Lithium Battery Upgrade

[RV Squirrel]

Two months ago I started a thread on entitled "Does Bluetooth work in an aluminum camper?  Victron vs. Renogy."  I was thinking of upgrading my existing lead acid batteries to lithium ones, and was comparing two vendors. 

This thread ultimately became a long and fruitful discussion about how to do a lithium battery upgrade.  It had a lot of great input from #DavidM and #Merlin, with some additional input from #Pinstriper.  It went way beyond Bluetooth!  Had I known that the discussion would become so inclusive, I would have named it something like "How do I go about getting a Lithium treatment?"

My old lead acid battery (93Ah) could no longer hold a charge, and my AC power converter was making a making an annoying sound whenever it charged the batteries (ever wonder what tinnitus sounds like?... this is the same!).  I had a PWM solar controller that I wanted to upgrade to MPPT, and I was never quite satisfied when charging of my lead acid batteries from my tow vehicle.

An upgrade to Lithium batteries would give me a number of benefits.  Not only would I have more storage capacity, but I would also be able to charge the batteries more quickly.  I decided to go with two 100Ah batteries so that we could make better use of campsites with "no hookups" when we stayed at our favorite parks or went on an extended trip.  Although we could have kept the existing power converter (for lead-acid batteries), this would have never fully charged the batteries (because of lower voltages) and would have likely reduced the lifetime of the batteries (since the battery management would not have a high enough voltage to do cell balancing).  My old PWM solar controller did not support Lithium, and I wanted to upgrade to MPPT anyway.  Since Lithium batteries can recharge very quickly, they can draw a lot of charge... this made it wise to get a DC to DC controller that regulated the current from the TV and ensured that adequate voltage was being used to charge the batteries.

After a lot of discussion on the "Bluetooth" thread, I finally decided on particular components.  A block diagram is shown below. 



I've purchased nearly all of the equipment.  I still need to find a way to connect MC4 connectors through the side of the travel trailer, since I'm currently in the habit of moving solar panels around the trailer.  Also, I haven't purchased any equipment associated with the inverter.  Installing the components will require some work.  Some of that has already been done.  This thread will begin with what I've done so far.  As I make progress, I'll add notes.

The "Bluetooth" thread includes a lot of information from forum members in response to my questions about different components and manufacturers.  I ultimately purchased the following:
-    Battle Born BB10012 batteries (100 Ah).  There are other options for a lower price, but the Battle Born batteries appear to have consistently received good ratings.  The battery management keeps the unit from being charged at freezing temperatures, which can ruin the battery.  The batteries are "designed and fabricated" in the U.S...  Although the cells probably came from someplace outside the country, customer support is more available and there appears to be a wider knowledge base.
-    Progressive Dynamics PD9145AL (AC to DC) Power Converter.  This is a deck-mount version that supports Lithium batteries only.  Progressive Dynamics makes a Lithium compatible unit that could have been inserted directly into my existing PD4045 Power Control Center, but that unit only supports "single stage" charging.  The PD9145AL supports "two stage" charging, which can help extend the life of Lithium batteries. 
-    Victron BlueSolar charge controller MPPT 100|50.  A MPPT 100|30 would have probably done the job just fine (even if I added another couple of panels to the three 100W panels that I already have), but I ample room in case I decided to use even more panels.
-    Victron Orion-Tr Smart Isolated DC-DC charger.  It was this beast that initiated the "Bluetooth" discussion, because there is no other way to configure it other than with a Bluetooth connection.
-    Blue Sea Bussmann Surface Mount 187 series DC circuit breaker (80A).  I could have also used a panel mount circuit breaker, but I wanted to be able to see as many of the connections as possible, literally "above the board".
-    Blue Sea MaxiBus BusBar (Pat No. 2126).  I got two of these, one for positive and one for negative.  Each bus bar has six 5/16" posts.  This may be a little overkill, but it allows me to troubleshoot components without removing the cabling for other components. 

Did I mention that I have a Livin' Lite Camplite 14DBS?  Although this is great for my wife and two dogs, space is at a premium.  I was reluctant to give up an entire cabinet, and my wife didn't want to give up space for the dog food.  Nonetheless, I wanted to keep the batteries indoors... to keep them from freezing, and to keep honest folks honest!  The other components mentioned above also take up more space than what was there before.  Therefore, I decided to make a "circuit board" that would fit in the space that divides one of the cabinets under the sink with the "power/fuse/breaker" cabinet directly adjacent to it.

