Adding A Second Battery

Thanks. I hadn't seen your reply when I replied to Rotus8. I should probably try to learn from what you learned from Cary and Craig. We have a weeklong trip in April, but I doubt that I'll have the time to implement anything (other than some battery troubleshooting) before then. I'd be interested to know what my TV's "output" is before I implement a solution that may not be necessary. If my TV's output is fine, then the battery issue is/was likely the result of other abuse of the battery (i.e., using battery to power fridge when the TD was on shore power, a no-no which I did not realize until recently). If my TV output is fine, then I simply need a new battery which has no memory issue.
-Al

 

Al-

How would plan to go about measuring the charging output, short of getting the Trimetic battery monitor ($150 or more) and going through the learning curve (I'd like to know our charging output, if there is a relatively easy way to figure that out)? Come to think of it, a former neighbor of mine is an electrical engineer. I'll drop in on him one day soon to see if he can help. He has an Airstream and a 2016 tow vehicle, so he might be curious whether this will be an issue for him at some point...

We've been good about putting the Dometic on shore power (whenever we are on shore power). Our AGM battery isn't that old, but we seem to struggle with power issues if we don't connect to shore power every few days, or supplement with the generator periodically.

 

I have no idea. I've sent an email to the place that has done some light maintenance on the TD for me, as they want to discuss it with their battery supplier. I don't know if this trailer service place has a Trimetic monitor....

I recall that someone (same trailer place?) checked my TV's output (likely from the 7-pin connector). I need to look through some paperwork to see if they wrote it down....but my recollection is that the # was much lower.

 

I just measured the voltage at the TV battery while the vehicle was running. I also checked the voltage at the trailer battery while plugged into the TV and vehicle running. There was a little voltage drop but not much. If you have a battery isolator it takes a certain amount of voltage just to get that device to allow current to flow to the trailer battery from the TV. I was concerned about that for a while and bypassed it, but even my low TV voltage output is enough to energize the isolator.
You can check voltage pretty easy with an inexpensive voltmeter, it's sure not rocket surgery.

And as regards charging with solar while going down the road, if you have the roof rack option (we don't) it would be easy enough to mount a panel to the rack and just plug it into the built in jack on the trailer. What I don't know is if I'd have to somehow disconnect the vehicle charging circuit in order to do this.

r dickson
550 #645
34,645 miles, 148 nights

 

Not having any clue about how alternators work, I wasn't sure that the voltage might be different at various engine RPMs or based upon what sort of loads there might be on the vehicle's battery at any given time? But I will throw our volt meter on the 7 pin connector to see what I get. I would do it now, but I managed to leave it on last time we used it, so I'll need to go out and buy a new 9v battery for it first...

The downside of mounting the solar panel to the roof rack would be that it would put the panel up high enough to create what I would suspect would be a significant amount of drag (there is also the issue of how to keep the charging cable from flopping around and banging against the camper's aluminum skin, if the goal is to charge while the camper is moving). The rack crossbars are 60" long by 3/4" round (45 sq in surface area, and there are two of these). The rack with empty Yakima Frontloader bike trays creates significant drag, which gets much worse when you add a bicycle. I would guess that a panel mounted on the rack's crossbars would create drag similar to the empty trays, perhaps more, since it would likely cause more wind turbulence under the panel (assuming a 100 watt-sized panel). Solar mounted directly on the tow vehicle's roof would likely result in much less drag, as you would only have about a 2" x 24" face (less than 48 sq in) exposed to the wind (with a rigid panel, a flexible panel would only be around 1/8" x 24" [3 sq in]), and the front of the vehicle would already be deflecting some of the wind above the panel.

 

Hmmm.... I get no reading from the charge line while idling, nor do I get a reading from any of the other lines when the running lights are on. I suspect there is something obvious that I am overlooking, but, just as obviously, I have no clue what that is. The meter is functioning, because the extra 9v battery that I purchased reads 9.7+ volts, and when I check the camper battery directly it reads 12.99 volts. I should have some knowledgeable help later today or this weekend, so hopefully they can help me figure this out, if nobody on the forum has any thoughts.

 

You should see voltage when testing 7 pin connection on truck. Check other pins to see brakes light and turn signal changing voltage. If you don't see that when brakes and or signals applied, it may be faulty 7 pin connector or operator error with the volt/ohm meter.

 

It appears that part of our issue may be that the ground on our 7 pin is not connected (or it has become badly corroded). By grounding directly to the van I get a reading of 14.99 volts at the battery, and the electric brakes on the 7 pin reads 14.25 volts (I am guessing that is due to line loss, and not because the brakes would somehow be getting a different voltage reading). I'm still not getting a reading on our charge pin, but I need to visit U-Haul next week and will ask them to check it while I am there. I'm wondering if our charge line has become corroded, though I would think I would still get some sort of reading (unless the line got severed somehow)?

I stopped in for a visit with my ex-neighbor who is an electrical engineer. He was familiar with the issue with AGM batteries not getting optimal charging voltage from the more efficient alternators in newer vehicles.

