Why TM batteries are not (normally) charged by Tow Vehicles

[rickst29]

I write this post as a new Thread, providing definitive answers for two questions which appear frequently - and have been answered incorrectly upon most occurrences. The way that it all works is complicated.

#1 "Will my Tow Vehicle (unmodified) re-charge the TM batteries while towing?" and

#2 "WIll the Fridge, while running DC12V, discharge the TM batteries during Towing"?

The answers are slightly different. Short answers: NO , an unmodified Tow Vehicle will never re-charge TM batteries while towing; and YES, the Fridge will usually draw on the TM batteries "first", discharging them. Details follow, and "solutions" are at the bottom.
- - - -
"Will my Tow Vehicle (unmodified) re-charge the TM batteries while towing?"
The answer is NO, and upgrading to a somewhat bigger "Trailer Battery Charge" wire will not help this by much.

In the Tow Vehicle, the actual voltage of "+12VDC" is determined by a regulator - typically integrated into the Alternator, which sets the Output Voltage on the Alternator as a whole (it's variable). The regulator uses a "sense" wire connected into the under-hood fuse box bus. When voltage on the Sense wire is tested, the Alternator's regulating coils are fed more or less current to increase or decrease the output.

For a short period after running the Starter, the Alternator will run at very high levels of Voltage and Output current - creating a system Voltage well above 14 Volts, which will help the TV battery recover quickly. But, after the TV Battery has recovered to a fairly high State-Of-Charge, the system Voltage will be allowed to drop - to a value which is typically less than 13.5 Volts.

Even with an upgraded "Trailer Battery Charge" wire, and with further upgrades to the TM Battery wiring connections, 13.5 Volts (or even less) will not provide significant charging power into the TM batteries. The problem is Low Voltage right at the beginning, and avoiding Voltage Drop along the wiring path doesn't provide much help with this problem.
- - - - -
"WIll the Fridge, while running DC12V, discharge the TM batteries during Towing"?
Generally YES, although this IS a function of Voltage Drop along the wires. The DC-Powered Refrigerator will draw about 12A from the "best" source of Voltage into the DC Load Center. Within the TM, Voltage can be provided from TM batteries at 12.8 -> 12.4V (as the batteries discharge). At the TV fuse box, Voltage can be provided at about 13.3V. But the TM Batteries are the preferred source, unless wiring form the TV is substantially modified, or the 13.3 Source Voltage is increased.

Long calculations follow, you can skip over if you want:

With large current, there is significant Voltage Drop between the TM batteries and the "12VDC Load Center within the converter unit. There is also Voltage drop from the Load Center to the Fridge, but this is the same in both configurations. (Power "from the TV" and power "from the TM batteries" uses the same pair of wires to reach the Fridge from the DC Load Center, unless you modify this).

For the "TM Battery" path, there is typically 7-12 feet of #10 between the battery and the DC Load Center (more with batteries on the tongue - less with batteries in the rear compartment). We'll assume this path as a round trip of #10, even though the path from front-mounted batteries back to the DC Load Center "Grounding Bus" is actually made through the Frame). A 10ft round trip on #10 wire, carrying 12A, will loose about .25 Volts on this path. If the TM batteries fall from 12.8V to 12.4V during drastic discharge, they can maintain the Load Center at 12.55 -> 12.15 volts while the Fridge is running.

Now compare with the use of the TV path: Within an unmodified TV, the "30A" fused "Trailer Battery Charge" wire is typically only 14 AWG, sometimes even 16AWG. (The grounding "return" wire is usually larger, but I'll ignore the difference, just as I ingored the differnce for the "Battery Path" return through the frame.) There's about 14 feet of this to reach the Bargman Plug. At 12A, the Voltage drop in this segment is about 0.4V Volts. The next segment is the Bargman Cord inside the TM itself, and along the hitch A-frame. (It ends behind the Fridge): About 15 feet of #10, with a starting Voltage of about 12.9V, the Voltage drop on this segment is almost another .4V, yielding 12.5V at the nut which terminates the Bargman "Trailer Battery Charge" wire. But from there, we have another 8 feet of #10 to actually reach the Load Center. The ending Voltage will be about 12.3VDC, if we lost nothing in all of the connectors and fuses. With some additional resistance occurring in various wire nuts, wiring harness pairs, and the Bargman - I will further reduce this Voltage by an additional 0.1V (SWAG). Result = 12.2V.

