Tow Vehicle Battery and Alternator

I'd run dedicated wiring so at least your trailer battery gets nearly the same voltage as the engine battery.

The voltage your engine battery gets is decided by the vehicle's voltage regulator, and none of these are programmed intending to quickly and fully recharge depleted batteries.

Mostly they are designed to never overcharge, and as such, will drop to mid to high 13's relatively quickly, but all vehicles will vary in the max voltage and for how long it is held.

Only if your alternator was incapable of meeting all the loads applied to it would increasing its rating prove beneficial, and likely only at higher rpm, not idling.

Likewise a larger better starting battery is not going to assist trailer battery charging.

I'd get the following 6awg harness or similar, and run it to alternator + and a - mounting bolt. Disconnect the connector manually when parked for any length of time so trailer cannot discharge engine starter battery.

landyacht318 wrote:
I'd run dedicated wiring so at least your trailer battery gets nearly the same voltage as the engine battery.

The voltage your engine battery gets is decided by the vehicle's voltage regulator, and none of these are programmed intending to quickly and fully recharge depleted batteries.

Mostly they are designed to never overcharge, and as such, will drop to mid to high 13's relatively quickly, but all vehicles will vary in the max voltage and for how long it is held.

Only if your alternator was incapable of meeting all the loads applied to it would increasing its rating prove beneficial, and likely only at higher rpm, not idling.

Likewise a larger better starting battery is not going to assist trailer battery charging.

I'd get the following 6awg harness or similar, and run it to alternator + and a - mounting bolt. Disconnect the connector manually when parked for any length of time so trailer cannot discharge engine starter battery.

Thanks for the response. 2 follow up questions. If my DC to DC charger is rated at 40 amps (Renogy has one), would it be a bit taxing on the 150 amp OEM alternator? Any benefit to upgrading to a 220 amp alternator in this case?

Secondly, do these dc to DC chargers act just like a conventional onboard RV multiphase converter? Bulk to absorption to float? From my understanding, with this setup, I would rarely get above 80% stage of charge after a 3 hour drive, right?

I would stick with the existing charge wire and just get the DC/DC booster mounted on the trailer. You will need to be 10 amps rated lower than the charge line. Most of these lines are fused at 40 amps so you are looking for the Renogy 20 amp DC/DC charger. The manual says you can use the 40 amp rated on a 40 amp fuse but I don't buy that.

Three hours at 20 amps is actually a good charge into the battery. The existing alternator will do just fine. Yes it does the three stage charging routine putting full voltage right on the battery terminals and will be a huge improvement. Use the setting for 14.7 volts on the pair of GC2.

Charge to 90% is fairly quick at this voltage and the last 10% takes some extra time no matter how many alternator amps, charging capacity or wire size.

CJM1973 wrote:
If my DC to DC charger is rated at 40 amps (Renogy has one), would it be a bit taxing on the 150 amp OEM alternator? Any benefit to upgrading to a 220 amp alternator in this case?

Secondly, do these dc to DC chargers act just like a conventional onboard RV multiphase converter? Bulk to absorption to float? From my understanding, with this setup, I would rarely get above 80% stage of charge after a 3 hour drive, right?

How taxing it would be to the alternator would depend on how long your depleted batteries could suck up 40 amps, and how much other loads the alternator is having to power, like headlamps@~15 amps, blower motor on high @~18 amps, ect.
My 1989 engine at 2k rpm requires 12.2 amps to run ignition and fuel pump, not including field current to alternator. Other vehicles will vary.

Heating of the alternator is the issue, and how hot it gets depends a large amount on the vehicle's engine bay, and the speed one is travelling. I can max mine out at 120ish amps@ 65mph and the stator does not exceed 138f, but idling parked at 50 amps max and it shoots upto 220f in a few minutes.
An alternator with a bigger rating 'should' be able to handle or exhaust the heat better.

I have no experience with the DC to DC chargers sold as such, but I do believe they take whatever vehicle voltage is allowed by the vehicles voltage regulator and try to do 3 stage battery charging stepping it both up and then down once it has decided to 'float' them.

I'd not want to be limited to the Renogy 40 amp max, but I have an externally regulated alternator and use a modified voltage regulator so I spin a dial to choose a voltage and watch voltmeter and ammeter respond accordingly.I had to trick my engine computer in order to do this without setting off the check engine light, but it has been a great reliable modification.

I do make use of Dc to DC voltage boost, and voltage buck devices, but I have not tried to add one to a 7 pin trailer harness. I did buy a '1800' watt voltage booster to experiment with, but it was so poorly soldered and things went pop, and smoked a wattmeter rated upto 60 volts when I had it set to 24v with little load, then changed the load.
I have a 150 watt voltage booster that works well considering it was sub 3$, there are 300 400 and 600 watt ones sold too.

I've used this booster to equalize one battery at 16.2v from another at 12v which had a regular charger on it. Some of these have current limiting potentiometers so you could limit it to ~15 amps to save the 7 pin wiring harness.



Some have done the inverter on engine battery, powering a high amp charger in the trailer, over a quality 120vac extension cord, in your situation. If the inverter can draw more than the alternator can make then it feeds off the engine battery when charging the depleted trailer batteries at a high rate. In this case a larger engine battery gives more of a cushion when rpms are low or one forgets to turn off inverter when parked. One needs to keep in mind engine starting batteries do not appreciate being cycled, and perhaps employ a marine battery instead when one is due for a new engine starting battery.

There's lots of ways to get more juice from alternator to depleted trailer batteries, and no 'one' single correct way, no matter how loudly the opinion is typed.

