Why Tow Vehicle Charging Amps Low?

[Justinfinity]

This topic has been discussed at length in other posts and the main points are:

1. Different tow vehicles send current out the 7-way connector according to different management rules embedded by the TV manufacturer.

2. Regardless, the 7-way is only ever going to provide a trickle of current because it is restricted by the wire gauge.

3. To get high bulk charge current, you will need a separate "umbilical" using heavy 2/0 gauge wire to carry the high current the full distance of the circuit (assume ~50 feet from TV engine compartment to trailer batteries and back).

4. On the TV side, your alternator(s) needs to be beefy enough to provide higher currents for longer periods of time (assume most "civilian" vehicles are set up to power lower currents for shorter periods of time).

5. On the trailer side, depending on your battery chemistry (lead acid vs lithium), you may require battery management of some kind, a combiner/isolator, a dump load, etc.

6. You don't automatically need a DC-DC charger and this alone won't overcome the limitations of the 7-way connector or your TV alternator.

I have a 2/0 umbilical going from the TV to a Victron Cyrix Li-Ct combiner on the trailer and then to LiFePo4 batteries (with internal BMS). I get ~60A charge current from the TV (Ford F250 Super Duty 6.7 diesel with dual alternators).

[Meallen]

This is a good post. I have wondered about this and have even read that if needed, you can connect jumper cables between your TV and your trailer battery for a quick charge (assuming non lithium batteries on the trailer). I do wonder how much current would flow, especially in cases where the trailer battery is near depleted. My Audi Q7 does not charge the trailer at all on the Aux trailer pin. I was contemplating connecting the umbilical direct (with a fuse) from the Audi battery to the Zamp connector on the trailer (an SAE plug essentially which has a 30 amp fuse already) but I don't know if 30 amps is enough. My guess is that I'd be OK unless the trailer battery was very low of the had lots of things consuming power.

[wulfraat]

Originally Posted by GCinSC2;

So it’s seeing input vs output? Output while towing is parasitic plus fridge maybe .2-.3 amps?

BMV measures current inbound and outbound from the battery. My trailer at “idol” with batteries connected draws 2-3 amps for what it is worth. Fridge, a couple lights maybe, stereo on standby, Pepwave WiFi router, Verizon hotspot, Victron BMV/ccgx/BMS, propane detector, etc.

[GCinSC2]

Originally Posted by wulfraat

BMV measures current inbound and outbound from the battery. My trailer at “idol” with batteries connected draws 2-3 amps for what it is worth. Fridge, a couple lights maybe, stereo on standby, Pepwave WiFi router, Verizon hotspot, Victron BMV/ccgx/BMS, propane detector, etc.

Data from this AM. Master switch on, not connected to shore power, no lights, stereo off, fridge with gas system actually running. BMV-712 says current draw 0.5 amps.

And now back to our regular programming, As the Truck Charges brought to you by the OverFused & UnderWired Network.

[Justinfinity]

Originally Posted by Meallen

This is a good post. I have wondered about this and have even read that if needed, you can connect jumper cables between your TV and your trailer battery for a quick charge (assuming non lithium batteries on the trailer). I do wonder how much current would flow, especially in cases where the trailer battery is near depleted. My Audi Q7 does not charge the trailer at all on the Aux trailer pin. I was contemplating connecting the umbilical direct (with a fuse) from the Audi battery to the Zamp connector on the trailer (an SAE plug essentially which has a 30 amp fuse already) but I don't know if 30 amps is enough. My guess is that I'd be OK unless the trailer battery was very low of the had lots of things consuming power.

Careful. You haven't specified the battery chemistry of the batteries in your trailer and, regardless, you'll want to manage the connection with a combiner/isolator. Finally, generally speaking, 30A is a decent amount of current to charge with but, depending on the battery chemistry and the Charge Acceptance Rate of that chemistry, you can't expect to get conventional lead acid batteries to charge quickly from 85% to 100%. Further, you mentioned something about your trailer battery being "near depleted": if you are using lead acid batteries, you should not be running them below 50%; that's a sure way to kill them. With LiFePo4 (lithium) batteries not only can you run them down to about 20%, they will charge all the way back to 100% without the declining charge acceptance problem typical of lead acid batteries.

[Life is a Highway]

Exactly, the Australia folks figured this out years ago hence the invention of the DC-DC safe converter. Smart Alternator logic if your new vehicle has it makes DC-DC a must. Old school charging systems will trickle 3-7 amps to you trailer battery bank.

Originally Posted by wulfraat

On the tundra you are limited to the current that can travel on the small guage constant 12v line on your 7-way. 2-5 amps is typical. It’s designed for a maintenance charge current only.

You need to rewire your 7-way with a heavier 10ga wire for constant 12v hot and common ground, or run a dedicated +/- line to your DC-DC charger.

Your alternator will be the next bottleneck, potentially. I’m not sure what your truck is specked with…

[Meallen]

Thanks for the responses to my question about just using "old school" hardwiring back to the trailer. All valid points. I better understand now it effectively joins the TV and TT batteries together and if they have different charging chemistries, charge levels, or charging requirements this can be an issue given the TV alternator becomes responsible for charging them all. I can definitely see the value of these DC to DC converters and isolators. I will rely on the sun to charge while I drive a while longer as a research these options.

[pteck]

Originally Posted by Justinfinity

This topic has been discussed at length in other posts and the main points are:

1. Different tow vehicles send current out the 7-way connector according to different management rules embedded by the TV manufacturer.

2. Regardless, the 7-way is only ever going to provide a trickle of current because it is restricted by the wire gauge.

3. To get high bulk charge current, you will need a separate "umbilical" using heavy 2/0 gauge wire to carry the high current the full distance of the circuit (assume ~50 feet from TV engine compartment to trailer batteries and back).

4. On the TV side, your alternator(s) needs to be beefy enough to provide higher currents for longer periods of time (assume most "civilian" vehicles are set up to power lower currents for shorter periods of time).

5. On the trailer side, depending on your battery chemistry (lead acid vs lithium), you may require battery management of some kind, a combiner/isolator, a dump load, etc.

6. You don't automatically need a DC-DC charger and this alone won't overcome the limitations of the 7-way connector or your TV alternator.

I have a 2/0 umbilical going from the TV to a Victron Cyrix Li-Ct combiner on the trailer and then to LiFePo4 batteries (with internal BMS). I get ~60A charge current from the TV (Ford F250 Super Duty 6.7 diesel with dual alternators).

Great post, and covers just about all of it.

Thinking outside the box, for those that have not installed solar or have smaller arrays, it may be more worthwhile to spend the time and effort there.

With a 300-400W system, for 95% of users, you will never have to worry about charge again. Whether camped, towing, whatever. It'll be like a perpetual power machine. Even with standard lead acid batteries.

[Justinfinity]

Originally Posted by pteck

Great post, and covers just about all of it.

Thinking outside the box, for those that have not installed solar or have smaller arrays, it may be more worthwhile to spend the time and effort there.

With a 300-400W system, for 95% of users, you will never have to worry about charge again. Whether camped, towing, whatever. It'll be like a perpetual power machine. Even with standard lead acid batteries.

To that point, I have 6x100W panels on my Globetrotter and have almost never needed or used shore power, except when "moochdocking" at the in-laws in FL where having air conditioning is a blessing.

Conversely, I spent two months dry-camping in Canada last winter and the batteries (400AH LifFePo4) were consistently back up to 100% by the middle of every afternoon. I used my Honda 2000 gas generator only once or twice after a couple of days of no sun. I ran the furnace every night (and, yes, I went through a lot of propane!).