The "circuit board" is made out of sheet PVC.  I used this because it is non-metallic and easy to work with.  Also, you can glue pieces together (think plywood) in order to make it stronger.  Since PVC contains chlorine, it can generate dioxin if it burns.  However, it is my understanding that PVC has a much higher ignition temperature than wood, and does not stay lit unless something else (like wood) is burning it.  CAVEAT:  I'm not a expert on this, so you should investigate this yourself if you consider using sheet PVC for this purpose.  Since I have an aluminum camper, I figure that I will have other things to worry about if the camper frame is hot enough to begin to burn.



The circuit breaker is at the top left corner of the circuit board.  The solar charge controller and the DC/DC converter are in the middle, and the bus bars are on either side.  Holes below the charge controller and the DC/DC converter allow for cables from the AC/DC power converter, the DC distribution panel, the TV DC entrance, and the solar panel entrance.  The notch in the lower right is for the wheel well of the 14DBS.  The funny squiggle above that is to allow room for the hot/cold PEX tubing that goes to the galley sink.

This is the rear side of the circuit board.  There are up to three layers of sheet PVC, each 3/8" thick.  The sheets are held together with plumbing PVC cement (I didn't use the PVC primer, but I did clean the pieces with acetone).  Components are held in place with tee-nuts (which were glued in place with cyanoacrylate).  CA and PVC seem to love each other... I tested one tee-nut before using this approach, and the nut actually deformed rather than come away from the PVC.

The user manuals for the solar charge controller and the DC/DC converter recommend having more space than you see from the front of the board to allow for air flow across the heat fins on the back of the components.  For this reason, I cut out the portions of the PVC immediately behind the components to allow for more air flow.  I also chamfered the top/bottom edges of the second layer to facilitate air flow.



Since the components have a high profile, they partially blocked the opening of the cabinet in which they were being installed.  I managed to offset them with "spacers" made out of layers of sheet PVC.  The layers were held together with PVC glue and CA glue.  The edges were filed away in some places to account for welds in the adjacent aluminum tubing.



This is where the components will be installed.  Due to the "spacers" mentioned above, the components will actually occupy space behind or above aluminum tubes. 



This is what the circuit board looks like after install.  The lithium batteries are sitting on the wheel well.  Ultimately, they will need to be held in place with a battery tray (which I haven't made yet).  The components are recessed enough that they did not stand out beyond the edges of the cabinet door, and there is enough space the slide the batteries in and out if I need to.



This is from the inside of the cabinet.  I installed the shunt (can I mount this directly to the bus bar?).  I have not installed cables to the batteries yet.  Holes below the charge controller and the DC/DC converter allow for cables from the AC/DC power converter, the DC distribution panel, the TV DC entrance, and the solar panel entrance.  I'll be adding these cables after I install and connect to those components.



Finally, this is from the top.  The existing PD4145 (with DC distribution and fuse box) is at the bottom of the picture.  The PD9145AL will be installed right where the flashlight is sitting.  This is above the wheel well.  I'll need to make a mount for this as well.



Many thanks to those that have provided help so far.  Thanks also to those that made it possible to imbed pictures within the text of posts on this forum!

My next step will be making the battery tray and the mount for the PD9145AL AC/DC converter.  I'm hoping to use some tips from #Pinstriper for this.

[DavidM]

Tom:

The most commonly used connection to go through the trailer wall for portable solar hookup is the SAE connector, sometimes called the Zamp connector. Here is a page of SAE sockets and male connectors on Amazon: https://www.amazon.com/s?k=sae+connector&ref=nb_sb_noss. You have to be careful about the polarity with these connectors.

Use MC4 cables from your solar panel to the connector as MC4 cables are available in various lengths and up to #8 gauge and then splice to the SAE connector to go through the trailer wall.

David

[RV Squirrel]

Two steps forward, one step back...

First step forward....
I finally assembled and installed trays for the lithium batteries and the AC/DC charger.  I've posted comments and pictures in a separate thread (started some time ago) at Can I put batteries indoors on wheel wells??

I now have all of the major components installed for the lithium upgrade.  I've added cabling to connect the batteries, AC/DC charger, DC/DC converter and solar controller.  For those that are wondering, YES... I do have both lithium batteries connected together without switches between them.  Once I fully charged them individually, I cabled them together in a "till death do us part" configuration so that they will be charged/discharged as a single unit in wedded bliss. 

