How long would this last?

[bassjunkie]

Ok, this may be a completely stupid question to those that know about electronics - but I'm trying to work out whether the work is worth it.

I have a radio going straight to battery - it's a 2 way radio that receives only, so my guess is it would not draw much ampage in general use. If I was to leave the radio on overnight accidentally, and my battery was fully charged beforehand (on a new car), would this leave my battery too flat to start?

Thanks in advance

[mancow]

Likely not but a few figures need to be known... mainly the amperage consumption of the radio. Most car batteries can power a radio on RX only for quite a while. However, there are some that are hungry like the dual band dual deck DIU Motorola Spectras.

[bassjunkie]

Having a little trouble finding how much amperage it would draw on RX. It's a Tait T2020, though I'm not sure that would help as I've done quite a bit of searching before

I've seen a few documents saying that the radios have low consumption when on standby or receiving only, but no figures.

If it's a likely yes, however, then that's good enough for me - I don't plan on leaving the radio on, just more so I know whether or not I'll be calling my service company if I do.

Thanks

[PA_Firefighter]

Another consideration is how much traffic it will receive. Makes a big difference in how much power it will consume. If this is a consideration, spend the money and invest in a Charge Guard. It can be programmed to shut down in 15 minute, 30 minutes, an hour, whatever. another solution is to wire it to an ignition sensing circuit. Get a fuse tap and find the fuse for the vehicle's stereo. Just make sure it is on the side that is supplying power, not the fused side. That should remove any fears.

[mrweather]

It will depend on the kind of radio, the amount of traffic it receives, etc.

My Kenwood TK790/890 setup draws almost 800 mA at idle (no signal received).

[KI6ABZ]

It's a very easy question to answer.

Pick up a multimeter, if you don't have one already. Set it to the 10A range and connect it in-line between the radio and battery: Disconnect the radio's red lead. Connect the meter's red lead to the battery and the meter's black lead to your radio's red lead.

Now turn your radio on and turn the squelch all the way down so that you're getting static.

If you get a reading of less than 1 amp, switch to one of the mA modes, then multiply that number by 1000 to get the amperage reading.

Assume you can draw 25 amp-hours from the battery. Divide 25 by this figure, and that's how many hours you could probably safely leave the radio on.


So let's assume that you can safely draw 25 amp-hours from your battery and still start the car. Now if your radio is pulling 500mA, or .5 amps, it will take 50 hours to draw 25 amp-hours.

[bassjunkie]

Originally Posted by KI6ABZ

It's a very easy question to answer.

Pick up a multimeter, if you don't have one already. Set it to the 10A range and connect it in-line between the radio and battery: Disconnect the radio's red lead. Connect the meter's red lead to the battery and the meter's black lead to your radio's red lead.

Now turn your radio on and turn the squelch all the way down so that you're getting static.

If you get a reading of less than 1 amp, switch to one of the mA modes, then multiply that number by 1000 to get the amperage reading.

Assume you can draw 25 amp-hours from the battery. Divide 25 by this figure, and that's how many hours you could probably safely leave the radio on.


So let's assume that you can safely draw 25 amp-hours from your battery and still start the car. Now if your radio is pulling 500mA, or .5 amps, it will take 50 hours to draw 25 amp-hours.

Thanks, perfect reply

I haven't got a multimeter, but I think my father in law may... I shall ask around.

[rescuecomm]

You should be okay with most radio gear if your battery is in good shape. I have left the CB and the Kenwood TMV-708 dualband on for a 10 1/2 hour shift and the car cranked right up. The parking lot is about a 1/4 mile from an interstate highway. On the other hand, years ago I had an old Motorola Motrac on 6 meters that ran down a size 24 battery in about 6 hours (they used a vibrator power supply for the transmit that I didn't wire through the key switch). I quickly rewired it after nearly being stranded on a mountaintop. Fortunately,they don't make stuff like that anymore.

Bob

[daveMICU]

Originally Posted by KI6ABZ

It's a very easy question to answer.

Pick up a multimeter, if you don't have one already. Set it to the 10A range and connect it in-line between the radio and battery: Disconnect the radio's red lead. Connect the meter's red lead to the battery and the meter's black lead to your radio's red lead.

Now turn your radio on and turn the squelch all the way down so that you're getting static.

If you get a reading of less than 1 amp, switch to one of the mA modes, then multiply that number by 1000 to get the amperage reading.

Assume you can draw 25 amp-hours from the battery. Divide 25 by this figure, and that's how many hours you could probably safely leave the radio on.


So let's assume that you can safely draw 25 amp-hours from your battery and still start the car. Now if your radio is pulling 500mA, or .5 amps, it will take 50 hours to draw 25 amp-hours.

This information is true, however one would also need to know what battery capacity is needed to start the car. Your answer does not take into account that a car will not start with a battery in 5% charge condition...
-Dave

[KI6ABZ]

That's true. The magic figure to know is the battery's reserve capacity or the amp-hour capacity.

I was assuming a 50 amp-hour battery being discharged to 50%. A 50-amp-hour battery is pretty small. From what I've read, car batteries are anywhere from 40 to 100 or more Ah.

You can deduce Ah by looking at the reserve capacity, if the Ah rating isn't listed. Reserve capacity is the number of minutes that you can draw 25 amps. So if you multiply that by 25, that's the number of minutes you can draw 1 amp; then divide by 60 for hours.

For example: if the reserve capacity is 30 minutes: 30 * 25 =750. 750 / 60 = 12.5. Cut that in half, and you can run your scanner for 6 hours or so safely. But don't do that too often. Starter batteries don't like being discharged too far too often.