Charging The Batteries From The TV While Running The Refrigerator On Battery

[Bill]

Originally Posted by ThePair

It was a solid state relay (SSR) for DC - DC.

Unlike a mechanical relay, an electronic (solid state) relay has a fairly high resistance when switched on. Although I have no specs for the one you used, in the general literature I commonly see a significant fraction of an ohm, or more. And of course if you run 10 amps of current through a quarter ohm of resistance, that means 25 watts is dissipated as heat. And that's a lot of heat. All of the literature says that this is a weakness of electronic SSRs in general - they really, truly, need a heat sink.

Think of the isolator in your tow vehicle, the one that prevents the TM from draining your tow vehicle battery. It is either electronic (a diode in this case) or a mechanical relay. If it is a diode, it is mounted on a hefty heat sink. On the other hand, if it is a mechanical relay, it is a cube about 1 inch on a side, costs $3, and requires no heat sink. To me, that would be an ideal solution here.

At any rate, from what I recall, I think that the PID approach was probably sound, though quite elaborate. But if I were doing it, I would use a mechanical relay on the output, at least for the DC side.

Remember, these are general observations. I don't have the specifics of your circuit. But at a minimum, a hefty heat sink, and good air circulation to get rid of the heat, is certainly necessary with an SSR.

Sorry you got to be the guinea pig. But if you decide to go back and try a relay, I would be glad to work with you on that aspect.

Bill

[ThePair]

I already have the AC side done. Untested extensively, but done... Used an SSR for it, but we did the calcs after my little mishap, and feel that the box won't need any additional heat sink/cooling.

Thanks for the offer, though, I do appreciate it. I'll post when I've tested the circuit to make sure it's working as intended--I have a few wires that need to be a bit longer, and then I can move on to another project :P

[scrubjaysnest]

Attached is a short pdf of how I did this mod. A couple of notes:
1. The easiest approach for the 2008 Dodge RAM is replace the factory Bargman connector with a standard universal bargman. Then replace the factory +12VDC wire with at least a 10 gauge wire and add an isolation relay. The way Bill did his with 10 gauge extension will work fine. Under refrig splice a 10 gauge wire both +12vdc and ground to TM bargman pigtail and run straight to the TM battery. Use breakers at the TV and TM.

2. If you go the route I did, separating out the TV fusible link from the alternator you may void your warranty!!!!

The pdf goes into a little more detail with pictures. Voltage at the TM battery with the refrig on DC settles out at 14VDC. Starting out with a fully charged battery and driving 3 to 5 hours between campgrounds should have your TM battery in great shape.

[scrubjaysnest]

So far the DC changes I made are working well, we are getting 8 days on a battery without recharge and it extends out to 13 days with a 3 hour charge between campsites. I am not going below 60% discharge before swapping batteries.

Can't beat these for battery connectors.

http://www.amazon.com/Moroso-74201-B.../dp/B000COO1QI

or these,

http://www.amazon.com/Johnson-Outdoo...sr=1-2-catcorr

Koz

Originally Posted by Pitbull

Can't beat these for battery connectors.

http://www.amazon.com/Moroso-74201-B.../dp/B000COO1QI

Koz

I agree.

I use those for a jumper cable between my two ATVs.They are idiot proof. They can not be connected with reverse polarity.

They come in different current capacities. Not knowing how much current an ATV starter draws, I used the 100 amp connectors with 6 gauge wire.

BTW, the cheapest way I found to purchase around 6 to 8 feet of two conductor 6 gauge wire was to purchase jumper cables and cut the ends off.

[rickst29]

Originally Posted by scrubjaysnest

Attached is a short pdf of how I did this mod. A couple of notes:
1. The easiest approach for the 2008 Dodge RAM is replace the factory Bargman connector with a standard universal bargman. Then replace the factory +12VDC wire with at least a 10 gauge wire and add an isolation relay. The way Bill did his with 10 gauge extension will work fine. Under refrig splice a 10 gauge wire both +12vdc and ground to TM bargman pigtail and run straight to the TM battery. Use breakers at the TV and TM.