I guess the new question that I have is what sort of voltage will come out of the charge controller when hooked up to a solar panel? My understanding is that the voltage off a typical 12 volt panel is around 19 volts. Does the controller step it down, or does it only act to disconnect from the battery when voltage is either too low or the battery is at or near capacity? The reason I ask is that if it steps voltage down to anywhere around what the alternator is putting out, then you would still have the issue of the voltage being too low to fully charge the AGM battery. Ideally it would be supplying voltage high enough to assure as full a charge as possible. Anyone know the answer?

 

I think you are correct that a 12v panel with output 17 to 19 volts to the controller. There are different controllers, MPPT and PWM controllers and they both do different things. MPPT controllers are expensive and maximize amps going to the battery and do have a "step up" feature due to circuitry. Most likely you have a PWM "pulse width modulation" controller and the main function is to prevent the battery from over charging. The battery, when connectioned to either the CampInn battery tender or to a solar controller will only take the volts and amps it needs. Thus if you had battery at 50%, years would see lots of volts and current going into the battery on a sunny day or when connected to tow vehicle charging system. Conversely, if you had 100% battery, you would see very little volts and amps going into the battery because the solar controller and or battery tender would sense battery 100% condition and limit volts and current to a trickle. I think this case would be called float mode. This protects the battery from over charging or cooking of the battery.

Hope this helps.

 

Steve and Alea

As I read your post I understand that you are not getting any power to your charge pin, on your 7 pin connector on the tow vehicle. I had the same problem last year. On my car there is a charge wire running from the battery to the rear of the car. There is a wire coming off of the back of the 7 pin that connects to the wire from the car. That connection is under the rear of the car where it picks up everything being sprayed up at the rear of the car. That connection is what corroded in my case. Northern Michigan winters likely caused the corrosion. It turned out to be a simple repair. My ground wire connection was fine.

 

What I have learned thus far: Even if you ground is perfect and you have the property gauge on the charging wire, AGM batteries are incompatible with charging from the TV. There are accessories you can add to help the TV charge the TD AGM battery, but it looks to be a PITA. Better approach may be to use more typical battery.

Of course, the fridge would prefer the AGM battery over a few days.

So there really isn't good compatibility with the AGM battery and the TV and the fridge, especially if you leave the fridge in the TD and connected to the battery while you drive. That seems to be extra-problematic.

Not sure what the solution is.

Now I know why so many of you folks have generators.

Now I know why so many of you folks use two coolers and no fridge.

Now I suspect I know why so many of you folks carry your fridge in your TV while you drive.

One solution would be a normal battery (charges better from the TV) and a propane-powered fridge.

Solar works great with the AGM battery, but that doesn't help if you arrive at Campsite B in the late afternoon.

Of course, I may be very wrong about all of this....

 

[QUOTE Of course, I may be very wrong about all of this....[/QUOTE]

I don't think you're wrong, in fact I think your analysis is spot-on and closely matches my experience.

I would say, however, that (according to the trimetric monitor) we get a substantial charge while driving, even with the dometic running in the trailer. The problem is that it's not a full charge and simply can't be for an AGM battery no matter how long you drive. So when you pull into your campsite B for the weekend you may have enough battery capacity for the first night, but without good solar exposure you're sure in trouble after that. We've had that issue a number of times.

Hauling the fridge in the TV is certainly an option but if you're like me you'd like to solve the problem rather than treat the symptom. Plus the fridge in the TV presents its own set of problems such as what happens when you stop for a good while when on the road. And the dometic is pretty heavy so moving it from TV has to get old (like me ). And then there's still the problem that the TV won't fully charge the AGM.

The solar-on-the-roof RVs probably put the panels there because they've got big roofs but they've sure got an advantage over those of us with tiny roofs. But while we tow around vintage-looking teardrops our electrical systems can be pretty advanced. I remain very interested in installing teardrop roof-top solar but in the meantime won't hesitate to unplug the dometic and use the cooler if I think I'm about to harm the battery. If we do enough dry camping that situation will certainly occur, even with solar panels on the roof.

r dickson

 

Imteresting discussion. I keep the Dometic plugged in the galley while towing and have always arrived at my campsites with 100% charge. My TV is. A Ram 1500 truck with a factory tow package. Does charging voltage through the 7 pin vary by TV?

Randy

 

Earlier comments stated that newer vehicles seem to be more efficient and as a result output lower voltage than older vehicles. If true, then charging would be better with an older vehicle (not sure where the cutoff is [and it probably varies by make and model], but certainly vehicles manufactured in the past few years are likely to have this issue.)

 

This is something that can be fixed with a little electrical knowledge and some work. The way I would do it would be to add a heavy wire with a fuse from the TV battery to the 7-way connector area of the TV. Then add a relay controlled by the wire that would normally be the 7-way battery connection. Wire the output of the relay to the 7-way battery connection. The result will be switched full battery voltage direct to the 7-way battery terminal, eliminating the TV circuit that drops the voltage.

If you have the battery isolator, the relay isn't strictly necessary, you can just connect the new fused wire to the 7-way; personally, I would use the relay anyway.

Not a trivial thing to do, but not too hard. If you don't have the inclination to do it yourself, any competent trailer or independent auto repair place can do it easily.