UHF power consumption in amp hours?

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UHF power consumption in amp hours?

Post by sneaker » 26 Jul 2017, 3:45 pm

G'day,

I've got an old UHF radio by Uniden, can't find the model as the sticker is long gone.

Just a normal size one, 5w, and connected to 6db antenna if it matters.

I have an 85 amp hour starter battery, I'd like to know how much the UHF radio is drawing and how quickly it will drain this.

I've looked up books for similar radios and stuff and it talks about power in voltage and watts, but not the consumption in Ah so I'm not really getting an idea.

Can someone tell me what a typical UHF radio uses in Ah?

Thanks.
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Re: UHF power consumption in amp hours?

Post by AusTac » 26 Jul 2017, 4:13 pm

0.41 A i reckon
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Re: UHF power consumption in amp hours?

Post by BRNO_Bigot » 26 Jul 2017, 4:21 pm

Ok, I'll take a go at this .... not that I know what the damn' thing actually draws, but ...

Power is Volts times Amps (and is measured in Watts) - so a 5 Watt appliance, switched on and drawing its 5 Watts is drawing about 5/12 (Watts/Volts) = about 0.5 Amp

Since the Battery capacity is 85amp-hours - this means (all theoretically, BTW) that the battery will either deliver 1 amp for a period of 85 hours or 85 amps for a period of one hour, but don't try this at home, delivering 85 amps will surely screw your battery.

Anyway and 85 amp-hour battery delivering 0.5 amp should last about 170 hours according to my calculations.

Anyone with actual experience who can gainsay this?
Last edited by BRNO_Bigot on 26 Jul 2017, 7:47 pm, edited 1 time in total.
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Re: UHF power consumption in amp hours?

Post by YoungBuck » 26 Jul 2017, 6:15 pm

I think the 5w draw is when transmitting, not when receiving or being idle. Receiving or being idle will probably use around 1w (guessing here).
This would mean you would get a lot more than 170 hours from the battery but would vary depending on your transmission use.
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Re: UHF power consumption in amp hours?

Post by Chronos » 26 Jul 2017, 6:27 pm

YoungBuck wrote:I think the 5w draw is when transmitting, not when receiving or being idle. Receiving or being idle will probably use around 1w (guessing here).
This would mean you would get a lot more than 170 hours from the battery but would vary depending on your transmission use.


Yep, SFA when idle/receiving and a little more when transmitting, my GME handheld 5w unit has a 2200mw battery and lists a 24h standby battery life.....an 85AH 12v battery should have about 1020000mw available giving you about 463 days of standby :lol:

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Re: UHF power consumption in amp hours?

Post by darwindingo » 28 Jul 2017, 2:13 am

Just remember that there is other things that need to be considered, Cyclic Discharge V/S Expected Cycles. ( the less you discharge between cycles the more cycles you will get ) .. There is even more to it than that... Look up peurket's law... Discharge of a battery is not as linear as one would expect depending on the amp draw ... Look, I'm two inebriated to attempt to explain it. Its not quite as simple as it first appears. Also Ah ratings of batteries are somewhat deceptive... Divide them in half to remain within the manufactures recommended discharge range... Its a bit of a marketing ploy .. You will be surprised as to the effect discharge depth has on the expected cycles..... I'm also curious if this battery you have still needs to start a vehicle ?

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Re: UHF power consumption in amp hours?

Post by doc » 28 Jul 2017, 10:54 am

I think there's a lot of confusion and misinformation here in regards to currents. Don't be confused with the 5w Rating! This refers to the Transmit power through the antenna - NOT the current drawn / required from a power source.

As an example - the specs for the RV-M7-U data band radio have receive mode set to 90mA. (On 12v this would be 1W). HOWEVER - the specs for Transmit on this same radio (at full transmit power of 5W) is 2.7A. This is around 30W of power for a 5W transmission!

Thus (for this unit) - to achieve 5W transmit, 30W input is required from the battery / power source.

The best way to find out (if you don't have spec sheets available) is to use a multimeter and measure the draw current - but performing calculations based on the antenna transmit power (aka 5W) will be grossly misleading.
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Re: UHF power consumption in amp hours?

Post by gillian » 01 Aug 2017, 8:18 pm

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Re: UHF power consumption in amp hours?

Post by Daddybang » 02 Aug 2017, 6:35 am

Can't tell ya all the technical aspects but I've often sat in camp with the uhf left on(no other accessories)occasionally sending and receiving for 5-6 days and still had plenty in the battery to kick the landie over :thumbsup:
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Re: UHF power consumption in amp hours?

Post by AlanK » 02 Aug 2017, 11:48 am

Chronos wrote:an 85AH 12v battery should have about 1020000mw available giving you about 463 days of standby :lol:


I think if your mates don't turn up by the time that goes flat you can officially say you've been stood up :lol:
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Re: UHF power consumption in amp hours?

Post by Lorgar » 02 Aug 2017, 1:42 pm

BRNO_Bigot wrote:Anyway and 85 amp-hour battery delivering 0.5 amp should last about 170 hours according to my calculations.

Anyone with actual experience who can gainsay this?


I've never run mine out to test how long before it goes flat but if waiting at a camp site for friends to rendezvous or if someone in our party is wondering around I leave my mounted unit on until contact is made.

Have done 6-7 hours of idling without any dramas starting the next day. I wouldn't leave it on for days unattended but for practical periods like that I've never had it show any signs of being drained.
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