Powering radio while mobile from car battery

[VK3LDR]

Greetings,

On the weekend I went up to the inlaws and finally threw up a 40m inverted V as a quick test, worked a treat. Nice low non existant noise floor, and exchanged reports with US stations during the IARU contest. My problem was power supply.

While checking SWR/PWR I found the radio would only put out about 40-50w... I started the car, and presto, full power.... I'm a bit puzzled now. The car battery is only about 9 months or so old as I killed the last one by leaving the radio on in the car overnight and all day

The radio is an Icom Ic706MKIIG that demands around 10-15 amps on full power TX and the car is a 2000 Holden Commodore V8, so it's not exactly a small battery. I have run a dedicated power wire from the battery to the interior of the car specifically to power radio equipment bypassing the cars electrical system (both lines are fused).

So it seems the battery does not have enough grunt to sustain continous heavy current draw??? The central locking system in the car uses close to 20amps when you activate/de-activate... Is this normal that I can't get enough power to the radio? Is this a sign of this battery on its way out?? Do I need to consider running a second battery in the boot wired into the car's electrical system (what about ventilation then?)?

Just to make sure I didn't kill the battery while using it I plugged a car battery charger into the battery as well which delivers up to 8A while charging.

Next time I operate from the inlaws QTH I'll just take my power supply with me an an extension chord and operate using that, better yet, take all my radio gear OUT OF THE CAR and operate in the comfort of being inside the house too! However, this still worries me when operating mobile elsewhere...

comments appreciated,
Dave

[VK2ZRH]

Dave,

Best you seek some evidence . . .

Get a digital multimeter with max/min capability. Set up the function, choose the right voltage range and attach the DMM leads directly to the battery terminals.

Go through your on-air routine without the engine running (and no battery charger attached).

Examine the results on the multimeter's display. If the max reading is within what the car manufacturer says should be the battery voltage range (if known - otherwise assume the range is 12.5-14 volts), all well-and-good, provided that this is within the required operating voltage range for the IC706. If the min reading is greater than 1 V (one volt) lower than the DMM's max reading after (say) 15 minutes operating, then it's indicative of a problem.

Anyway, there's a place to start.

If you don't have/can't get a DMM with max/min, you can use whatever meter you've got and ask another person to perform the max/min function and write the values down.

73, Roger Harrison VK2ZRH

[VK3BA]

Dave,

I've had these dramas for years. These days in the Hilux 4WD (current model), I run the factory battery (Cranking capacity) plus an ARB dual batt system which utilises an electronic isolator and Exide Extreme (hybrid Deep Cycle) which cost a fortune but after years of regular jump starting batteries, changing batteries, etc, I'm glad I finally went this way. For years I've always ran a few radio's (and these days an Engel fridge) partially because of my role as a volunteer in an emergency services organisation, I've regularly killed batteries. The thing with normal cranking batteries is that once you flatten these a few times, they're finished. Go get a new one. But with high capacity deep cycle batteries, you can run these flat time after time (there are limits!) and you'll generally recover them after charging. These actually prefer to see a bit more discharging and charging than what your regular cranking battery does.

I know it's hard to install a dual battery system in a regular car (I installed one in my previous XR8 ute), it's worth investigating. If you've got a 4WD, I definitely reccommend a dual batt system - get the best batteries you can afford.

Cheers,

[VK2OMD]

Greetings,

On the weekend I went up to the inlaws and finally threw up a 40m inverted V as a quick test, worked a treat. Nice low non existant noise floor, and exchanged reports with US stations during the IARU contest. My problem was power supply.

...

Dave,

Firstly, you are subject to the characterisitic of the battery. Is it Lead-acid, or a Calcium battery? (There is a direction to replace Lead acid batteries with Calcium batteries which will require a different charger, and they will probably migrate to 24V.)

I am not (yet) familiar with the Calcium batteries.

Lead acid batteries carry a surface charge which is depleted quite quickly by load current after cessation of charge. This initial rapid drop in terminal voltage will see reduction of around 2.1V per cell to around 2.0V per cell. Voltage will then decrease fairly linearly with discharge Ah until a rapid decrease again occurs below about 1.8B per cell. At this voltage, you might regard that you have extracted the available charge at your load current.

Additionally, there is voltage drop in the conductors and accessories (connectors, fuses etc) between the radio and the battery. These are easily measured with a DMM, but differently to others, I suggest you find a load other than a radio transmitter to measure them... lots of multimeters are adversely affected by RF and until you prove them accurate, measurements in a high RF environment should be regarded as unreliable.

