Battery Power

From: eudamac3
To: Ray (VU3ORN)
Sent: Wednesday, 13 July, 2005 6:24 AM
Subject: Battery Power

You should be able to do these calculations yourself.Battery capacity is rated in ampere*hours. A substantial car battery is good for 65 AH. That means 65 amps for one hour, or one amp for 65 hours. That's a SIZABLE battery. Some smaller car batteries are but 25-30 amp hours. An old battery can be as poor as 5AH and still start a car, unless the car needs a LOT of cranking. Old batteries are deficient in amp hours. If "mom's old car battery" was replaced because it was not really starting the car reliably, then it's probably way low in amp-hour capacity.

Anyway, here's how the calculations worked for me one year.Rig was a Kenwood - don't remember model, one of the early solid state rigs. This was a "full power" 100W solid state radio.

RX current consumption: 2.5 amps TX current consumption: 22 amps at 100W CW carrier Got those numbers from the owner's manual.

The one added number that would be nice to have is "TX current consumption with no carrier". That's when the radio is in transmit mode, but you're not actually pushing on the key (or speaking into the microphone). For the radio I was using, I measured this value at 5 amps.Then, figure some duty cycles in percent.For hard-core contesting, RX=50%, TX=50%During the transmit time, figure "no carrier" for 50% and "full carrier" for 50% (that's the statistical norm for CW - SSB is actually a bit more gentle on the battery, but we'll figure this conservatively) - that means 50% of the time we're in RX, 25% of the time it's TX no carrier, and 25% of the time TX with full carrier.Multiply the current draw times the % of time that you're drawing that much. Total % must add up to 100%. The number you'll end up with is amp-hours required PER HOUR. Then multiply by # of hours of operation desired.RX: 50% X 2.5 amps = 1.25 amp-hours per hourTX no carrier: 25% X 5 amps = 1.25 amp-hours per hourTX full carrier: 25% X 22 amps = 11 amp-hours per hourTotal here equals 13.5 amp-hours per hour. If we want to run a full 24 hours on Field Day, then it's 24 X 13.5 amp-hours, or 324 amp-hours total required. I would need five large car batteries for my entire weekend of operating. Maybe only four, since I did do this very conservatively (actually spend more than 50% of the time listening, for instance).Now, this was for an FD I did WAY back in 1977. I did not know the actual car battery amp-hour capability. So, here's what I really did. I had a car, and a spare battery. I added "jumper posts" to the car - 1/2" bolts, attached to a phenolic plate, located some distance from the battery, but still under hood. These went in parallel with the cables connecting car battery to alternator. I brought two sets of jumper cables. I drove the car to the FD site, and set up camp. I set the rig up on the spare battery. I removed the "main" battery from the car, setting it on a hunk of wood a few feet from the car, but jumper-cabled back into the car. I then ran the radio until the "spare" battery died. At that time, I started the car (the "main" battery was jumper-cabled in, remember?). I then used the second set of jumpers to attach the "spare" battery to the car. I was careful to make the final connection to the jumper post under hood so the spark did not occurnear the battery. Now I have the car running, with a "full" and "empty" battery connected. I disconnected the "full" (otherwise known as "main") battery, being careful to avoid sparks near the battery. Now, put the "main" battery at the operating station, and go at it. After an hour, shut off the car. Operate until "main" battery dies, and do the car-start/battery-swap routine again. I only had to battery-swap twice.One variable that I did not include - at what terminal voltage does the rig quit operating? Some "full power" HF rigs are famous for insisting on at least 12V, and will shut down quickly below that.Now, just for grins, let's do this again, but use the Elecraft K2 QRP rig. I'll do this thing conservatively again. I picked the K2 only because Elecraft gives more detailed specs and technical data than almost anybody else, and the K2 seems to be used by many contesters, not just QRP operators.RX current consumption 0.25 ampsTX current consumption 2 amps (at 10 watts output)Elecraft's website does not specify the current consumption of the K2 when in TX mode but not transmitting, so I'll dispense with that analysis.RX: 50% X 0.25 amps = 0.125 amp-hours per hourTX: 50% X 2 amps = 1 amp-hour per hourTotal = 1.125 amp-hours per hourFor a 24 hour field day, the total is 27 amp-hours.This should be within the capabilities of a medium-sized car battery in good condition.You could test your car battery, by the way. Charge it fully, and connect an automobile headlight to it, and see how long you go before the battery voltage drops below 11 volts. Most auto headlights have a wattage rating on them. Divide that number by 12 to get the amps draw. For instance, my VW's headlights are marked "60 watts", so they are 5 amps. If my car battery runs them for only 2 hours before the battery voltage is below 11, then I only have 10 amp hours in the battery - which is a pretty dead battery.Assuming a fairly efficient QRP rig, if you only got two hours of operation, this suggests the battery is delivering less than 3 amp hours of capacity. You'd have gotten more lifetime by stringing 8 "D" alkaline cells in series together and it's time to scrap that battery!In fact, I'm quite certain that I can get roughly 6-8 hours out of my 6 watt 2 meter rig, with a heavy amount of transmitting (relaying information for a bicycle race) with a single charge on my 2.2 AH NiMH batteries.It's worth spending $30 to $40 for a brand new car battery, to be kept trickle charging in the garage (never in the house), or maybe $20 for a 7AH sealed lead acid battery.Boy, this is reminding me of the days when I tried to homebrew a battery powered go-kart using old car batteries. I learned very quickly that by the time a car battery seems weak in a car, it's useless for anything else, too.Yeah, get a new battery.Thanks for letting me ramble......FM