Coach Battery Wiring

Keelandb · December 15, 2015

My Greyhawk 31FK needs a new and larger coach battery so that I don't have to run the generator so much while boondocking. A larger or additional battery will not fit in the original Jayco location so I placed two Lifeline GPL 8DL 12 volt AGM batteries in the basement compartment at the back of the RV. Hooking the AC system was easy, but I'm not so sure about the 12 volt system.

At the original battery location a wire with blue tape leads from the positive post of the battery to some type of fuse/breaker, then to some type of electric canister. Could that be part of the coach battery shutoff that is switched just inside the RV, by the front door? From the electric canister a wire with blue tape seems to go to the 12 volt fuse box, again indicated by blue tape near that end of the wire. If this wire at the 12V fuse box is indeed the lead to the 12V system that is fine, but where/how do I hook the negative wire to the 12V system?

Kirk W · December 16, 2015

While it is possible that what you describe is accurate, there is no way for us to know based upon what little you have given us. It would help if you were to post a picture or two. On the negative cable, that is normally tied to the RV chassis ground. The way to know for sure that you have a good and proper connection is to use a meter to verify that your new negative cable has new 0 ohms resistance between it and the one at your present battery.

Let me suggest that you start the project by first reading "The 12V Side of Life" on Mark Nemeth's website.

Keelandb · December 19, 2015

Thanks for the reply Kirk, but a friend who is an electrician on an offshore oil rig stopped by to help me. I was right in that the blue tape identified the wiring from the battery to the 12V system of the RV. What I did not know was that all of the wires that were originally connected to the positive post on the original battery had to be connected together to have power from the generator and alternator going to the batteries/coach. Everything seems to be working now, even though I now have a rube goldberg of wiring going from the old coach battery location (about halfway down the length of the RV) to the rear of the RV where the new batteries are located.

Kirk W · December 19, 2015

What I did not know was that all of the wires that were originally connected to the positive post on the original battery had to be connected together to have power from the generator and alternator going to the batteries/coach.

That is normal for most motorized RVs but the way in which it is done varies quite a bit. Electrically, a continuous electric path is the same as having everything connected at one common point, if we ignore any small amounts of resistance. As long as all of the loads needing +12V are connected to the positive post of the battery and the output from the alternator is also connected to it at some point as well as the output from the 120V/12V converter, it really isn't critical where each of those connections are physically located. The important thing is to use a larger diameter wire as the distance gets greater in order to minimize power loss over distance. All wire does have some amount of resistance so the longer the run the more power it uses just getting where it is needed. As wire diameter increases the resistance per foot decreases.

Remember that power is current times voltage (Amps x Volts = watts). This is the reason for much heavier wire used in 12V systems over what is needed for 120V systems.

oscarvan · December 19, 2015

What I did not know was that all of the wires that were originally connected to the positive post on the original battery had to be connected together to have power from the generator and alternator going to the batteries/coach. Everything seems to be working now, even though I now have a rube goldberg of wiring going from the old coach battery location (about halfway down the length of the RV) to the rear of the RV where the new batteries are located.

I suspect that is 12V power TO the generator, for the starter. The generator output is generally 120V or 240V

oldjohnt · December 19, 2015

Hard to say not being there, but FWIW I tend to agree with Oscar above, and here's how my motorhome was wired (Does NOT mean yours is that way)

From the Positive post on my house battery bank (via connectors and buss etc) I basically have THREE big cables leading away.

a) One to My Onan Generator which supplies 12 VDC to the Generators Starting Motor plus supplies its low power 12 VDC control circuits. The Genertators 120

VAC output wires to my 120 volt 30 Amp Generator Output Receptacle located in the box where I store my RV's shore power cord and plug. In a bigger unti it may

wire to a ansfer Switch

One to my RV's 12 VDC Distribution Panel which powers all the fused 12 VDC circuits throughout the coach for lights and water pump and vent fans etc

c) One to my engine starting battery + via a mechanical Isolation Relay (with time delay) so my engines alternator supplies some degree of charging current into my

house batteries as I'm driving down the road.

Of course, the cable size depends on the amps and distances, but this is NOT any place to use too small cables that could drop excess voltage. Best I recall my cables are like maybe 4 Gauge to Onan Generator and Engine Battery, perhaps smaller to DC Panel. BIGGER (within reason and practicality) IS BETTER

John T

Kirk W · December 20, 2015

I suspect that is 12V power TO the generator, for the starter. The generator output is generally 120V or 240V

I suppose it depends upon what generator? If the on-board generator set, then I'd agree but it he refers to the chassis alternator, then what I indicated. But you may have picked up on a good clue.

Wish that he would come back and respond to let us know if we are going in the right direction.

oldjohnt · December 20, 2015

Keeland spoke of power from the "generator" and "alternator" going to the batteries. As an RV owner and a retired sparky, when speaking of an RV I tend to think and talk like I think Keeland was IE WHEN TALKING ABOUT AN RV I CONSIDER "GENERATOR" AS MY ONBOARD ONAN GENSET AND WHEN I SAY "ALTERNATOR" IM TALKING ABOUT THE ENGINES ALTERNATOR THAT CHARGES THE ENGINE BATTERY.