I've bypassed the old lead-acid battery, and connected the DC system to the distribution panel that was part of the AC/DC converter.  Although I have the solar controller installed, I still need to install an entrance for cabling from the solar panels.  Although the shunt is installed, I need to connect the sensor wires and reprogram the device.  However, the AC/DC charger and DC/DC converter are fully connected.

I've since been able to charge the batteries from shore power.  Since the batteries are connected to the DC distribution panel (through a breaker), I can actually use my trailer.  In fact, if I were to fully charge the batteries, I'd be able to take it out for a long weekend (or more) without worrying about recharging. 

Second step forward...
This past weekend, I rerouted the cabling that originally went from the AC/DC charger (in the trailer) to the lead-acid battery (on the tongue of the trailer).  This cabling now connects the input of the new DC/DC converter (in the trailer) to the junction box for the 7-way trailer connector (on the tongue of the trailer).  At first, I was thinking of replacing the existing #10 cable with #8 cable, but noticed that things were VERY TIGHT in the junction box for the 7-way trailer connector.  Instead of 5/16 bolts, connections were made with #10/32 screws.  I would never be able to fit #8 lugs inside the box. Sooo, I stuck with the #10 cable. 

Once everything was in place, I conducted my first test.  I connected the 7-way cable connector to my truck, and started the ignition.  Inside the trailer, I had disconnected the batteries by turning the breaker off.  I checked the voltage from the DC/DC converter, which was 14.2V (appropriate for the lithium battery preset).  Appliances like the fan and lights seemed to be working properly.  I used a clamp ammeter to measure the current coming into and out of the DC/DC converter, and everything seemed as it should be.  I was able to monitor input/output voltages using a Bluetooth connected cellphone.

One step back!...(hopefully only one)
I then turned the breaker on, so that the lithium batteries would be connected.  The batteries were nearly fully charged, so I did not expect any large current draw.  However, I noticed right off the bat that something was wrong.  The ammeter showing that the output of the DC/DC converter would go from 20A to 0A every several seconds.  The input of the DC/DC converter would go from 30A to 0A.  The voltage into the DC/DC converter would go from 14V to 9.8V.  At that point, the cellphone app would say that the charger was "disabled" due to undervoltage.  At times, it appeared that not only was the charger disabled, but that the unit had actually turned itself off entirely so that it did not appear in the cellphone app!  All this was happening every several seconds.

Therefore, I repeated the test.  This time, I did not turn the circuit breaker on.  Instead, I turned on individual appliances (fans, lights, etc.) while recording voltages and currents into and out of the DC/DC converter.  By steadily increasing the number of appliances, I was able to change to draw on the output side of the converter from 0.6A (phantom load) to 10.3A.  As I did this, the current going into the converter changed from 0.6A to 15A.  The input voltage changed from 13.2V to 10.8V. 

While doing the test, I had to disable two features on the DC/DC converter.  The first was "Auto Engine Off Detection", which assumes that the ignition is off if the input voltage falls below a certain value (around 13V).  At this point, the charger would normally be disabled, so I had to turn off the feature to keep the charger working.  The second feature was "Input Voltage Lockout", which turns the whole system off if the input voltage falls below a certain value (around 12V).  I had to turn off the feature to keep the charger working.  I would think that if everything was hunky-dory, I would not have had to disable either of these features.  The current that I was drawing was only half of what the DC/DC converter documentation said that it could provide.

It appeared that I was drawing a lot more current from the tow vehicle that I had ever expected, and was bringing the voltage down as well!  I wasn't sure if this was a problem with the DC/DC converter, or if it was a problem with the cabling.  I came up with a quadratic equation for the required input current and resulting voltage drop from the tow vehicle, using the efficiency factor (87%) from the converter manual and the resistance for #10 cable (from David's WireSizer program).  Even if I assume a 50' round trip for the length of the truck and the camper, the current draw should only be around 11.2A (instead of 15A) and that even with a ensuing voltage drop, the voltage should higher than 10.8V.  I played around with the numbers a bit, and noticed that the calculations seemed to line up with the actual values if the resistance was 3 to 5 times greater than what I thought it should be.

I know that my cable harness is a mess... I dragged it for around a mile a couple of years ago, and wore away a good amount of plastic... but the conductors seem to be intact, albeit a little rusty and maybe not as tight as they once were.  I was surprised to see the small terminal screws in the junction box for the 7-way trailer connector... they are nothing compared to what I used to connect the batteries and other components.  I wonder if these connections could be contributing to resistance, and if there is some way to "clean up" the connectors... particularly the 7-way connector.

