be it the old cw or the newer types like pk31.

jcheil · October 2013

Re: help with math

kc6cnn wrote: »

I will try to go by there and ask them what they did if you would like.
Thanks
Gerald

Yes please do. I can't imagine what someone could do to a unit to suddenly make it consume less energy (unless somehow the output of the unit is being reduced). But Even if that is the case it is exactly what I would be looking for since I only need a 3000BTU unit.

kc6cnn · October 2013

Re: help with math

Okay, I am not an air conditioning guy so I wrote down what they told me.
They took my window unit and replaced the fan motor with a smaller one, took the compressor out and replaced it with a FCF-5000 marine compressor they had from a boat repair job. Then installed a hard start kit to insure low power drain on start up. The guy says my unit pulls 4.4 amps max even at start up. I asked him about getting a 3000 btu air for you and he said that unless you use a FCF marine air conditioner or a split unit from a something, there are not a lot of choices in ac units under 5000 btu.
Hope this helps, I do not know what it would cost to have one done like mine, my Daughters boy friends dad works there and they did it for me.
Thanks
Gerald

jcheil · October 2013

Re: help with math

kc6cnn wrote: »

Okay, I am not an air conditioning guy so I wrote down what they told me.
They took my window unit and replaced the fan motor with a smaller one, took the compressor out and replaced it with a FCF-5000 marine compressor they had from a boat repair job. Then installed a hard start kit to insure low power drain on start up. The guy says my unit pulls 4.4 amps max even at start up. I asked him about getting a 3000 btu air for you and he said that unless you use a FCF marine air conditioner or a split unit from a something, there are not a lot of choices in ac units under 5000 btu.
Hope this helps, I do not know what it would cost to have one done like mine, my Daughters boy friends dad works there and they did it for me.
Thanks
Gerald

Thanks so much. I kinda figured it was pretty serious changes like that!

help with math

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