Akom's Tech Ruminations

Various tech outbursts - code and solutions to practical problems

Getting RV Solar and Shore power to coexist nicely

Posted by Admin • Tuesday, August 4. 2009 • Category: RV

I have an RV (20' Sunline T1950 travel trailer) Yay! Like most, it came with a tiny battery (23Ah @2.5hour rate??) and a Power Converter (Centurion 3000) which doubles as a battery charger, but it's hardly a good one - it's not even a 1-stage (that would be just bulk charge) - instead it is a float-only 13.1V power supply. Charging a sizeable battery using a float charger would take a very, very long time (and is hardly good for the battery). According to documentation it's supposed to output 13.4V but I guess it's +/- 0.5V :-)

My primary interest is in boondocking (dry camping) so for me battery performance is of utmost importance. This means good battery utilization, and good battery charging when charging can occur. Stock, the RV does neither.

First thing I managed to do was to secure a small solar system including a Solar Charge Controller (got an awesome deal on Craigslist). It's 100 watts and the CC is a 2 stage (absorption and pulse-float), 15 Amp unit (Mark PV 15). My first question was of course... how do I hook it up to the existing electrical system without causing problems?

After some experimentation I determined that simply hooking it up direct to battery is fine. The result looks like this:

Original Wiring Plus Solar

This even works!
However, there is a major issue. Solar only works when there is sun, obviously, and when I get to plug in, I'd like to use shore power for charging quickly and well (2 or 3 stage process). Instead, what we have here is a PC that fights the solar charge controller, reducing voltage to 13.1V - preventing it from charging correctly all the way, and bubbling the battery unnecessarily after it's full. Moreover... the Power Converter drains power from the batteries in the absence of shore power!!! Which brings us to:

Phantom (Vampire) Drains

I went out and measured the individual battery drains for the systems the RV came with (when not on shore power, obviously)
DrainHourlyPer Day
Power Converter150mAh3.6A
LP Detector75mAh1.8A
My Additions
Solar Charge Controller at night8mAh0.1A (half day)
Solar Charge Controller at night (bargraph on)17mAh0.2A (half day)

First of all, this is ridiculous. If I get a nice big 100Ah Group 27 battery, the trailer will kill it if I unplug it for 10 days - that's not using anything, mind you - sitting idle. Now clearly I can get rid of the radio drain by pulling the fuse, and that cuts it in half. I guess I'll leave the LP detector... but:

Why does the power converter suck power back from the battery? Doesn't make that much sense. So I decided to add some switches to eliminate the PC from the circuit when I'm in "Solar Mode". I considered buying a ready-made high quality power converter/charger (these seem to always also include an inverter and cost nearly $1000).... but that did not appeal to me.

Good Ideas gone to waste

First I realized that I already have a 2 stage charger... why should it care if it's powered by solar or a DC adapter? So I decided to hook up PC's output to the solar charge controller. To do this I'd naturally need to disconnect the batteries from the PC, otherwise the CC would be ... charging ... its own input? Here is the plan:
First Failed Attempt

This failed not so much because it isn't possible, but because the PC outputs 13.1V, and I need at least 14.4V + 0.6V dropout at the CC = 15V. I probed around the PC's circuit board but the transformer seems to be regulated by shunting it or something along those lines - there was no >13.1V to be found.

Then I gave up and decided to introduce a separate clean power supply for the Solar CC. This was the plan:
Failed attempt with a separate Power Supply

This was a good idea... except that the switching DC power supplies I was able to find (in my garage) are not well-isolated from their inputs (this seems to include a very nice open frame 150Watt power supply that I managed to get 17V out of, as well as a 19V 3.5A laptop power supply. The problem is that the AC that the power supply is plugged into shares ground with the DC battery, and that either confuses the CC and it starts to flip between charge/full, or it outright freaks it out if you attempt to separate their grounds. (CC starts to squeal and blink, telling you "turn it off now or I will do it for you"). Apparently a Solar Charge Controller is just that - a Solar charge controller. It's not designed to deal with shore power in any voltage.

Final Design

Finally I relented. I will deal with solar charging, and I will hook up a manual one-shot charger (Mystery B6 sounds good) to recharge when severely depleted. But I will not accept the ridiculous drain from the power converter, nor it getting in the way of solar charging when plugged in. This is my plan then:
Simplified Solar + Power Converter disconnect wiring

I guess I can use a battery selector switch to accomplish this ($20+)... Or perhaps a 20A 4PDT ($10) wired in parallel? That should do 80A, theoretically (EE's may argue that because the moment of contact will happen earlier on one of the poles, it will take the entire load and melt/weld/overload ... I would argue that I would never switch it with 80A of load running). Then again, I could make this automatic using a similar relay, normally on solar... Then again, I do prefer manual switching - simpler and I feel some sense of control. Who knows - I may want to connect things in some weird way to troubleshoot something.