That's why on my particular RV on my house battery bank I have a cable to the engine Battery/"Alternator" PLUS another cable to the Onan "Generator" (supplies 12 VDC to its starter motor) PLUS I think the same as Keeland, a third cable to the RV's DC Distribution Panel Input ALL THREE ARE NEEDED ON MY RV (his RV ???)

Keeland also mentioned the "negative" wire. Its possible to use the conductive metallic RV frame/chassis as the return current NEGATIVE OR use a heavy say 2 or 4 Gauge etc. cable for the joining the battery and appliance NEGATIVES. Same as the Battery + (s) above all need connected, so also do all the appliances and Battery - (s) WELL DUH LOL. In the event I use the RV conductive metal frame/chassis for return current and/or to join the negatives (or can use a cable mind you), I l like to have all the components bright and shiny and use start bite washers as well as spot weld the bolt/terminal to the frame/chassis to insure a good low resistance permanent electrical bond and then best I can I protect and preserve and seal the connection to prevent moisture and oxygen from contaminating it.

As far as the engines Alternator providing some degree of charging current into the house batteries while driving down the road (his cable from house batteries to engine battery/alternator), I prefer a relatively big cable such as say a 2 or 4 gauge etc. (subject to length and current) so as to reduce resistance and cable voltage drop and the use of a quality 100% duty rated minimum 100/200 amp rated isolation relay. DESPITE THAT it does NOT provide the quality and complete charge to my house battery bank as my Smart 3/4 Stage Battery Charger does, but the less resistance and voltage drop from the engine to house batteries the better in my opinion. I say that because I have seen dudes run a 10 or 12 gauge wire from engine battery back to house battery bank. On my RV I have a "time delay relay" that waits a time period after my engine starts before energizing my isolation relay which connects my engine battery/alternator and house battery bank together. That allows for a stabilization period for my engine battery and alternator before connecting up to my house battery bank, which can be (subject to state of charge) a fairly big additional load.

John T Soon headed south for the winter. RV is full of gas ($1.66 recently) plus a few additives and LP Gas and serviced to the hilt, plus recent new Tranny and 10,000 tow rated aux tranny cooler and new elec in tank fuel pump and filter plus an ignition tune up and new furnace blower motor. I may need to float a loan this winter lol

Keelandb · January 19, 2016

I have obviously been remiss in checking this thread that I started. When I say that I have several wires running from the positive post of the old coach battery I mean that I have

1) a relatively heavy wire that goes to the ONAN GenSet for starting the generator and providing any needed 12volt "operations" voltage. Yes, the generator provides 120 volt power that goes to the automatic transfer switch.

2) One slightly smaller wire that carries 12 volt power to the 12 volt system. That is the wire that has blue tape around it near the coach battery and where it connects to the 12 volt distribution system.

3) One of the smaller wires that originally went to the Converter,

4) One of the smaller wires that originally went to the original inverter.

Somewhere in this mass of wires is one of the smaller wires coming from the alternator (on the engine). It does provide some charging of the coach battery while driving, but it is not much.

Some of this I figured out by looking at the TriMetric 2025 BMS. The "Rube Goldberg" of the wiring at the old coach battery location comes from still having wires from/to the old converter and inverter. I'm not sure which wire is which so I've just left them all connected together. That may not help in any way, but it will not hurt as long as the ends of the wires are covered to prevent sparks/discharge of the batteires. The new Magnum inverter is back where the new coach batteries are located, so the old inverter wires are no longer needed.

My solar panels are now connected and so I get much more charging from them than from the alternator, when the sun is shining. Two more solar panels to install and that is done.

Kirk W · January 20, 2016

If it were me, I'd consider installing some sort of terminal block with a common to all connection for the lead to your new battery location and then individual terminals to connect the other leads to as this could be very helpful in the event that you ever need to trouble shoot the electrical system. Something of the sort in the picture might be a good choice from Amazon.

SCClockDr · January 20, 2016

If it were me, I'd consider installing some sort of terminal block with a common to all connection for the lead to your new battery location and then individual terminals to connect the other leads to as this could be very helpful in the event that you ever need to trouble shoot the electrical system. Something of the sort in the picture might be a good choice from Amazon.

I wanted to organize all my 12VDC and eliminate the inline fuses.

I flattened a piece of copper gas tubing and punched a series of holes down its length. I then fastened an auto resetting breaker to each hole and attached each inline load to the appropriate breaker. Tied the buss bar to the primary 12VDC junction point on the distribution board. Works a champ. One could go the more spendy route and purchase a bonifide copper buss bar, it would look more professional. Now all I need to carry are a few 15 amp mini fuses for the PD distribution panel and a 150 amp for the battery bank.