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   Home   Help Search  
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Author Topic: Additional car battery hook up. (made easy ?)  (Read 3081 times)

Posts: 339

« on: February 04, 2003, 05:16:22 PM »

I am going to try some hf mobiling. Upon taking a close look at my car, I soon discovered that there is no practical or easy place to  run the radios power leads through the firewall to the battery. So, I thought I'd run an extra battery in the trunk and connect the radio to that one instead. Now , to keep this battery charged, my question is, using a parallel connection, can I simply attach a lighter plug to a pair of leads from the rear battery and plug it into the lighter recepticle to keep the extra battery charged ? This way there is no holes to drill and working from the trunk is much easier on this car. Am I going to burn something up or damage my electrical system ?

Posts: 21764

« Reply #1 on: February 04, 2003, 06:03:04 PM »

There are standard methods used to accommodate two batteries with one alternator, but just wiring the batteries in parallel (even via the cigar lighter outlet, which would *really* be a bad idea!) is not one of them.

There are diode steering blocks and switches available for using multiple car batteries, and such items are sold in RV and camping supply outlets.

Rather than go through all that, why not just drive your car into a local 2-way radio shop, or a shop that installs car alarms and high-powered audio systems, and ask them to run the wiring for you?  There is ALWAYS a way, you just haven't found it for yourself.

I've never seen it take more than two minutes for my local "car stereo" shop to pull a large gauge power wire through a firewall from the passenger compartment to the battery of any car, truck, SUV or anything else.  It's their job, and my local shop does about fifty installations a day, so they're pretty good at it.


Posts: 5480


« Reply #2 on: February 04, 2003, 06:04:28 PM »

> I am going to try some hf mobiling.

There really isn't any way to dip your toe into this one, you get sucked in pretty quick!

> discovered that there is no practical or easy place to run the radios power leads

I find that hard to believe, unless you are unwilling to drill a hole or punch through a seal.  Understand when you start to make compromises in the correct installation of HF mobile gear, performance and hence your satisfaction in the operation will suffer.  You won't hurt anything by doing it right.  If you can't find a hole, make one.

>  I thought I'd run an extra battery in the trunk and connect the radio to that one instead.

Fraught with problems.  The battery has to be adequately restrained, requiring it's own 'installation'.  Charge wiring of adequate current carrying capability would need to be run to the engine compartment, and isolation between the starting and trunk battery would need to be installed.  It's expensive, you have to purchase a deep cycle battery and isolator.  The positive side of this is you can run the 2nd battery dead and still start the car.  A cigarette lighter cord won't cut it, the alternator will try and pump dozens of amps into a discharged battery to try and bring it up, way beyond the capacity of a cigarette plug.  Not to mention it's just as much or more trouble to get the wiring from the trunk to the passenger compartment as it is to get it to the engine compartment.

Run some 6 gauge wire through some tubing or loom under the car from the engine compartment to the trunk and save the grief of installing and maintaining a 2nd battery.  Drill as many holes as you need.  It will not affect the value of the car and allows you to do a proper installation, which is actually less time, effort and sometimes inadvertent damage than trying to get around it.  

Mark K5LXP
Albuquerque, NM

Posts: 527

« Reply #3 on: February 05, 2003, 05:55:03 AM »

It all falls back to basic electricity. There are many
unknowns in your idea and it seems that you may be
about to re-invent the wheel. The responses of some
of the other folks seem to be right on the mark. But
the best way is K.I.S.S. Keep It Simple Stupid.
This is not an insult directed at is the
old saying as is. It is the truth. And don't forget
Murphy's Law, anything that can go wrong will go wrong.
It seems that,among many other things, Murphy was a
HAM. We can all explain to you how we have our mobile
rigs and power sources done but in the final analysis
it will be your own ideas that you will accept.
73 and good luck - Tim

Posts: 1003

« Reply #4 on: February 06, 2003, 01:35:31 PM »