At this point, I'm also wondering what size cable is used between the alternator and the 7-way connector on the truck, and if it would be appropriate to increase it to something greater (#8 AWG? Maybe #4 AWG as Dave mentioned in a previous post).  I also wonder if I should bypass the 7-way connector entirely and use an "Anderson" type connector, or something like what Dave mentioned above for the solar setup. 

[RV Squirrel]

Today I cleaned the pins on the 7-way connectors... both on the tow vehicle and on the trailer.  I first sprayed them with "tuner control cleaner and lubricant"... I bought this a long time ago when radios had something called "knobs".  I then gently rubbed emery cloth against the surface of the connectors.  Afterwards, I used a small flat screwdriver to bend the connector blades so that the connections would be a little tighter.

The change was significant.  The DC/DC converter was able to function with the lithium batteries connected, without cycling the charger on and off.  Things seem better, but I'm still not out of the woods.

Admittedly, the batteries might have had more charge on them today then yesterday, and therefore might have drawn less current.  So I reran the tests that I had done yesterday, while turning on appliances and measuring voltages and currents.  Yesterday, it seemed as if the cables/connectors were providing 3 to 5 times the resistance of 50' of #10 cable.  Today, the same setup (with a cleaner connector) was only providing around 2 times the resistance.  Much better.  Yesterday, when I had the maximum number of appliances on, the output current from the DC/DC converter was around 10.3A, the input to the DC/DC converter was around 15.0A and the input voltage was around 10.9V.  Today, the output current was the same (10.3A), but the input current was reduced to 13.0A, and the input voltage increased to 12.4V.  This suggests that there was less loss in the cable run, so the DC/DC converter did not need to draw as much current, which helped to reduce the voltage drop.

When I switched the batteries in place, The DC/DC converter provided 18.9A to charge the batteries.  However, it drew 27A from the tow vehicle (dangerously close to the 30A fuse limit).  The input voltage to the converter was 11V, and the output voltage was 13.7V (since the battery was charged enough to be in float mode).

When I re-enabled "Auto Engine Off Detection", the charger shut itself off due to the low voltage input to the DC/DC converter.  At this point, the DC/DC converter seemed to act as a piece of wire... instead of providing 27A, it drew 4A to 7A (it bounced around a lot), and only provided 4A to 5.5A to the batteries.  The input voltage was around 12.9V to 13.2V... if it got any higher than this, the charger would get enabled, draw more current, which would reduce the input voltage and turn the charger off again!

Sooo... I need to figure out a way to reduce the resistance more.  Things work now, but I'm drawing 27A which is scary close to the 30A fuse limit.  And I have to disable the "Auto Engine Off Detection" to do this.  And things may get worse with depleted batteries.  I plan to deplete the batteries more and rerun the test.

Any suggestions for how to reduce the resistance more would be appreciated (recabling with thicker gauge, better connectors, etc).  I know that some recommendations were made in the initial thread... I'll go back and reread these (again!).

[DavidM]

Here is how to find bad connectors that may be causing your voltage drop:

Hook a digital voltmeter to a good ground, using a long cable that will let you reach the entire circuit. Then put your load on and starting with the source, the TV's battery presumably, check each connection, in and out. Any measurable drop, other than long wire losses should be investigated.

You obviously need to get to near the predicted wire voltage drop.

BTW, you are using the round trip length for Wiresizer, right?

David

[Merlin]

From your detailed description so far, I suspect you need more work on the 7 pin connector and it's terminal block. It's hard to clean up rust to make a good connection. Have you considered replacing the 7 pin harness with new?  How is the 7 pin connector in your tow vehicle?  Anything bent or corroded? 

I'm bothered by the 27A draw even when the batteries are fully charged. More to think about there.........

[RV Squirrel]

David,
Yes... I am using the round trip.  I'm estimating that the combined cable run through the truck and the trailer is around 25' in one direction, or 50' in both directions.  Right now, the return in the trailer is via a (white) cable back to the terminal block for the 7-way connector.  I suppose that I could ground the return to the chassis, near to the DC/DC converter, and that this might reduce the resistance for the return current.  However, using a cable to the terminal block seemed more straightforward.

