Charging battery with tow vehicle

Five years ago or so I installed parallel 6 gauge wires, both positive and negative, with a solenoid controlled by one of my up fitter switches that goes to a plug in the bed, a plug like you see on winches. I then ran 6 gauge wires directly into my 4 125 amp hour batteries in my Alpenlite.I now arrive wherever I'm going with fully charged batteries. I have a residential frig so I run an inverter while traveling. Not only does the heavy duty alternator keep up with the inverter draw from the frig but it also fully charges the battery bank.

RoyB wrote:
The typical Truck Alternator setup was only typically designed to charge up the ONE Truck Start battery.

My understanding has always been that the alternator is there to maintain a state of charge, not recharge a low battery. This is an important distinction, as the alternator doesn't have the circuitry in it to properly charge a deeply discharged battery. Sure, driving around for a while will eventually "fill" the battery back up, but it doesn't do it in a way that maximises battery life.

The intentions of the high amp alternators, and dual alternator setups is for vehicles that have higher than normal electrical loads, such as emergency vehicles with a bunch of added lights. electric hydraulic pumps, bigger cooling fans, etc.

It's always been my understanding that the 12V charge line was intended for maintaining small batteries, such as those that power the breakaway system on trailers, not charging up the house battery (or batteries). Even then, you're supposed to charge the brake system battery to full with a battery charger, then the TV maintains that charge.

https://www.optimabatteries.com/en-us/experience/power-source/fact-alternators-are-not-designed-to-charge-dead-batteries/

How many watts to get 15amps in the morning?

The thing about solar is that it doesn't take a break, it's there producing power if the sun is up. I live in MN and have my trailer parked (for the most part) during Winter. When in storage, the module is tilted so the snow falls off and I have a better angle for insolation. You would be surprised what a single panel can produce in a day, it is significant. If my batteries are discharged, I am usually seeing 15 amps of power in the morning. It doesn't take too long to get the batteries charged back up.

I'll have to check but my guess would be that I have a 135 amp alternator in a LT package with rear heat and A/C. I can't imagine on a 1/2 ton Suburban that dual batteries would have been an option so even a 160 amp alternator might not have been possible without an aftermarket upgrade. Sounds like take what amps I can get leaving the Suburban stock and plan on using the generator when I stop. My first thought was solar but I do a lot of September-November camping while hunting and I don't think solar will do much then in Minnesota and the Dakotas. Not enough high angle sun to justify the cost of solar.

beemerphile1 wrote:

RoyB wrote:
There may be a doubled edge sword here if you beef up the charge cable from your Truck Alternator system.

The typical Truck Alternator setup was only typically designed to charge up the ONE Truck Start battery. Each battery in a system when being hit with 14 PLUS DC charging voltages could demand as much as 17-20AMPS DC Current from the source. This could be more DC Current demand than the standard Truck Alternator setup can produce....

You are correct to an extent. One needs to know their vehicles capabilities. My new truck has a 220 amp alternator. I haven't looked yet to see what gauge wire they used in the factory tow package.

Getting two alternators was also an option.

I've been seeing the 220 as part of snow plow prep option on Ram 2500's as I window shop for my Outdoorman replacement... it had rekindled my thoughts of an auxiliary charge line to my trailer's battery bank.

RoyB wrote:
There may be a doubled edge sword here if you beef up the charge cable from your Truck Alternator system.

The typical Truck Alternator setup was only typically designed to charge up the ONE Truck Start battery. Each battery in a system when being hit with 14 PLUS DC charging voltages could demand as much as 17-20AMPS DC Current from the source. This could be more DC Current demand than the standard Truck Alternator setup can produce....

You are correct to an extent. One needs to know their vehicles capabilities. My new truck has a 220 amp alternator. I haven't looked yet to see what gauge wire they used in the factory tow package.

Getting two alternators was also an option.

There may be a doubled edge sword here if you beef up the charge cable from your Truck Alternator system.

The typical Truck Alternator setup was only typically designed to charge up the ONE Truck Start battery. Each battery in a system when being hit with 14 PLUS DC charging voltages could demand as much as 17-20AMPS DC Current from the source. This could be more DC Current demand than the standard Truck Alternator setup can produce. Using the standard #10 gauge wire back to the trailer connector most likely will compensate heavier load by having some voltage drop which in turn may protect the truck alternator better from too big of DC load providing less charge current of course.

Also some of the older trucks have this charge line always connected which may run down your truck start battery if left connected for a long period of time without the truck motor running. This is why the newer trucks have this charge line under ignition key ON only control...

Adding the smart relay to the truck alternator setup to only be connected when the truck alternator is in CHARGE MODE ONLY will in most cases be governed by the truck start battery being so close proximity to the Truck Alternator. The Trailer battery may be too far away distance wise to effect any charge mode changes to the truck alternator sensing setup. In other words the Truck Alternator will sense the Truck Start battery meet its charge mode state and then drop the smart relay back to its OFF state. The trailer battery would then stop being charged by the truck alternator and may not have met its charge state yet... It may take a longer time to get both batteries fully charged to their 90$ or 100% charge state so they can be used again...

The Trailer Rv Batteries can withstand a 50% charge state and can be recharged backup to their 90% charge state and be used again whereas a Truck Start battery can only withstand a 80% charge state and in many cases when this has happens your truck start battery never really returns to a good state after being recharged again.

As always one of the golden rules is to watch what you do with your Truck systems as this is usually the only way to get back home on...

Good planning prevails here...

Yes - I have been caught with my batteries down many times in my off road camping trips... My 2KW hand Crank Honda generator is my number one PLAN B on my trips...

Just saying...
Roy Ken

I replaced the original charge line in the TV with 10 AWG and paralleled it with 8 AWG through a separate connector. Added a Shure Power isolator and breakers.
Paralleled the 10 AWG of the trailer with 8 AWG. This was mostly due to the 40 foot run from alternator to batteries. Breakers were also added at the trailer end. The result 14.5 to 14.8 volts at the batteries in the trailer with the reefer on DC. FWIW.

The bigger the better but 10 gauge minimum.