Bench Power vs Car Battery

[WRTH574]

I saw something about a bench power supply needed to be about 30 Watts output at 13.8 V to have a 50 Watt radio actually transmit on 50 watts. Now I don't know a thing about this, but I'm wondering if connecting a 50 Watt radio to a car battery has enough juju to let it transmit at about 50 watts.   In other words I guess, I'm wondering if the mobile radios are designed to output their max power when connected to a vehicle? I would assume and hope so. I have no idea how much wattage is output by a car battery.

[BoxCar]

Car batteries have more than enough "juju" to run your radio at max power.

[SteveShannon]

59 minutes ago, WRTH574 said:

I saw something about a bench power supply needed to be about 30 Watts output at 13.8 V to have a 50 Watt radio actually transmit on 50 watts. Now I don't know a thing about this, but I'm wondering if connecting a 50 Watt radio to a car battery has enough juju to let it transmit at about 50 watts.   In other words I guess, I'm wondering if the mobile radios are designed to output their max power when connected to a vehicle? I would assume and hope so. I have no idea how much wattage is output by a car battery.

You probably meant amperes or amps in the part of your quote I underlined and bolded.  For my 100 watt amateur radio transmitter I have a 30 amp 13.8vdc power supply.  At full power the current is around 23 amps I think.  For a 50 watt transmitter you probably need at least a 15-25 amp power supply just so you’re not running the power supply at full power output.  Watts out can never exceed watts in.

A car’s electrical system puts out 13.8 volts when the car is running in order to charge the battery.  When the car is turned off the voltage of the battery drops down to somewhere around 12.7 or 12.8 volts DC.  I routinely charge several sealed lead acid batteries and they are all very close to 12.7 or 12.8 volts when I need to use them (charged and sitting around for a few weeks on a shelf).

You may not get 100% of the power from your mobile radio when it’s strictly on battery, but you probably will never know the difference.  

As far as wattage, it depends on the capacity of the battery, but nearly any of them will put out many amps of current.  8 amps at 12 volts is nearly 100 watts, . You only need that while transmitting. Receiving takes much less power.

[dosw]

30 minutes ago, WRTH574 said:

I saw something about a bench power supply needed to be about 30 Watts output at 13.8 V to have a 50 Watt radio actually transmit on 50 watts. Now I don't know a thing about this, but I'm wondering if connecting a 50 Watt radio to a car battery has enough juju to let it transmit at about 50 watts.   In other words I guess, I'm wondering if the mobile radios are designed to output their max power when connected to a vehicle? I would assume and hope so. I have no idea how much wattage is output by a car battery.

Radios that are set to output 50w are designed to do so when supplied with 13.6 or 13.8v, and at that voltage might draw 13A while transmitting. When the vehicle is shut off, the battery is going to supply closer to 12.6v when fully charged, and at that voltage, the radio will draw fewer amps; maybe 10-12A. And that means you will probably be transmitting at a little under 50w. You would have to measure to find out more precisely. The FCC won't approve a GMRS radio manufactured with the capability to transmit more than 50w at 13.8v. That means manufacturers aren't going to risk producing one that outputs 50w at 12.6v, because it might exceed the FCC limit when being run in a vehicle with an alternator putting out 13.8v.

 

That means, if you want full power output from your radio, it needs to have a power supply that produces the correct voltage, and that can meet the amperage needs of the radio. For your 50W radio, you would want a power supply that can produce that full 13.8v, AND that can provide 15A current (which is probably what the radio is fused at).

[KAF6045]

5 hours ago, WRTH574 said:

I saw something about a bench power supply needed to be about 30 Watts output at 13.8 V to have a 50 Watt radio actually transmit on 50 watts. Now I don't know a thing about this, but I'm wondering if connecting a 50 Watt radio to a car battery has enough juju to let it transmit at about 50 watts.   In other words I guess, I'm wondering if the mobile radios are designed to output their max power when connected to a vehicle? I would assume and hope so. I have no idea how much wattage is output by a car battery.

As mentioned, you probably meant "30 Amps". A 30 Amp/13.8V supply is sufficient for a 100W Amateur transceiver with a margin for overrun.

Kenwood TS-2000

• Supply Voltage: DC 13.8V +/-15%
• Current: Transmit (max) 20.5A or less; Receive (no signal) 2.6A or less

Kenwood TS-590SG: same for transmit, idle is better at 1.5A or less

Icom ID-5100 (50W VHF/UHF)

• Power Supply: 13.8V DC +/- 15%
• Transmit: Maximum current drain =< 13.0A
• Receive: Standby =< 1.2A; Maximum audio =< 1.8A

Many of these are also based upon very short transmit/receive cycles with long idle periods (For handhelds I've seen specification of 6sec transmit, 6sec receive, 48sec idle, when estimating battery life). FM mode is a "100% duty cycle mode" -- no matter what you are saying, the radio using the full power level it has been set for. The SSB mode used in HF gear typically runs a 20-50% duty cycle (depends on how much compression/processing one is using) -- when there is no sound, SSB uses no power to the antenna (it does, of course, still need power to keep the circuits running). A rough rule of thumb is to assume that the radio requires two to three times the output wattage due to circuit overhead (that's why so many also have cooling fans -- to dissipate the heat: 100W out implies 200-300W draw, with 100-200W generating heat in the box).

For the above three radios, the wattage comes to: 100W out/283W in [2.8X] (both Kenwoods), 50W out/179W in [3.6X] (Icom). Depending upon the internal circuits, these radios may draw more current (more amps) as the battery voltage drops -- maintaining the same wattage. Also -- unless you intend to operate with just the battery sitting on wood spacers, you have an alternator that provides most operating current and recharge of the battery while the engine is running (and likely is providing around between 13.8 and 14.2 volts).