The Voltage which can be supplied by TM Batteries (at the DC Load Center) falls from 12.55 down to 12.15 while the Fridge is drawing DC from TM batteries. The Voltage which can be supplied by the Bargman path, if it was supporting the same load, is constant, but only about 12.2V. From a Battery Voltage (at the Batteries) of 12.8V, down to a Battery Voltage of about 12.5V the TM Batteries are the better source, and power will be drawn from them almost exclusively. The Bargman-TV Path will begin getting used when the TM Batteries have been "pulled down" to about 50% SOC.
- - - - -
The "fixes"
- - - - -
The most effective fixes raise the TV Voltage in various ways, and run a higher-Voltage Bargeman (or Auxiliary) power wire into a Solar Controller, or into the Converter as 120VAC input:

• Buy a purpose-built Redarc or Ctek, which Boosts "Tow Vehicle Voltage" after it is received in the TM. They include the Charge Controller (to avoid over-charging your batteries). They cost a lot. (I did this, and it works - but I'm switching over to "bullet #3".)
• Create a new "Charge from TV" method (using a 120VAC cable connncted to Converter AC input) fed by a 120VAC "Inverter inside the TV. This is fairly cheap, but requires another cable between TV and TM at the hitch, and running all the way back to the Converter. (And it's a high-voltage cable. .)
• If you already have a Solar Controller, the you can do a similar "Voltage Bump" while still using the Bargman Cable "Trailer Battery Charge wire": Increase TV-supplied Voltage to 24V, and use a Relay in the TM to determine input Voltage and switch the TV "Trailer Battery Charge" wire to (a) it's original 12V connection; or (b) the Solar "Panel Input +" connection. This involves a lot of Switches and Relays, in both the TV and the TM. It also costs more than the 120VAC solution - but it avoids the second electrical cable between TV and TM. (I am personally doing this one.)
• Reduce Voltage drop by pulling a better "battery charge wire" within the TV, and (perhaps) also making a fused connection between Bargeman "12V trailer battery charge" and the TM Fridge. That new connection behind the Fridge eliminates about 12 feet of wire (with Voltage drop), otherwise needed to reach the Fridge through the DC Load Center. But ripping out TV body panels is somewhat involved, and this does nothing for battery charging - it only fixes Refrigerator Operation.
• Increase TV Voltage by modifying the Alternator Sense Wire. Simply insert a Diode into the wire. (A standard silicon diode, NOT a Shottky.) This will introduce a Voltage drop of about .7 Volts, and raise the TV operating Voltage by the same amount. Very quick, very easy - but I do not recommend this, because it will overcharge both the TM and TV batteries if left in place for "too long".

[scrubjaysnest]

That pretty much covers the problem in most cases.

For those that want to try the diode method look at this article from Smartgauge:Smartgauge.UK
They also have a nice article on how and the why behind correctly wiring multiple batteries.

[LoveToCamp]

Once again, Rickst29 gets into his long-hair electro-talk, which is not my area of expertise. But, (thank you!), Rick explains so I can understand the problem and solution.

I had read the many threads saying I could run my fridge on battery while traveling, and started doing that last summer. (My first TM ran fine on propane while traveling, but the 2014 TM's flame blows out while on the road.) While I have two batteries and (at the time) a single solar panel, my batteries were not at full-charge when I arrived at camp, even though I drove across Colorado and Utah in full sun. I was scratching my head, wondering if I interpreted the threads correctly.