I like the idea of using a voltmeter and looking at voltage drops.  I suppose that I could do this at the truck battery, the fuse box in the truck (maybe), the terminal block for the 7-way connector on the trailer, and then at the DC/DC converter in the trailer.  However, it would be great to be able to do this just before the 7-way connector on the truck.  It's hard to get to the connectors when the cable is connected though.  I was hoping to do this today, but spent too much time matching wits with a groundhog.  I lost.  I did manage to get the shunt connected and the ammeter calibrated.  Let's see the groundhog do that!

Merlin,
I did clean the 7-way connector on the truck.  It didn't look corroded or bent.  I've thought about replacing the 7-way connector on the trailer since I did drag it a bit.  Even though I finagled the blades back into position with a screwdriver, I'm wondering if they don't fit as tightly as they should.  However the replacement parts that I've seen at the RV store (where you just replace the connector, not the cable) seem like they are cheap plastic.  The wiring harnesses available online seem a little better, but the power and return seem to be at most #10AWG, which is what I already have, and the terminal screws accept small crimps (not like the lugs that I'm using in the camper), and the 7-way connectors are copper but not tin plated.  Should I be looking for something that has at least #8 for power/return, larger terminal screws, and tin-plated connectors?  Does something like this exist?

It does look kinda silly to have #8 cable coming out of the DC/DC converter, with only #10 going in.  The cables going into the DC/DC converter are much longer and carry more current.  However, I'm concerned that I will not be able to get a #8 lug to fit inside the terminal block.  The screws inside the block are 10/32, and it is not easy to find a #8 lug for that size screw.  I found a 60 piece kit that has five such connectors... it seems strange to buy a whole kit for 5 connectors.  I guess that I have to keep on looking.  Presumably crimps are smaller than lugs, but I've not seen #8 crimps for a #10 terminal screw.

I plan to do David's test, but I wonder if I'll need to resort to using a separate connector (like Anderson) to carry the power.

[Merlin]

I don't think there are any 7 pin connectors that take more than 10 gauge wire for power. You could replace the entire thing all the way back to the junction box and then use dielectric grease on the connections. A molded connector would be your best bet. From your descriptions of mechanical damage and corrosion, I bet the 7 pin you have is a high resistance.

https://www.etrailer.com/Trailer-Wiring/Bargman/54006-059.html

[RV Squirrel]

Despite the steady rain, I was pretty enthusiastic about doing David's "voltage drop" test.  I got everything set up, only to notice that the DC/DC Converter would not stay engaged due to a "low voltage lockout".  I could have disabled this, but I noticed a really low voltage on the dashboard of the TV.  It was around 12.6V.  Normally the voltage from the alternator is around 14.4V when I first start the engine, and then drops down to 13.2V to 13.6V a couple of minutes afterwards.  There are times though (I noticed this a couple of days ago) when the voltage is only 12.6V at the alternator.  Why does this happen?  Does the truck reduce idle?  Does the alternator go into a different mode?  I had "tow mode" on.

Merlin,
Thanks for the link for 7-pin cable, as well as the tip for dielectric grease.  I notice that there are some 7-pin connectors labeled "RV style"... does this mean that they have a special pinout for RV's?  Or is there some other benefit?  I measured my cable length, and think that I may be able to get away with a 6' cable, which appears to be significantly cheaper, while still being a Bargman.  Is Bargman better than Hopkins?

I did notice that the is one wire that extends from the 7-pin connector, through the junction box (without stopping there), and into the trailer breakway battery.  I'm thinking that this is a 12V line... If I replace the 7-pin connector, can I daisy-chain this breakaway battery wire to the 12V line in the junction box?

[DavidM]

Let's talk about the breakaway battery. There is an old thread that also discusses this. The breakaway battery supplies DC to the brakes when the cable pulls the switch on the tongue closed in an emergency breakaway. It is independent of the TTs house battery. This is a problem, particularly for those like me who don't camp for the winterish 6 months. The small 5-10 Ah AGM battery runs down and it is toast.

The solution is to get rid of the breakaway battery. Inside the junction box on the tongue, you can get to the wires to the battery. Disconnect them from the battery and wire the positive from the switch to the pin #4 circuit- the one that keeps your house batteries charged and which goes to your DC to DC charger once it is all finished and working.

This change eliminates the breakaway battery and uses the more robust house batteries for that purpose.

David

[RV Squirrel]

David,
I could do that, but that would mean that I would have to run a separate set of cables from the house batteries to the breakaway battery.  When I installed the DC/DC charger, I used the cables that originally connected the battery (outside the trailer) to the AC/DC charger (inside the trailer) so that they now connect the junction box (from the 7-pin connector) to the DC/DC charger (inside the trailer).