 

Now, answering the battery side of the question:

For a car battery... Consider that a moderately new (and upper end) car battery is rated to provide "cold cranking amps" in the 500-700A range (there's a reason battery cables are so thick). That rating assumes a max of 30-seconds. Many also have a rating for run-time if the alternator fails -- Reserve Capacity. This is minutes until the battery drops below 10.5V with a constant draw of 25A.

But for your purposes, the more useful rating is Amp-Hour. Not often found on "engine start" batteries, but many smaller (UPS, Emergency lighting) batteries will have this rating. Theoretically a 14 AHr battery could provide 14A for 1 hour, or 1A for 14 hours -- in actuality, the first combination will probably kill the battery in less than an hour (and the heat from that current load may /really/ kill the battery -- dig up a D-cell alkaline and hold a thick jumper wire to both ends... and be ready to drop it when it starts burning your hands).

Peruse https://www.motortrend.com/how-to/car-battery-specs-sticker-numbers-explained/

Now, using some of those common radio test times (6s Tx/6s Rx/48s idle) and for the Icom... in one hour you have 6 minutes of transmit -- 1/10th of an hour => 1.3A average, 6 minutes active receive => 0.18A, and 48 minutes of idle/silence => 0.96A. Total draw for the hour:  2.38AHr. 5.8 hours with a 14AHr battery. But, if you are rag-chewing, with say 20 minutes transmit, 20 minutes receive, and 20 minutes of idle per hour... 5.3AHr average, battery life 2.6 hours (probably less due to higher average draw -- Amp-Hour is based on a 20-hour draw down, and for that 14Ahr battery that means only 0.7A draw).

[OffRoaderX]

OP: What time is it?

"Some People": [6 paragraphs on the theory of time and how clocks are built]

[marcspaz]

2 hours ago, OffRoaderX said:

OP: What time is it?

"Some People": [6 paragraphs on the theory of time and how clocks are built]

 

Actually, you cannot both measure time and know exactly where you are in time.  In fact, the more precise your measurement of time is, the more unlikely you are to know exactly what time it is. You can either know what time it was... or experience time in the moment... but you can't do both.

[KAF6045]

11 hours ago, OffRoaderX said:

OP: What time is it?

"Some People": [6 paragraphs on the theory of time and how clocks are built]

• Local civil time (with or without daylight savings?)
• UTC
• GPS (their clocks do not run on UTC, and have to be converted to UTC for clients)
• Local Solar
• Sidereal
• Bessellian year
• Julian year

 

[WRTJ886]

12 hours ago, OffRoaderX said:

OP: What time is it?

"Some People": [6 paragraphs on the theory of time and how clocks are built]

I appreciate the additional technical details.  The OPs question was answered with a single paragraph in the first reply, so it's not like he was left hanging or had to wade through pages of technical content.  I get the "some people" stuff, but this doesn't seem to be the case here.

Edited to add: I don't want to come off as argumentative with my first post on this board.  You're the reason I got into GMRS and I really appreciate your videos.  Keep it up!

Edited July 18, 2022 by WRTJ886
Content

[Coffeemaker]

On 7/17/2022 at 4:48 PM, WRTH574 said:

I saw something about a bench power supply needed to be about 30 Watts output at 13.8 V to have a 50 Watt radio actually transmit on 50 watts. Now I don't know a thing about this, but I'm wondering if connecting a 50 Watt radio to a car battery has enough juju to let it transmit at about 50 watts.   In other words I guess, I'm wondering if the mobile radios are designed to output their max power when connected to a vehicle? I would assume and hope so. I have no idea how much wattage is output by a car battery.

You cannot get 50 watts out with only 30 watts in.  It will probably take 100 or more watts in to get 50 out.  There is no such thing as a perputal motion machine. 

[MichaelLAX]

9 hours ago, Coffeemaker said:

You cannot get 50 watts out with only 30 watts in.  It will probably take 100 or more watts in to get 50 out.  There is no such thing as a perputal motion machine. 

Those are different "watts" in your example.

Which weighs more: a pound of gold or a pound of feathers?

[SteveShannon]

1 hour ago, MichaelLAX said:

Those are different "watts" in your example.

Which weighs more: a pound of gold or a pound of feathers?

What do you mean “those are different watts”?  His statement is correct. It’s impossible for a radio to emit more power (or even as much) as it consumes. There are always losses. Thus his comment about perpetual motion.

A radio that is drawing 50 watts of power will emit a portion of that as heat and some portion of that as RF power.  

[WRHS218]

Does anybody really know watt time it is...

[marcspaz]

Me thinks thisem' sarcasm in these posts. Not sure. 

 

Just in case, I'll use my power supply as an example.  When any of my 50w radios run full tilt on the bench, to create 50w out of the final PA, it uses about 95 watts of DC power.  However, for my power supply to be capable of creating that 95w DC, it burns about 165 watts of AC power.  At about 60%-65%, those are about average efficiencies for this type of good personal use power supplies.  And private use radio efficiencies just kinda suck, too.

 

[KAF6045]

I just finished installing an ID-5100 in the rust bucket (along with wiring up the MXT-115, and an old Cobra CB [the one where the entire radio is the speaker microphone, except for a small power/antenna coupling box]).

The manual for the ID-5100 (50W max) show it has 15A fuses, AND specifies that the power source should be capable of 20W.

13.8V x 15A => 207W draw. 13.8V x 20A (presume an instantaneous peak that doesn't blow the fuse) is a whopping 276W.

I suspect it doesn't really operate at 15A, more likely at 10-12A, which brings the wattage down to 138+W

[Chilango]

All the discussion about wattage is humorous when the op most likely simply confused the power supply's output of 30A with 30W.  

Now I'm craving some A&W root beer.  (That's the truth.)