eHam Forums => Mobile Ham => Topic started by: K2FR on March 14, 2010, 11:00:53 AM

Title: 12v - 12v battery charger
Post by: K2FR on March 14, 2010, 11:00:53 AM
ok, so I am working a setup for my new Subaru Forester,  I just carried a HT in my mustang so I had no need for a full setup.

However I am looking at a 706 for the Forester, and would like to just tuck everything away in the back and run through the remote head unit.

I have a 12v 120 watt cigarette lighter plug in the back, so heres my idea.

I dont want to run a main line off the battery all the way back,  So i was thinking of strapping down a battery box maybe a 75 amp gel cell, or whatever I can find.  and run the rig off that directly.

I was wondering if there is a 12v type battery charger that I could use safely for this setup.  Use the Cigarette lighter plug for a 12v source or 14.4 when the cars running to trickle charge the 2nd battery and thus keep the two systems seperate.

is there such a device?  any way to safely do what im thinking about?

this way id never have to worry about leaving the radio on and draining the main system out, or put more wear on the alternator by putting that large load on when in use.  

Thanks for the input!
Andrew - K2FR

Title: RE: 12v - 12v battery charger
Post by: K0BG on March 14, 2010, 05:20:02 PM
Best advice I can give you; don't!

About all you're doing is complicating the installation, and adding a bunch of failure points.

Wiring from the front to the rear, and any modern vehicle, isn't a 10 minute job for sure, but it isn't difficult. The difficult part is making time to do the job correctly. I suspect if I bought a Forrester, I could wire a rear mounted radio in less than 2 hours, and you wouldn't see the wiring, even if you knew where to look. You can also rout the remote head wiring in the same place, and never see it either. Again, it just depends on how good an installation you want to have, once you're do. Remember the old cliché; If you don't find time to do the job right the first time, how are you going to find time to do it over?

Title: RE: 12v - 12v battery charger
Post by: K5LXP on March 15, 2010, 08:38:22 AM
Not to mention the expense- all of the stuff to do that costs a whole lot more than a piece of wire and some terminations.

Mark K5LXP
Albuquerque, NM

Title: RE: 12v - 12v battery charger
Post by: KH6AQ on March 16, 2010, 10:39:46 AM
Yes that will work. You will want to current limit the cigarette lighter to gel cell path so it does not blow the fuse. Let's see what can be done with a passive circuit.

The charging system will run at 14.2 volts. Let's say the gel cell drops to 12 volts under load. To limit that to 10 amps we want a 0.22 ohm resistor.

Now if the cigarette lighter fuse is 20 amps we could have a situation where the 0.22 ohm resistor must handle 20 amps. That is 88 watts. You could use a 2 ohm, 225 watt resistor. Digikey has these for $12.90 part number HL22507Z2R000JJ, Vishay.

What else can be used as a charging resistor? An automobile headlamp. A 50 watt lamp will be about 4 ohms hot and 0.3 ohms cold. That is a nice positive temperature coefficient resistor. During normal use it should not glow and can be left in the open.

Title: RE: 12v - 12v battery charger
Post by: KH6AQ on March 16, 2010, 12:33:10 PM
For higher charging current parallel headlamps.

Title: RE: 12v - 12v battery charger
Post by: K2FR on March 16, 2010, 01:03:05 PM
thanks for the input everyone.. Seems Westmountain radio has exactly what I needed..

I think thats pretty much what I was thinking about.

all in a nifty little thing thats pretty much plug and play.

Im still thinking about running a bigger main line back just for safety and not using the cars wiring.

Title: RE: 12v - 12v battery charger
Post by: K0BG on March 16, 2010, 02:39:58 PM
Well, I think you're on the wrong track. Let's start over.

You cannot connect to, or isolate, non-common battery types without jumping through a lot of hoops. It just isn't worth the expense. You can justify it of course, but that in itself doesn't mean much.

The suggestion to parallel the headlight circuit isn't such a good idea either (the headlight circuit is also tied into the engine CPU. Read on about that). If you have to have a hookup, then buy a trailer hitch assembly from your dealer. Included in the package, is the requisite wiring. And, part of that wiring is a circuit typically called B+. It is meant to charge trailer batteries. Lead acid ones of course.

The problem with isolators is that you end up putting a discharged battery on-line with one that isn't. Every late-model routes the alternator output through the engine CPU. This is done to control emissions during both closed and open loop conditions. When the aforementioned hookup occurs, a lot of engine control systems will assume something is amiss, and turn on the Check Engine light.

If you just have to use a second battery (it is recommended in some cases, using an amplifier?), they should both be lead acid, and hooked in parallel. The only isolation needed is a fuse to protect the wiring.

Incidentally, the unit from West Mountain is provide a convenient way to parallel a battery, and a power supply. If the mains go off, then the battery takes over. It was not designed for, nor intended to be, a battery/battery isolator.

If you do use a second battery, make sure it's an AGM. If you don't know about these, you might want to visit my web site. Look under Alternators & Batteries. It'll give you the straight scoop.

Title: RE: 12v - 12v battery charger
Post by: KH6AQ on March 16, 2010, 03:11:31 PM
Did someone say anything about a headlight circuit?

I said to use headlamps as positive temperature coefficient resistors. Done it. It works.

Title: RE: 12v - 12v battery charger
Post by: KD4LLA on March 17, 2010, 09:44:01 AM
Looks to me that instead of spending time and $10 for wiring you would rather spend $200+ to have a battery and additional electronics taking up space in your car.


Title: RE: 12v - 12v battery charger
Post by: KF5APV on May 05, 2010, 11:58:23 AM
I've used a 60A 12V "Ice Cube" relay and a 30A fuse for years to charge auxiliary batteries from the alternator.  Use the normally open contacts on the relay and wire the coil to a "Hot in Run" wire in the harness so that the relay closes after the engine starts.  (You don't want the relay to close when you're cranking or you'll blow the fuse). Then feed the fused switched line to your auxiliary battery.  Relays and sockets are commonly available from hobby electronics parts outfits like All Electronics, or from JCWhitney.

Title: RE: 12v - 12v battery charger
Post by: K0BG on May 05, 2010, 01:11:01 PM
David, there is a potential problem using relays (of any ilk) to switch in a second battery.

Almost without exception, all modern vehicles route the alternator through the engine computer. There are several reasons why, but the bottom line is to meet emission standards. When there is a sudden, unexplained load, place on the alternator, the CPU thinks this is a fault, and will light the Check Engine warning. This is why, most of the time, it's best to parallel batteries; as long as they are both lead acid, the electrolyte notwithstanding.

If you operate portable, then the safest was is to use a trailer towing setup. They include what most call the B+. It is usually a 30 to 50 amp circuit (depends on the vehicle), that is relay switched by the engine CPU. This separate circuit is also monitored for obvious reasons.