-Fat copper to trailer batteries and whatever voltage is allowed by vehicle, is what it is.

Dc to DC chargers, over adequate cabling.

Inverter on engine battery to feed charger on trailer battery over a good ac extension cord

a simple dc voltage booster on the 7 pin wire.

All are valid ways, and the choice can be influenced by what voltage the vehicle allows when the alternator is hot.

The big enemy is voltage drop, as depleted batteries only getting 13.6v at their battery terminals will be accepting 1/3 to 1/2 that they would had 14.7v been reaching the terminals.
I get a lot of bile about this 1/2 to 1/3 cobservation, but I have ammeters and voltmeters on my dash next to my voltage control dial, so this is actual repeatable observation and the main variable over the last 4 years has been the battery condition, 1/3 when newer, now closer to 1/2.

When disbelievers call me out its like one is travelling 60mph, as indicated by a radar sign, highway milemarkers and a clock, and ones own speedometer, but someone else is saying you are only doing 48mph based on their beliefs alone.

1/2 to 1/3 the amp flow is real data, when 13.6v reaches the terminals compared to 14.7v. Other batteries might vary some, but not a lot.

as far as how much charging you can get done driving in x amount of time, it is all dependent on how low your batteries were when you started charging/driving, and how many amps you can feed them and this is dependent on the voltage reaching their battery terminals.

regardless, it basically is impossible to recharge lead acid from 80% to 100% charged in less than 3.5 hours assuming ideal voltages reaching battery terminals, But 2.5 hours at ideal charge voltages in the mid 14's can at least get one in the 97% state of charge range, where 13.6v might get one from 80% to 84% in the same amount of time.

The more aged the batteries the longer the 80% to 100% takes, especially at lower charging voltages.

I forgot to ask; if I do proceed with the DC to DC charger, do I wire it directly to the + battery terminal or + alternator lead? The renogy diagram shows multiple methods.

Also, what type of connector is preferred from TV to trailer? Wire will be 4 or 6 awg.

The original wiring from alternator to starting battery and its fuse was likely not intended to carry an additional 40 amps.

Can it, should it, will it be fine? Depends on the gauge.

SO I usually recommend the alternator (+) stud when it is easily accessible and one can stack another ring terminal on it. There is an advantage in that if one takes power from starter battery, there should be an additional fuse added near battery + terminal, but if one takes it from the alternator there is already a OEM fuse between alternator and starter battery and no additional fuse is needed, unless perhaps the product instructions demand it.

That winch style connector on that first link I posted, is a good one, but is not moisture proof unless one makes it so by filling gaps where wires enter housing, and filling the contact area with dielectric grease.

Anderson makes those types of connectors in the SB style, not the smaller powerpole 15/30/45 amp flavors that are limited to 10awg.

https://powerwerx.com/anderson-sb-connectors-sb120-120amp



perhap$, a better option:
https://www.millsupply.com/medium-duty-truck-parts/lift-gate-hardware/liftgate-plugs-cables-sockets/

Proper wire termination is always important for long term reliable low resistance connections, so factor that into your connector choices

As stated earlier, your best bang for the buck to increase the amount of current getting to the trailer batteries is to start by running a heavy duty charge line from your truck to the trailer batteries. Huge reduction in power loss compared to most factory trickle charge lines.

CJM,

Without knowing the specifics of the TV and TT, I fear you are headed for a disappointment. Even if the the TT house bank is on the tongue, we are talking a long run of cable. Remember, to make this work right, you will not be able to count on the frame of anything as a conductor.

With the entire life of a Lead/Acid battery between 12.0 and 12.6, and peak charging limited to 14.4 (or .6), there is no room for voltage drop. So a huge alternator will be a lost cause even if you run welding cable from the alternator to the house bank.

A Renogy charger will be a big help, but you are still stuck with the recharge time being limited by the battery chemistry. Even AGMs, though will charge faster, still have limits. They just get to 90% a lot faster if you can feed them juice.

You do not say what the installed converter is in the TT, but if it is not a modern 3 or 4 stage unit like Progressive Dynamics or Iota (there are others now) that does a boost, bulk and float, I suggest you replace it first.

Then, go an buy an inexpensive little genset. Try not to buy a "Contractor" unit because they are designed to cover up the noise of hammers and Skilsaws. There are more than a few on the market. Then, you will also have the portable for other uses. (You will find some.)

I know of a guy that boondocks with a long cab TV and a TT that ties his generator down in the bed and runs it when underway. He just runs the shore power cable to it. (With some duck tape for control.)

This will all be more simple and reliable than the alternatives, you will just have to put up with some noise some times.

CJM1973 wrote:
I know I can install a DC to DC charger in the trailer.

Do this FIRST ! Use the standard 12V line on your pin trailer harness. I think you will be surprised at how much better it works. Whether it can completely recharge your battery bank depends on 1) the size of the battery bank, 2) the "state of charge" (SOC) when you hit the road and 3) how many hours you drive.

Matt_Colie wrote:

With the entire life of a Lead/Acid battery between 12.0 and 12.6, and peak charging limited to 14.4 (or .6), there is no room for voltage drop. So a huge alternator will be a lost cause even if you run welding cable from the alternator to the house bank.

First, it takes AT LEAST 13.2V to put energy into a lead-acid battery, PERIOD !

I have mentioned this in a least a dozen other posts. Modern cars, light and most medium duty truck (Class A) use "smart charging". In short, the voltage is regulated AT THE VEHICLE BATTERY to about 13.2V for 99% of the time the engine is running. After the voltage drop to your house battery bank, there is simply not enough voltage left to charge it !