One of the ham myths is that the negative lead should go all the way from the radio to the battery, rather than using the body for a return. Measure it, work it out for yourself. Measure the voltage drop in conductors and fuses. A good installation for the IC7000 should have less than 200mV total voltage drop at 20A to the radio terminals. This is DC... don't complicate it by introducing RF into the equation.

Ok, so you address these issues...

Your radio might deliver 100W into a non-ideal load with 13.8V supply on its terminals... but its ability to do that is diminished at low voltage, so high antenna VSWR is likely to have more impact on power at reduced voltage.

One answer is to use a DC-DC converter to develop 13.8V at supply voltage down to 10V... but they are expensive. In time, these will become cheaper... particularly when cars migrate to 24V and you use DC-DC converters to run your 13.8V equipment.

Another solution is to float charge the car battery using an AC charger... but in-vehicle charging of batteries has risks in modern cars.

And another solution is to power the radio from an AC power supply as you suggested.

If you can quantify the problem, ie to express it in numbers, you are much closer to finding a solution that suits your needs.

Does this help?

Owen

PS: The pics in the article at http://www.vk1od.net/dc/Commodore.htm might give you some ideas.

[VK2OMD]

Dave,

I should perhaps have given you a hint on measuring the voltage drop in the distribution system.

If you make measurements from rail to rail (ie +12 to -12), you are subject to the change in battery voltage under discharge... so try to avoid it as it is error prone.

Connect a substantial load, 10A or better, to the load end of your distribution system. Measure the actual test load current.

Measure the voltage drop from the battery post (not the attached terminal) to the the load end of rail using a DMM. You will need to extend one lead of the DMM to do this, so a 6m long insulated wire with croc clips will be handy. There is almost no current flowing in this wire (~1µA) so voltage drop is insignificant. Write it down.

Now do the other rail, and write it down.

Add the two together. Now calculate the drop at 20A (the full load current of the IC706) by multiplying by 20/ActualTestLoadCurrent.

Does that make sense?

Now you can do the same for components of the distribution... measure the drop across the fuse (you MUST have one). Does the lead for the Icom have two 3AG fuses, one in each leg? Review this after you measure the voltage drop.

Is the DC lead used with the Icom the out-of-box length? No one likes to cut them... but measure the voltage drop.

If you used the body return path for the negative lead, what was the voltage drop on the -ve side compared to +side, what conductor size did you use?

There is an afternoon's fun and learning in store!

Owen

[VK3LDR]

Thanks for all of the responses.
I don't operate mobile enough to warrant an advanced battery system at the moment.
I will endeavour to carry out some measurements next time I'm up there I think and check the voltage drops under load etc. I think the only steady heavy drain current I'll be able to use will be the power windows (or maybe a big dummy load for the radio). I will also outright check the voltage levels as well, I didn't think to do that but didn't have my DMM with me.

I just would have expected a car battery to have a fair reserve of ah in it to drive a 100W transmitter. Oh, and ironically, of course I didn't note this problem in the past as the last few times I've operated mobile I was running low power only while restricted by foundation limits!! That's why this has become a new problem. The ic706 has even powered itself off when TXing.

Re SWR, the dipole was surprisingly broadbanded and the SWR was practically 1:1 most of the way through 40m. This was also ironically using the suspect feedline and co-ax choke that I was having trouble with on 20m. Obviously at this point the feedline and co-ax choke are in full working order. the apex was only however elevated about 5m off the ground with the end point on one about 2m above ground and the other about 1.5m above the ground.

Cheers,
Dave

[VK2OMD]

Thanks for all of the responses.
... The ic706 has even powered itself off when TXing.

Dave,

You seem to have attributed this to low voltage.

IC706's are infamous for shutting down when they get RF into the power supply leads... so the cause of the shutdown might not be as you at first think.

Can you see why involving RF in trying to understand the DC regulation has disadvantages?

Owen

[VK3HZ]

Most 12V powered rigs (PA's etc) are quite sensitive to voltage variations. The drop from 14V with engine running to under 12V after a few mins on battery alone can make a substantial difference to power output. Most rigs need to be run at 14V to meet spec, and some even higher than that ...

I suspect there's nothing wrong with your battery.

[VK3AUU]

My Icom 706MkIIG was FMing on 144 Mhz SSB when I used the supplied leads connected to the cigarette lighter socket without the engine running so I installed another battery behind the seat and kept it charged through the lighter socket. Still had the same problem without the engine running.

Solution was to reduce the length of the leads to the minimum and the problem went away. Power output also went up.

Given the same amount of drive power, the output is roughly proportional to the square of the supply voltage. If the same also applies to the drive device, reducing the supply from 14 volts to 12 volts at the set, can result in a significant power reduction.