I installed a second 100w solar panel, and my subsequent trip seemed to allow me to arrive at camp with more juice in my batteries. Bottom line seemed to be, sufficient charging does not seem to come from tow vehicle, nor tow vehicle and one 100w solar panel.

Rick's explanation validates my observations. However, instead of doing any of the mods indicated (I am not an electrician, so those mods would be my last resort), I may simply add a block of ice to the fridge at the beginning of the trip, and set the fridge temp a little warmer, so it doesn't cool as much. Still enough to keep the food chilled, but not enough to draw as much juice from the batteries. On the road, beer will be kept in a cooler with ice, instead of the TM fridge.

Thanks, Rick, for your explanation. It is greatly appreciated, as it helps me understand more about the entire TM electric system.

[Keith Wire]

Hi Rick,

You said you were doing this one...

Originally Posted by rickst29

• If you already have a Solar Controller, the you can do a similar "Voltage Bump" while still using the Bargman Cable "Trailer Battery Charge wire": Increase TV-supplied Voltage to 24V, and use a Relay in the TM to determine input Voltage and switch the TV "Trailer Battery Charge" wire to (a) it's original 12V connection; or (b) the Solar "Panel Input +" connection. This involves a lot of Switches and Relays, in both the TV and the TM. It also costs more than the 120VAC solution - but it avoids the second electrical cable between TV and TM. (I am personally doing this one.)

Could you explain how you are increasing the Tow Vehicle voltage from 12 to 24 volts?

I am assuming there is is a DC to DC Converter and you have tied it in the charge line after the Bargman Plug so the converter is located on the TM. Can you share the make and model of the converter you are using?

Do you have a wiring diagram showing your relays and switches for this setup? And again any manufacturers and model numbers for the relays and switches would be helpful.

Thanks, Keith

[rickst29]

Originally Posted by Keith Wire

Hi Rick,

You said you were doing this one...
Could you explain how you are increasing the Tow Vehicle voltage from 12 to 24 volts?
I am assuming there is is a DC to DC Converter and you have tied it in the charge line after the Bargman Plug so the converter is located on the TM. Can you share the make and model of the converter you are using?
....
Thanks, Keith

Because the Booster can draw almost 40A on the "12VDC" input terminal (if the Solar Controller is charging batteries in "Boost mode", with high output current capability), we locate it inside the the TV. These provides two advantages. #1, current through the Bargman and TV wiring is cut in half dramatically reducing Voltage Drop and Power Loss in the TV and Bargman "Trailer Battery Charge" wires. #2, it allows the Driver to control use of the Booster, versus use of the genuine Solar panels, with the flick of a switch in the dashboard. (It's switchable at any time, including driving down the Interstate.)

Don't forget, a Solar Controller is mandatory to avoid blowing batteries and TM equipment (which will blow up if the "24V" is put directly into the batteries or DC Distribution). The purpose-built Redarc and Ctek units contains both parts - an MPPT Solar Controller, and a Voltage Booster. They are installed in the Trailer only, and they cannot be switched off from the TV cabin. The Redarc and Ctek "trailer-only" devices pull up to 25A down the wires Bargman wires, causing much higher I-squared-R power losses.

So Built-from-scratch is better, if you have a high-power Solar configuration to use when the sun shines brightly. (I've got one). Another TM owner and I have WIP on this conversion, making this same upgrade on both of our Trailers (and Tow Vehicles). We put the 12/24 Converter in the Tow Vehicle, under the control of a switch in the dashboard. The switch scenario goes like this:

• Dragging any other Trailer: Leave the switch off (run the Bargman at "12V").
• Pulling the TM in excellent sunlight: Leave the switch off (TM Solar Panels do the work).
• Under clouds or Night Driving: Turn the switch ON.
• In a Boondock campsite, when Solar Panels haven't been helpful enough: Connect the TV Bargman to the parked TM, turn on the TV, then idle for 20-30 minutes with the switch ON. (Recharge the batteries in camp, using the TV as a 24VDC "generator".)

