Adding a tongue battery on a rear-battery TM

[Kimball]

I have a 2010 2720sl...want to install an auto jack and install a 12v battery on tongue for power to auto jack...is this feasable?

[Bill]

By "auto jack" I presume you mean an electric tongue jack? Many have done it by running #10 wires forward from the battery in the rear compartment.

Adding a second battery on the tongue, and wiring it in parallel with the existing battery in the back, can be somewhat counter-productive. The problem is that the tongue battery will tend to hog most of the charge current that is coming from the tow vehicle while you are driving, leaving too little to fully charge the rear battery. And the same thing will work in reverse when you are operating on shore power. The rear battery will tend to hog most of the charge current from the TM's built-in charger, starving the front battery. This happens because of the voltage drop as current flows through the long wire leading from one to the other - the two batteries do not have quite the same voltage. This doesn't mean that the farthest battery will not charge at all - but it will be slower, because the battery nearest the charge source will have a somewhat higher voltage, which will signal the charging source to shut down before the far battery is fully charged.

If you do decide to add a front battery, and connect it in parallel with the rear battery, remember that you will need to include a fuse on both ends of the interconnecting line. If the connecting line develops a short to ground, you have to isolate both batteries from the fault.

Whew! Did that make more sense than it felt like?

Bill

[Shane826]

I has a question... Wouldn’t the rear battery hog the charging voltage all the time since the charging circuit from the tow vehicle is routed through the converter, which as you said is closer to the rear battery?

But yeah, Bill is right. Just run a wire directly off the battery up along the frame to the tongue to power your jack. Don’t forget to put a fuse within a few inches of the battery.

[Bill]

Shane -

You are right. I was thinking (incorrectly) that he would tap into the charge line up near the front of the TM, before it goes to the back. This would not be a good idea, if only because it opens the cable up to weather. Thanks for catching my error.

Bill

[Bill]

Originally Posted by rickst29

Unless some kind of 'charge from the TV' upgrade has been installed to boost the Tow-Vehicle output voltage, then the Tow-Vehicle WILL NOT be charging the trailer batteries very much

Rickst -

I think that your statement above represents a pretty good summary of the situation that the OP is asking about. See if you agree with this explanation.

People forget that the tow vehicle has an alternator, a voltage regulator, and a battery, all located close together under the hood. The purpose of the voltage regulator is to control the alternator's output voltage. When you start the engine, the starter draws a lot of current from the car's battery, but for only two or three seconds. The regulator then cranks up the alternator's output voltage to recharge the battery. But the recharge may take only 15-20 seconds. Then the regulator cuts back the alternator output voltage, to avoid overcharging the battery. The alternator is now putting out the "float" voltage of the battery, not the charge voltage, and the battery receives a net zero charge. Since the float voltage is too low to charge the tow vehicle battery, it is also too low to charge the distant TM batteries, way back at the end of the long resistive wires. Making the wires bigger, or shorter, will help a little, but not much.

Back at the TM, the batteries see at most the low float voltage from the alternator, and will receive only a very small current that is insufficient to really charge them.

As you suggested, the only way to remedy this situation is to make the voltage available at the TM higher than the voltage under the tow vehicle's hood. Since you can't crank up the alternator output voltage, the only way to do it is to boost voltage at the TM, using a voltage booster device.

Bill

[rickst29]

The other question is: Will the tongue battery charge "too much", or "not enough" in comparison to the rear batteries? My answer is: the difference in charging may start out large, but the "lower" battery(s) will catch up near the end. of charging. It is comparable to having one battery slightly "older, more worn out" than the other. It is best to replace lead-acide batteries at the same time, so that they are roughly identical in charging and discharging characteristics - but small differences are not fatal.

On the tongue, the +12V battery terminal needs to be connected (through a 30A fuse or circuit breaker AT THE BATTERY) to a fused 30A terminal at the DC "power distribution" circuit board. This distance is about 15 feet, one way, and the wire size should be #10 AWG. The negative lead needs to be connected into a grounding lug which is firmly drilled into the frame, using #8 wire.