I did replace the breakaway battery last year, and was wondering how to keep from having to do it again, so this info may come in handy.  I was wondering if it would be necessary to replace the #10AWG going from the junction box to the DC/DC charger with something like #8.  If I did, then I could use the #10 for the breakaway battery.  However, as your test indicates, and as Merlin suggests, I may have other fish to fry.

The rain let up, so I managed to get outside and do the "voltage drop" test.  I did two tests, one with a bunch of appliances turned on, but the lithium batteries disconnected, and the second test with both the appliances AND the lithium batteries connected.  Here are the results:

Appliances only:
Current into DC/DC charger: 12.0A
Current out of DC/DC charger: 8.7A
Voltage at TV battery: 12.59V
Voltage at junction box: 11.65V (drop of 0.94V... 78% of total drop)
Voltage into DC/DC charger: 11.39V (drop of 0.26V... 22% of total drop)
Voltage out of DC/DC charger: 14.17V (absorption mode?)

Appliances plus lithium batteries:
Current into DC/DC charger: 30.8A (scary)
Current out of DC/DC charger: 19.3A
Voltage at TV battery: 12.55V
Voltage at junction box: 10.56V (drop of 1.99V... 74% of total drop)
Voltage into DC/DC charger: 9.86V (drop of 0.7V... 26% of total drop)
Voltage out of DC/DC charger: 13.52V (float mode?)

Both tests suggest that around 75% of the voltage drop occurred between the alternator and the junction box.  I had hoped that replacing the #10 cable in the trailer would help things significantly, but am now realizing that it would reduce the voltage drop at most 25%... and probably much less than that.

Sooo... it seems that my options are to purchase a new 7-pin connector and see if that helps, or to put in an "auxiliary cable" that would be accessible through a second connector, with a separate fuse.  The second approach could be another can of worms though.  I was trying to determine what size cable is used in the truck... if the cables were small, then the "auxiliary cable" might make sense.  However, much of it is wrapped in loam and tape, and I'd rather not disturb it.   

[DavidM]

Hmm! So lets look at in and out of the DC charger in the second case where the current is higher. The input to the DC to DC chargeris 9.86V at 30.8A or 304 watts. Output from the DC to DC charger is 13.52V and 19.3 amps (it is doing its job and limiting to 20A or is it 18?) or 260 watts. So it is only about 260/304 = 85% efficient. I would have thought it would do better.

Looking at the long 10 gauge wire from the TV battery to the junction box, about 25' one way, and 30.8 amps should result in a voltage drop of 1.6V. You are getting about 2 volts of drop. Maybe the length is more than 25' or the gauge is not #10 (doubtful) or Wiresizer is wrong, or there are connector losses- that I can believe. So check the connectors.

Finally 30.8 amps may blow the TV's fuse before too long but probably not. I would like to see it down to 25 amps though.

The DC to DC charger is doing its job- boosting voltage to where it needs to be, while using the available input voltage and limiting output current to 20 amps. But the input voltage is so low it pulls 30.8 amps to make that work. When the batteries are almost full the DC to DC charger should supply about 14.4 volts at the end of the charge cycle. That may really be a problem for the fuse.

David

[RV Squirrel]

Actually, the data sheet for the DC charger says that the efficiency is 87%... so 85% is not far off.

In my initial estimates, I was assuming 50' round trip for the entire cable run (including the trailer, since it is relatively short), but now that I've gone out there with a tape measure, I'd say that the round-trip run from the truck battery to the junction box is at least 50', with an additional 20' round trip for the camper.  As you mentioned, WireSizer says 1.6V drop to get to the junction box, and I measured around 2V.  For the 20' round trip for the camper, WireSizer says that the drop would be .64V, and I measured .7V. Together, the observed voltage drops are a little bigger than the calculated values (by a little less than half a volt), but I can imagine that this could be attributed to loss in the connectors, and the cable length in the truck could be greater than 50' round trip.

I was hoping that a new 7-pin harness might fix the problem, but now I am beginning to wonder if I (also) have to increase the size of the cable.

[Merlin]

Well, you've been doing interesting and methodical work so far, so I'd say continue with one step at a time and hope it's a forward one and not a backward one. Do the connector and see where things are.

[DavidM]

Well, you've been doing interesting and methodical work so far, so I'd say continue with one step at a time and hope it's a forward one and not a backward one. Do the connector and see where things are.

I agree. Methodical solutions are usually the best.

David