RV - Truck charging voltage with different Solar panel voltage

Mustang65 · May 2013

I have been following the Alternator AMP thread and figured I would start a new thread for a voltage question I have. I installed a 250 Watt Solar Panel on our travel trailer with a Morningstar MPPT 60 Amp Charge Controller (overkill, but I like the Wi-Fi monitoring). It charges the two Trojan T145 batteries with no issues at all. I will be adding a second 250 watt panel in the future.

Ok, prior to installing the solar panel, the Travel Trailer was charged by the F150’s alternator, a 220 Amp output, with towing package option and has a 30 amp fuse between it and the trailer plug. The most that the alternator produced voltage wise was 12.6 to 12.8 volts (at the camper); probably because of the gauge of the wire (on the truck). I have been running the camper (stored next to my house) on solar only and not using the onboard AC charge controller (turned off). The voltage goes through its cycles of 14.2 to 13.2 Volts (Float…). It averages about 13.2 all day. No Problems!

My concern is, when towing, the trucks voltage is only 12.6 which is in conflict with the 13.2 volts from the charge controller. Will the 13.2 volts override the 12.6 or will it average out? Can it cause any issues for the truck/trailer? Should I put a cut-off switch to stop the voltage from the truck, and just have the solar panel charge the batteries? While traveling I leave the Inverter on which supplies 120VAC to the fridge and Wi-Fi. I also installed a 30Amp blocking diode between the trailer and truck, which keeps any of the voltage/current from feeding back into the truck’s electrical system from the trailer.

Should I be concerned?

My first camping trip will be soon and do not want to hook up and cause any problems with the trucks electrical system.

Sorry for the long dragged out explanation.

Don

niel · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

sounds like due to voltage drops in the wires and such that you aren't getting sufficient charge to the batteries from the alternator. the solar will deliver its full charging abilities and offhand i don't see a problem with both sources on the batteries, but if the batteries are paralleled to the starting battery there could be some issues there as you will be attempting to charge differing batteries from the same source. it may already be isolated and that may also be a reason for the extra low voltage, but would not be the only reason it is low imo.

Photowhit · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

Your producing the electric at minimal cost through the truck while traveling, I'd suggest investing in some heavy gauge wire. and see if you can get the voltage up. Do you have any idea of the voltage coming off the Alternator?

BB. · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

The two voltages are fine for automotive or float voltage (a bit on the low side). And The amount of current flow it probably not going to be that great.

You might use a DC Current Clamp Meter to measure the current (and direction) under various conditions...

Lights on/off, motor running/not, solar charging/not, etc... I tend not to like a DC link between two systems because things can go wrong and I am not aware of it (loads in trailer draining vehicle battery, too much current flow pops fuse/damages wiring, etc.).

If you really want to charge the trailer--An AC inverter in the truck (properly wired) and a 120 VAC cord back to the the trailer for powering an appropriate AC battery charger would be more consistent and probably better/more power transfer (if desired) for when you are underway.

-Bill

Endurance · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

The diode was a good move. Because you have that, I see no problems.

Cariboocoot · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

Resistance works both ways: the drop in V from alternator to RV that causes it to charge too low will keep the higher V from the solar controller from pushing the vehicle Voltage high.

The vehicle regulated Voltage is usually around 13.8, btw, and the Trojans probably want 14.8. Check that both charge sources are providing what they should at their respective batteries, then see how much line drop there is either way. You can disconnect the vehicle battery and see just how much feeds back from the RV with various loads on (not ignition).

Mustang65 · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

I was going to pop the 30 amp fuse out of the truck's fuse panel and take a voltage reading of the trailer voltage feeding back into the truck, but I have the blocking diode installed. I probably could wire around it using a 10 AWG wire with alligator clips. I have to look into my spare parts bin and see if I have an amp gauge in there. Disconnecting the battery on the truck is a last ditch effort, as the electronics all get reset and the twin turbos act up until the computer re-learns my driving habits. I may run a 10AWG wire (30Amp ckt breaker) from the truck battery directly to the trailers 1500 watt inverter and turn on the trailers internal charge controller. I would de-activate the towing plug's 12 volt positive connection from the truck that goes to the trailer's battery, when using the inverter.
Let me see if I can get some voltage readings....
Thanks

BB. · May 2013

Re: RV - Truck charging voltage with different Solar panel voltage

Yea--Even though you are using the diode... Between the diode drop (~0.2 to 1.0+ volts) and wiring drops, it is difficult to get "useful amounts" of 12 volt current from the tow vehicle to trailer.

And, given that solar power systems are designed to bring battery voltages to ~14.7 to 15.5 volts (when equalizing), it is not a great thing to hit a computer controlled vehicle with.

By the way, diodes do fail shorted fairly often (as well as failing open)... A common enough failure mode that for UL/NRTL safety when charging NiCad and other rechargeable batteries in electronic systems, they require "two" devices for redundancy (two diodes, diode+resistor, etc.). Would it happen to you... Probably not. If there where 10,000 people that did the diode block, would it happen to one or more of them over a couple of years, probably yes.

-Bill0

bwatson · June 2015

This was exactly the type of thread I was looking for - hope you guys are still all around and can provide an update.

a) I saw above mention of the wire gauge from tow vehicle to the trailer to charge the battery during transit. If using the wire within the 7 way yes I was told simply not enough. Our plan was to run a separate heavy gauge line. Safe to say a heavier gauge will get the job done?

b) I then also read that if charging in this manner you can actually overcharge the batteries in trailer. In our case we have 2 Trojan T105s. If you do not overcharge your tow vehicle battery, can you overcharge another battery on the same line?

c) Just the other day we thought about changing gears and perhaps 100% getting away from the tow vehicle charge and going with solar. Thus now researching what type of panels we should get and what controller and what inverter. I have just started my research so please excuse me for now knowing that yet. If you folks here have prefs. please send them in.

d) Lastly, the controller that one uses for the solar panels. If I did have a feed coming from the tow vehicle could a person also run that through the controller prior to going to battery? I assume that would elim. any chance of tow vehicle over charging the battery.

Thank you everyone very much for your help.

Brian

RV - Truck charging voltage with different Solar panel voltage

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