I've NEVER seen a vehicle where you could
not get throught the firewall.  Best place to
start is on front passenger side.  Lift up the
carpet and look where the firewall bends to become the
floor.  You should see open areas.  Now take
a punch and tap a dimple where a good spot may be.
Now look on engine side for dimple and make sure it is clear of anything.  If obstructions, see about where you need to move and go back to passenger compartment.
Repeat above steps.  Once you find a good place drill
pilot hole.  Use a hole saw for appropriate size for
cable.  Put a rubber groment in hole.  Run
cabbles to battery bein' sure to avoid movin parts
of engine.   Fill hole with silocone.  Repalce
carpet.  Hook to radio.

Be sure wires are well clear of the catalytic converter.  It gets HOT!

73 de Ronnie

Posts: 7

« Reply #5 on: February 08, 2003, 11:59:30 PM »

I have the same request, but for a slightly different reason. I plan on mounting my mobile rig in the trunk (where the car battery is, btw) and would like it to run off a separate, deep cycle battery. I plan on using the mobile in a cross-band repeat mode while I go off hiking with a handheld, and don’t want it running off the main battery, potentially leaving me stranded with no way to communicate!  I was thinking of running it in parallel with the main battery, with a switch to isolate it, but figure there are all sorts potential problems - flipping the isolation switch, running the deep cycle down 50%, and re-connecting could cause huge current flows, starting currents presented to the deep cycle could buckle plates, etc. What is the best way to do this?



Posts: 115


« Reply #6 on: February 12, 2003, 06:17:09 PM »

Most of the RV/Marine type battery isolators use an old Ford starter solenoid to do the switching.  These are available at auto parts stores for $10-$15.  They then add a dpdt "center off" switch to provide 3 different modes of operation: "off" which completely isolates the 2nd battery.  "charge" which allows the 2nd battery to be charged(and used) when the ignition is on. and "on" which allows the 2nd battery to be used when the 1st battery goes dead.    If you were paying close attention, you notice that the 2nd battery would still go dead if you left the switch in the "charge" position and the ignition on and the engine not running.  

For a second battery, I HIGHLY recommend any of the new "spiral cell" technology batteries.  These things just don't die.  I've run mine all the way down several times, and it has always come back after charging.  Also, they use a gel instead of liquid inside, and are completely sealed and maintenance free.  you can mount them in any orientation you like, even upside down.  They are a bit pricey, but the warranty is alot better than most other batteries.

Off roaders have been using dual-battery setups for years, so check off-road websites for wiring diagrams and product reviews.

Good luck.  Ryan, KB9YNB

Posts: 6

« Reply #7 on: February 21, 2003, 10:01:05 AM »

You may wish to consider the following solution, which is not perfect but works for me:

I have a 100Ah battery in the back of my car which I use for my 500w amp. It is not connected to the alternator, and the amp is also in the back, so no messy wiring through the vehicle. When the battery runs down, I just change it for another one I have, and put it on the charger ready to change over next time around. Using the system about an hour a day, it powers the linear for between a week and ten days. Using it to power a 100w rig on a 50% tx/rx and 100w ssb, you should get about 18 hours use out of it.

If you use your rig a lot this is a pain, but if you are a casual user and don't want wiring hassle, this may be a solution. I personally use this because my alternator is not big enough to charge both batteries and my car battery not big enough to run both the rig and the amp.

Happy mobiling!
Duncan EA5ON

Posts: 635

« Reply #8 on: February 27, 2003, 12:13:38 AM »

there are several issues to consider here.
one is the placement of a second lead acid battery.
extreme care should be taken to vent highly explosive
gasses from battery area.
an enclosed un vented trunk would be a disaster waiting to happen. one spark and boom!
as far as charging such a battery, it could be charged
via some fairly small conductors from your main electrical system.
however, you would have to install a diode to isolate the second battery from back feeding the main system.
you would also need to use a large current limiting resistor as well as proper fusing of conductors to prevent disaster.
in short, leave something like this to a professional.
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