Building a RV battery charger line from the CTD

[rweis]

I do not think my RV has a battery charging line seperate from the usual 7 pin turn signal, running lights, brake power, plug. I am thinking of building one. I know that if the batteries are in a low state of charge then you would need to limit the charge current so you would not overload the wiring or the CTD electrical system. If the RV batteries are fully charged then there is not a problem. However getting the RV from storage the batteries are generally not fully charged.



I have been looking for some sort of a DC current limiting device. I am thinking that a SOLAR PANEL charger should do the task. The solar panel chargers take a wide variety of voltages and currents from the solar panels (in my case the CTD) and then manage the battery(ies) with various different "steps" of charging. Most use PulseWidthModulation (PWM) desulfating, balancing, float, and several levels of charge rates. A MorningStar 15 amp or 30 amp model is what I am looking at.



I would use the "solar input" from the CTD electrical system and then the output would go to the batteries of the RV. Most of the charge controllers have a current limitation based on the model you get.



Depending on the battery state at RV hookup the charge controller would then work its charging programs to handle the batteries.



The RV truck connection would be a seperate connection of both positive and negative so there would not be a "frame" ground that may or may not be a GOOD ground. Both positive and negative would run through the charge controller directly back to one of the CTD batteries positive and negative.



What do you think of the idea?



Bob Weis

 

[SBall]

Bob,

I understand your concerns about excessive charging current through the 12V and return wiring. I had similar concerns for my truck camper when I retrofitted it with two golf cart batteries. I ended up running 10 gauge wire and ground from the truck starting batteries (alternator), and immediately going through a 30 amp fuse on the firewall. My golf cart batteries (in series) have a 20 amp circuit breaker. So far this has worked very well for me and I've documented the maximum charging current into my golf cart batteries when down to about 25% is 16 to 18 amps with my CTD. It seems like a nicely balanced system. If you were to run some calculations you'll see that a 12V deep cycle lead acid battery with about 25% charge remaining has an open cell voltage of about 12V. Your charge system is let's say 13. 8V. The resultant current would be I=1. 8/R. R being loop resistance in your charging circuit. In the end you'll see that most RV charging systems have sufficient (maybe excessive) resistance, thus never fully charging the coach batteries. The bottom line is I don't think you have to worry about a sophisticated current-regulated charging controller from your truck alternator. Hope this helps.

Steve

 

[DValentine]

You could put in line a circut breaker, say 30-50 amp. Use a re setting one and that will protect your system. I have not had a problem with them yet.



Dave

 

[klenger]

Another way that I have thought of doing this would be to mount a 1000 watt inverter near the batteries of the truck and then run 120 vac to the trailer and let the RV converter charge the batteries. This would get around the problem of needing such large wire to the RV.



I have never pursued this as I have solar and a generator, so the batteries are rarely very low when we travel.

 

[K5IP]

Bob,

It's nice to see folks actually thinking about the electrical aspects.

It is not a good idea to use the current limiting capabilities of a solar panel charger because they will always provide a voltage drop and limit your maximum (trailer battery) charge rates, even when you want them at maximum. Steve has it right. The only thing I might add is when you first plug your trailer into your truck, you can leave the engine off. This will llimit the initial surge (because your truck battery terminal voltage is about 1. 5 VDC lower), and reduce the arcing which can eventually damage your truck/trailer connector.

After connections are made, the truck can again be turned on. The inertial surge (inrush) will only last a few seconds. This also saves fuses in the event of a seriously depleated trailer battery. It is how I do it (using two golf cart batteries in the trailer) and I've never had a problem

Rog