Parts In the TV:

1. One switch, which will carry only a low current. I will use this this one: http://www.ebay.com/itm/141816846277
2. A high current, 4-pin Relay (disables the new wire from the battery, whenever the ignition isn't "ON"). This one is overkill, but includes an indicator light: http://www.ebay.com/itm/182033514542.
3. Two high curent, 5-Pin Relays (12V coil). Like this one: http://www.ebay.com/itm/131374818673. There's two more in the Trailer, so maybe buy a 5-pack. (We only need 40A capability, but I love having the indicator lights.)
4. And of course, the Voltage Booster itself. You want the maximum output current capability of this one to be AT LEAST AS BIG as the maximum input current of your Solar MMPT, so that it doesn't become overloaded and "fry" under high demand. This one is a good choice for "20A" Solar Controllers: https://www.ebay.com/itm/1-DC12V-Ste...l/332164334824. KL Pauba chose this one, and it also works: https://www.ebay.com/itm/DC-Converte...t/323331462886.

From the battery, a fat wire (#8 or bigger) goes through a new fuse or Circuit Breaker at 40A. That +12v wire continues through the power leads of the ignition-controlled Relay, and then continues into the first of two Relays being controlled by the "Zombie Switch". (The input "control" leads for both of these 5-pin Relays are wired together, from "Zombie Switch" output.)

Relay 1: the "power" output from the ignition-controlled Relay comes in on terminal 30 (This is #8 wire.) The coil "pulled in" (switch engaged) output lead (terminal 87A) goes to the Booster +12V input (this is also #8 wire). The "inactive coil" output lead (terminal 87) goes directly to the matching "inactive coil" terminal on the second Relay (terminal 87). This wire can be #10.

Relay 2: The "inactive coil" lead (terminal 87) connects from Relay 1. The +24V output of the Booster comes in on terminal 87A. Terminal 30 (the common "power" connector) connects to the Bargman "Trailer Battery Charge". These wires can all be #10, because the current from the "high-power" connection" has been cut in half (i.e., when the voltage was raised by the Converter.)

All 5 grounding leads ("control -" from all 3 Relays, plus two leads on the Booster device) go to TV frame ground.

When the ignition is on, but the Relay coils are not yet powered up from the "Zombie Switch", "trailer battery" current routes through the short "jumper" connecting the #87 terminals - bypassing the Booster. Trailer Voltage is "12V" (actually, a bit more than 13V at most times.) But when the "Zombie Switch" is turned on, the current path goes through the Booster input and output "+" terminals -arriving into the 2nd Relay at 24V. - - - - -
Parts in the Trailer:

• A 4-pin (or 5-pin) Relay, with 24 Volt Coil. (Hint: Car alarms use these.) http://www.ebay.com/itm/380650078279
• * Two 5-pin high-current Relays, with 12V coils (same as used in the Tow Vehicle).
• A Solar Controller - MMPT is much better than PWM in this application.

In the Trailer, a special Relay is used to detect 24V, and it switches the other two "regular" 5-pin Relays. (In roughly the same way that the "Zombie Switch" controlled the two 5-pin Relays in the Tow Vehicle). This one: http://www.ebay.com/itm/380650078279 It's a 5-pin Relay, but we use only 4 pins: a low-current jumper from the Bargman wire end (behind the Fridge); a coil "Ground", and a 12V power lead from the battery or load center into terminal 30.
According to the painted diagram on this one, we connect the switched 12V output power lead to terminal 87 (not 87A). This becomes the Coil "control" input for the other two Relays.

The 2nd Relay is a high-current 5-pin Relay, taking power from the Bargman (terminal #30) and switching it to either the Load Center on terminal #87 (coil inactive, Bargman Voltage running at "12VDC"), or a short "jumper" leading to corresponding terminal on Relay #2 (coil active, pulled in: #87A on both Relays).

