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  #11  
Old 04-17-2013, 10:10 AM
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Great thread...I am in the process of installing a dual battery and Engel fridge system in my 80. I am using a National Luna split charger for the batteries and have already installed the DC outlets and wiring to the rear.

Went with Blue Sea fuse block and Blue Sea DC outlets that lock the plug/socket connection. I also opted to install the outlets in a somewhat "stealth" area in the back so that the OEM panel can cover them up when not in use.
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  #12  
Old 04-17-2013, 01:15 PM
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Getting your fridge on the 2nd battery is important, IMO, as it isolates it more from the voltage drop/spike cycle associated with starting the truck. I can't quantify exactly how bad that is, and realize with only one battery it's not even an option, but for whatever reason I've tried to avoid hooking up a fridge or a radio (Ham or CB) to the primary battery, just to keep its voltage input a little more constant. Curious if anyone cares to refute or support that..
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Old 04-17-2013, 01:31 PM
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I wouldn't (and don't) worry about it too much. A battery does filter but is relatively slow and not terribly effective for high frequency noise. That noise is always there when the alternator is running and can be quite large spikes. Think of the battery as a large bulk capacitor. You need to stick a couple of small value, low ESR caps in parallel to reduce the higher frequency noise.

If you are worried about it you should physically disconnect the power coming into the radio because regardless if it's on or not you are injecting some of that into it. The power is controlled by a microprocessor, so most radios are always on 'a little', so to speak, if they have power connected. It's a small draw, but not zero and there is some susceptibility of the power control circuit.

What you can do is put a 15V clamping Zener across the power input to shunt the cranking pulses. Some manufacturers do that already as part of their overvoltage and reverse voltage protection circuits. It's also part of the EMI/EMC reduction to keep power line noise from feeding through. Go back to your General questions and brush up on differential and common mode interference. If you know what your DC power is supposed to look like you can pretty easily filter AC noise, especially in cars. Had to deal with all of this designing hi-rel power supplies...

FWIW, I hook directly to the battery and sometimes not even bother turning off my radio when I start my truck, although I usually do have it off until the truck is running. I also wind the power leads through a ferrite toroid before they go into the radio.

It is very important that you run both lines together and to the battery. You can get into trouble if you feed the radio unbalanced power, coupling noise must be the same amplitude and polarity on both positive and negative sides for the filter to work. If you run a long positive and ground locally with a short lead, then you can easily get damaging spikes bypassing the filter.
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Last edited by DaveInDenver; 04-17-2013 at 02:31 PM.
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Old 04-17-2013, 01:57 PM
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Dave don't you know we are all too lazy to find this information for ourselves. We just post and wait for your response .
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Old 04-17-2013, 02:01 PM
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Quote:
Originally Posted by DaveInDenver View Post
It is very important that you run both lines together and to the battery. You can get into trouble if you feed the radio unbalanced power, coupling noise must be the same amplitude and polarity on both positive and negative sides for the filter to work. If you run a long positive and ground locally with a short lead, then you can easily get damaging spikes bypassing the filter.
Good stuff Dave.

Question about the above statement: I think I understand the importance of this for my HAM, and I have it connected directly to the second battery via the secondary fuse panel. All Pos and Neg wires being the same guage. Is that correct?

How about the 12V plugs that are also powered off that second battery? Can they just be grounded locally with a short lead? I was curious about this since that second battery isn't itself grounded to the chasis other than through the connection to the main battery.
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Old 04-17-2013, 02:23 PM
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Martin, if you float the whole system so that all the circuits return to the second battery without going to the charging system return, then you are actually better off. The battery itself and, if necessary, a filter can be used to make sure secondary power stays stable and clean.

When you connect to the power lead and reference of the main system, then you need to be careful. When power is floating unreferenced a spike will raise the whole voltage up. IOW, if you're running a floating 12V circuit that sees a 5V spike, the positive side rises to 17V and the ground to 5V when you measure against Earth ground (this is the principle behind common mode filtering). All the loads connected to the floating +12V and floating ground still only see 12V even if you observe something quite different referenced to Earth.

This is a major part of household wiring safety, the idea of Earth grounding and neutral returns.

To answer your question, well sort of. If you are trying to keep the power clean on the secondary battery then all feeds and returns need to be referenced only to that battery to prevent creating ground loops. However, things like a utility outlet would not need the matches length and gauge for noise immunity, but you would not want to tie them to the chassis and you might want to do the matched length to prevent them from additionally being sources for coupled unbalanced noise, though you'd have to be very concerned to bother doing that (think mobile HF maybe). It gets tricky because we are using the common Earth ground (e.g. the chassis) as a RF return for our antennas. So it's quite easy to shoot yourself in the foot unintentionally. Your radio's coax is connected to the chassis, so can't really truly float your secondary power easily and, worse, you might make your radio the single point ground and it's not designed to handle that.

If I had a second battery I would run secondary grounds only to it, never to the chassis. The second battery would be grounded with a heavy conductor (and a braided RF return, too, BTW). I would tie the battery to the charging system with a timed relay so that it disconnects anytime the voltage is outside 11.8V to 14.1V, so starting, winching, etc. I would leave it disconnected until the starting battery has gone beyond the initial recharge post starting, since the alternator will be trying to refill the battery from the major current draw, so the voltage will be relatively unregulated and probably noisy. You could probably just do it on time, give the charging system 5 minutes or something to stabilize.
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Last edited by DaveInDenver; 04-17-2013 at 02:47 PM.
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  #17  
Old 04-17-2013, 02:57 PM
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Thanks Dave, that answers my questions perfectly.

The National Luna Solenoid I have actually does do the timed disconnect you were describing. It disconnects the second battery from the charging system for the first 5 minutes after start-up. I have everything connected to that second battery grounded to the secondary fuse block, and that fuse block grounds to the battery.

You mentioned grounding the second battery with a heavy conductor and braided RF - Can you describe that further? What should I ground it to if it already has a ground to the main battery?
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  #18  
Old 04-17-2013, 03:19 PM
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DC-wise, the frame or main battery negative are fine. Do whichever allows the largest/shortest conductor reasonably. There is probably a slight advantage going to the main battery, if the run is short. That quasi-floats the secondary relative to the alternator. If the battery is mounted remotely, a large cable to the frame from the negative side of both batteries is for all practical purposes like connecting them together.

From an RF standpoint you should use your truck frame as Earth. Everything should be referenced to it with braids and a minimum of hops. Even if you skimp on the DC negative leads, braids to the frame will help tremendously with RF noise.
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