The existing "rear" battery +12V terminals are also connected to the DC "power distribution" board with 30A fuses (or circuit breakers), but their distance is shorter - perhaps 7 feet, one way. My own 2619 never had rear batteries, so I don't know if they are grounded to the frame directly, or use a pair of frame connections which exist between the "grounding bus" lugs within the WFCO (or other) and a pair of frame clamps (outside, within a small protector box, behind the waste dump valves). My 2619 uses frame clamps within that small box, with dual #8 wires back to the grounding bus lugs.

The 7 foot wire has less resistance than the 15 foot wire path (which has even more than twice the resistance, because it consists of two segments connected by a wire nut). During "Boost" Stag, and the following "Constant Voltage" Stage, the rear batteries will be favored. But when the batteries begin to be highly charged (more than 90%), the charging current into those rear batteries begins to fall. This is because the internal resistance of the batteries is increasing. The front battery, which was being "cheated" in the early stages of the charging process, is not as highly charged - and has less internal resistance. The front battery begins to catch up, until the charging voltage reaches the float threshold - and the batteries end up approximately equal.

If you wanted to reduce that imbalance, you could add about 4 feet of additional wire length into the rear-battery charging circuit. But also keep in mind: The amount of current on the forward wire is lower - it's only charging one battery. The charging current on the 'rear' wire slightly more than doubled during early stages of charging, causing resistance to be higher. (That's why you would add only about 4 feet, and not the full 8 feet of length difference.)

[[email protected]]

I stripped a jumper wire into my running lights to charge my camper battery on a dodge minivan once and almost burned up my van
I had to replace the wiring harness which burned up trying to charge the battery wit too small wires
My truck has designated wiring to charge the TM batteries now

[rickst29]

Originally Posted by [email protected]

I stripped a jumper wire into my running lights to charge my camper battery on a dodge minivan once and almost burned up my van
I had to replace the wiring harness which burned up trying to charge the battery wit too small wires
My truck has designated wiring to charge the TM batteries now

It's weird that you rminivan "running lights" were not connected through a fuse, limiting the current! (The fuse would be in the Minivan, not the TM.)

"Trailer Battery Charge" should have a unique fuse under the hood. And for TM, that fuse should be only 30A. Some trucks allow 50A, but the TM bargman cable would burn if all that current actually occurred. IMO, anyone who tows a TM from a truck with a 50A fuse should downgrade that fuse to 30A, to protect the TM bargman cable.

[tentcamper]

Never looked at how the wire from the 7 pin connector is run? Is it run as one group to the power center (converter), or is it one group to the area behind the fridge?

If its behind the fridge? You could just add your wire to the connector that connects the 7-pin battery charge wire as it leaves group of wires, and connects to a wire that heads back to the power center. So you would end up with one wire going to the 7-pin connector, one going back to the power center and the new wire headed to the new battery.

[rickst29]

Originally Posted by tentcamper

Never looked at how the wire from the 7 pin connector is run? Is it run as one group to the power center (converter), or is it one group to the area behind the fridge?

If its behind the fridge? You could just add your wire to the connector that connects the 7-pin battery charge wire as it leaves group of wires, and connects to a wire that heads back to the power center. So you would end up with one wire going to the 7-pin connector, one going back to the power center and the new wire headed to the new battery.

Behind the fridge, the "trailer battery charge" wire goes into a wire nut with two other wires. One wire is #10 to the load center. The other wire goes up front, but not to the battery - it it is an alternate power source for the breakaway switch.

The issue which I don't like in your idea is this: "Trailer Battery Charge" is a special circuit, which needs to have a 30A fuse into the load center. Some Tow Vehicles allow 50A, which could burn the bargman cable insulation and create a fire or short circuit. The breakaway switch has approximately zero current. The oad center connector allows only 30A. But if you add "Trailer Battery Charge" into this wire nut, the Bargman could easily become overloaded - with (example) 22A into the front batteries, while also pushing 12A into the electric fridge through the load center connection.

You are basically creating a second "power distribution center" behind the fridge, with multiple power sources and two possible consumers, It might not actually blow up, but it becomes harder to trace and control when something breaks. I don't like it because it 'feels' sloppy and weird.