The load center wire (on terminal 87 of the first Relay) MUST also be shared with Breakaway Switch input power lead - the third wire which used to be in a wire nut with the Load Center <--> Bargman "Trailer Battery Charge" connection.

The Third Relay in the Trailer is similar to the second Relay in the Tow Vehicle - it chooses which of two "input" wires (87A or 87) will be connected to the #30, which is the "Solar Panel +" Input on the Solar Controller. One connection is from the Bargman, the other is from the genuine Solar Panels.

[rickst29]

This beautiful diagram was created by my design partner. I have removed a "second stage" Boost which he will also try to add in.

[rickst29]

Although it is theoretically possible to use a DPDT Relay (in place of the two 5-pin Relays), there are no socket bases with sufficient current-handling capability (up to 30A on 12V "power" wires). When the switch is off (as shown), the Trailer "Battery Charge wire" runs at 12V, up to 30A. When the switch is on and the 5-pin Relays are switched, up to 24A goes into the "12V input" side of the booster, and the output/Bargman wires carry no more than 12A.

[rickst29]

It's exactly as the diagram shows, with all components except a 12V/24V selector switch under the hood. Two photos are attached:

(1) My dashboard "mode switch". Very subtle, and definitely in good taste.

(2) Three Relays and the Booster Module, with all wiring connected.

[klpauba]

Very nice, rickst29! About the only thing left for you to do is modify the equipment list in your signature!

[rickst29]

I haven't made it pretty yet (aligning within a box, or attaching Relays to the vertical MPPT Mounting post on the left. But here's photo, and I'll explain it:

The blue Wire Nut contains red wires. #1, the Bargman "Trailer Charge" cable from under the Fridge (a high-power wire, up to about 280W ~ 11.7A @ 24V ). #2, a low current "Jumper" into the 24V Relay on the left, as the coil switching "+" lead. And #3, a high current wire Delivering power to the MPPT "Solar Input +" on Relay #2.

4 wires on Relay #1 are:

1. Coil switching "+" (the jumper from Blue Wire Nut);
2. White Coil Ground. (all Ground wires connect to "Battery -" on the Solar Controller);
3. "12VDC" pulled from "Battery +" on the Solar Controller; and
4. Output to the second Relay "coil +".

The purpose of Relay #1 is to detect when 24V is present on the Bargeman, and then Deliver 12V coil switching Voltage into Relay #2. Nothing more. All 4 terminals are extremely low power.

The 5 wires on Relay #2 are:

1. Coil switching "+" (the "switched output" from Relay #1, at battery Voltage or nothing at all;
2. White "Coil Ground."
3. A shared OUTPUT LEAD to the Solar Controller.
4. The Solar Power Input from Panels (In perfect conditions, mine would be 5.3A @65V, although the MPPT won't accept full power.) This one is connected to the Output Lead (and Solar Controller) whenever the Coil does not have 12V power.
5. The 24V "power input" from the Blue Wire Nut.

When I activate the "Zombie" Switch in the 4Runner, a 12V coil switching Voltage is put into the second Relay. The Relay switches the connection from the Genuine Solar Panels to the 24V "power input" lead.

- - - - What Happened to the 3rd Relay ? - - - -

The Job of the 3rd Relay was to connect the Bargman, running at "regular" 12V, into the TM "12V" electrical system. But I realized, as a I reached for the Relay, that it's a slightly bad thing to ever make that connection if the TV is capable of "24V or Nothing" operation, and you always remember to flick the Zombie Switch when it's dark.

Here's why: If The Trailer batteries are seriously discharged, and "Bulk Mode" charging is desirable under good Solar Conditions, the connection reduces the Solar Battery Charge Voltage by "leaking" power back to the Tow Vehicle. (And of course, the TV doesn't need that power at all.)

If Solar conditions aren't great, just flick the Zombie Switch, and connect the TM at a Voltage which is sufficient charge batteries. Let the Solar Controller make al the decisions, all the time.