TrailManor Owner's Forum - View Single Post - Why TM batteries are not (normally) charged by Tow Vehicles

**rickst29** · 03-06-2016, 11:52 AM

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".

03-06-2016, 11:52 AM	#1
rickst29 yes, they hunt lions. Join Date: Aug 2005 Location: Reno, NV Posts: 1,318	Why TM batteries are not (normally) charged by Tow Vehicles 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". __________________ TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor). TV = 2007 4runner sport, with a 36 volt "power boost".