Nice job! Maybe it's time for another switch in my switch panel for the "6-way" fridge?
One quick question, though: Is there a new fuse protecting your new "into-the-fridge +12VDC" wire, or do you now depend on the Refrigerator's own 12V fuse alone? (I.e., the one between the terminal block and the Control Panel, along one of the yellow wires.)

[rickst29]

and not so much from running the Fridge from a bypass wire.

If we assume the total length of 10 AWG Fridge wire avoided (+12VDC) to be 15-20 feet, the the voltage drop along that length (at 11A) is about 0.26V. The power loss, for Fridge alone, is only about 3 Watts. The Key reason, however, why this mod (and its variations) makes such an enormous difference is the combined load.

11A for the Fridge, plus some and some amount of current which will ultimately leave the WFCO DC Distribution Panel on the battery circuit (charging batteries), are added together on the same wire for the portion from Bargeman back to the WFCO, (i.e., it's DC DIstribution panel). Suppose that battery charging load is, under your current "conditions", another 10A. If voltage really held up at 12.8V, into the WFCO, then there would be (roughly) .10 Voltage drop on the segment from WFCO to Fridge, and (roughly) .12 Voltage drop on the segment from WFCO to the Battery +12V inline fuse. But the segment from Bargeman to WFCO is loaded with 21 Amps, and (If I assume the +12V inside the cable to be another "skinny" 10 AWG, for length of about 15 feet) then the Voltage drop on that segment (alone) is about 0.5V, almost 4%.

5x as much Voltage drop getting to the WFCO with that wire carrying both loads - and, with so much less Voltage present at the WFCO panel on it's Battery and Fridge circuits, battery charging will be much slower. The fridge will also be a bit harder to keep cool.
- - - - -
My summary: Messing around with a new wire to the Fridge doesn't save a lot of power, or create significantly better refrigeration. (The external on/off switch is probably nice to have.) The Key to Everything is getting the battery charge current onto a separate path of wire(s) from the Fridge current. As early as possible, and definitely before running all the way down the TM to reach the WFCO.

[rickst29]

BTW, I think this adds a need for at least three (not one) new fuses. The "battery charging wire" should almost certainly be fused at 30A (that one's obvious). But your TM battery bank can drive that "battery charging wire" backwards, and the possibility of driving either side of the "Bargemen cable junction" into overload requires that it be fused appropriately. Safest would be One fuse along the "battery-to-bargeman" wire itself, but another along the bargeman-junction-to-WFCO segment, and another along the bargeman-junction-to-TV segment.

Here's why:
Consider an intermittent and not-totally-shorted-out fault between TV and the junction. The segment has an endpoint fuse at the TV power source, but it is not directly interrupted on the TV side of the "new charge cable" junction. When current runs from the TV, that is the only source, and the one fuse is adequate. But we have created a pair of paths from the TM battery - one on the new "direct battery charge" cable (I presume to be fused at 30A), and another on the 30A battery to WFCO connector, through the WFCO, out through the junction, and into that TV-to-bargeman segment. These flows are additive, the wire could be subjected to 30+30 = 60A before either of the TM fuses blows up. (If the wire ain't #6, then we need a fuse).

Next, consider a similar fault (<60A, more than 30A) inside the "bargeman" wire from junction back to WFCO. If the TV allows up to 30A, the batteries can also supply 30A into the junction (it's supplying the fault, rather than accepting charge). The wire segment between the junction and the fault runs at up to 60A. (And that wire ain't #6, either.) Fuse it at the "junction" end.

[scrubjaysnest]

Originally Posted by rickst29

Nice job! Maybe it's time for another switch in my switch panel for the "6-way" fridge?
One quick question, though: Is there a new fuse protecting your new "into-the-fridge +12VDC" wire, or do you now depend on the Refrigerator's own 12V fuse alone? (I.e., the one between the terminal block and the Control Panel, along one of the yellow wires.)

A note here I use breakers instead of fuses to avoid heat problems with the fuse connections. There are three breakers under the hood of the TV and three more back in the battery area of the TM so everything is covered. The original fuse at the refrig was kept as there is only one feed; the switch.