Hey! There is a way to use relays and make this flexible too!
Auto Switchover using Relays

This plan makes use of two 30/40 SPDT relays (this way their combined current capacity is decently adequate), controlled by override switches. I feel that I can get away with using relays here instead of a battery disconnect because in a travel trailer I am not trying to run a starter, nor do I really expect any loads above 20A at any one time (nor do I expect a charge current from my PC in excess of this number, no matter how low the battery gets). My power center may be rated for 60A total, but I doubt it's really capable of this...

So if override switches are both on, loads get switched to PC when on shore power, and removed from battery. If they are off, the loads switch back to battery, which is the NC relay position. If only one switch is on, one relay connects loads to PC, the other - to battery, meaning that now battery and PC are connected as before (should I ever want this). The only quirk in this "both directions" mode is that it will not shut off automatically :-) - if you unplug shore power, battery power will continue to power the relay! You actually have to flip the switch "off" for a second (But hey, what's the worst that can happen - I'll have a 150mAh drain?) There is a workaround for this problem: Use 120V coil relays and power them from the 120v wiring.... The alternative to that is to use a tiny wall-wart to power relays, which will in turn plug into 120V wiring :-)

Now that this craziness is done with... can I ask why 40A relay harnesses come with 16AWG wire? I think if I'm going to do this, I won't bother with harnesses - just make my own multi-12AWG connectors. (2x12AWG=9AWG, hehe)

The Alternative

Finally, to be honest... If I were willing to spend $200 on a high quality 3 stage charger / converter to replace my el cheapo Centurion 3000, none of this would be necessary. But that is its own can of worms.

Final Implementation

In the end, I walked into a Radio Shack and found a pair of 50A @12V rated toggle switches (SPST). They are obnoxiously huge, but I cut the paddles shorter. So that settled it. I made a little panel for them and mounted them below the charge controller. Also, I was not able to use the fuse panel for fusing batteries any more - the wiring has changed. Fortunately, I had a leftover self-resetting 40A circuit breaker leftover from the trailer wiring kit (I mounted it on the back of the power converter housing. I made very nice cables with ring terminals and 8 Gauge wire for all of this business, and I'm done. Yay! Here is the diagram:
This is what I actually did


A few years later I actually did upgrade the charge controller to a nice and affordable 3-stage unit. Surprisingly, this changed nothing, and my modifications remained relevant, because even the brand new charge conroller (Power Max PM3-45) still places a slight drain on the battery when not plugged in. But at least the battery is much happier and now I can leave the RV plugged in indefinitely.

1 Trackbacks

  1. We have yet to take our first trip, but I couldn't wait. Actually I found used panels and a charge controller on Craigslist and picked them up the same day we got the trailer! Someone was selling a pair of 50 watt panels and a 15 Amp charge controller.


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  1. Thanks for sharing your work! Love the extra information of what you tried and what did and didn't work. You have already helped me out by including the Failed First Attempt. I was thinking of trying something like that! (and trying to bypass the PC charge circuit and go straight to it's main DC voltage, I wouldn't have thought of the CC 15V requirement!
    My CC has internal 'load' monitoring that I want to use, so I'm scheming to route the battery back into the CC 'load in' and then from the CC 'load out' to the RV loads. Just gotta finalize my plan, then the hard part of talking my wife into letting me re-arrange the RV. LOL Thanks Again!!! Craig

  2. Thanks for your research and information.Quick question, on the final design could you use a single 50amp SPDT switch to choose shore vs. solar?

  3. Depending on what you have in mind, my guess is that doing this would prevent you from trickle-charging from shore. Unless you are thinking of some other configuration. But sure, you could do that.

  4. One more question: if I have a RV Converter with a 3 stage charger, can I wire this directly as in your first example?

  5. I've had so many issues with my generator and shore power. I'm about ready to go full solar. Not sure how to make this work on a sprinter van. Moral of the story don't buy new. I have started looking into Goal Zero. They are solar powered generators. I have a few panels right now, but they only charge one battery. I find it being drained constantly especially over night if I leave fans and the 12 fridge on. Always trying to find more options. Just had some more issues with my shore power while camping this week. Thanks for sharing all the great advice!

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