Here's the deal.....

I have a Ram pickup as my tow vehicle. In the bed of the PU I have a 100Ah AGM battery to power a portable refrigerator , a winch and other electrical demands when away boondock tent camping from the travel trailer. This battery is charged with a 120w portable panel and a 40a Renogy DC to DC alternator powered charger while the trucks engine is running

In the travel trailer I have two 110Ah AGM batteries charged by 420w solar on the roof.

Is there a way I can safely, without damaging the batteries, connect the trucks aux battery to my trailers system to increase 12v electricity storage. The trucks battery is not the same age, make and model as the two batteries in the trailer. 

I've searched this and am not finding any device that would accomplish this.

Thanks

You should be able to connect the truck agm to the camper agm batteries via 35 mm cables and a proper sized Anderson connector or the two pole Phillips connectors used on commercial semi trailers to charge lift gate batteries. The dc-dc charger will then charge the camper batteries while towing. 

The thing to avoid is connecting discharged batteries to a charged battery frequently it’s like boosting a starter battery all the time but it looks like you have good maintenance charging capacity on both truck and trailer so one or the other batteries aren’t flat all the time...

Thank You!....I'm going to use anderson connectors as I'm very familiar with them.

Being able to charge both trailer batteries and the aux battery from the trucks 220A alternator would be a huge win!

rb, as you already know its best when connecting multiple batteries (to achieve better load and charge balance) they be the same type and design and even age but sure it can still 'work" even if not. That being said, especially if there are long cable runs joining two batteries, the cables should be as large as possible and likewise any connections be quality and low resistance....If one battery is near the alternator but another farther away you can realize (line voltage drop and resistance) how the one may not get the same charge quality and completeness as the other.

I'm sure you're also aware they make both solid state as well as mechanical isolation relays intended for dual battery systems which allow the main battery to be charged as well as an auxiliary battery. Typically an automotive alternator and its charge control regulation is not as well defined and precise as say an RV type of Smart 3 or 4 stage charger and especially a smart charger which has provision for FLA or AGM or Lithium     

HOWEVER sure it will work its done often and the world goes on lol.. Quality cables and connections and similar batteries can just make it work better WELL DUH 

John T

Thanks John,

The dual battery systems of which I think you are referring, are a simple high current, continuous duty relay that is activated only when the truck is running and simply switches between the trucks battery and the aux battery. Or a simple set of large diodes in the more vintage devices.

Many modern vehicles ( since about 2006), might have a “ smart” alternator and these relay type systems or diode based systems will not work properly.

My 2018 Ram has a VERY smart alternator and therefore I’m forced to use a much more sophisticated Dc to Dc Charger that I sourced through Renogy. My understanding is that these DC to DC devices do incorporate some type of advanced charge profile. The reviews from the road have been extremely positive. We shall see when I head Southwest in a January.

Thanks for the update rb, Yeppers I was mostly talking about the basic concepts regarding parallel connection of two batteries (with or without any fancy multi battery isolators) and the necessity of large cables and quality low resistance connections. Indeed any "smart" technology in your alternator or DC to DC Converter makes for a better quality charge as well as charge balance...…..

You're well on top of this, good luck

I rely on my Smart 3 stage solar charge controller to charge my three AGM house batteries but do have an "emergency" jump cable relay in place so if needed my engine battery and its (non smart) alternator can provide at least some degree of charging (albeit not perfect or balanced or politically correct lol) back to my house battery bank. I can also use it if my engine battery happened to run down which has happened and I was glad I had the emergency jump feature built in yayyyyyyyyyy

John T   Boondocked and live from Padre Island Texas but headed to south central Florida for most of the winter

Auto alternators are size large now so they will produce sufficient amps at low engine rpm. My F350 (220 amp alt I think) gasohol camper truck runs 1500rpm ish at 65mph in road gear bob tail. I haul/tow in 5 and 4 ratios so it is 2000 or 2500rpm so there should be amps available.

A diesel will cruise at lower rpms / lower amps. I am making this up because I actually don’t know what alternator output would be on your modern truck at lower rpm. 

Towing, I'm usually at 2200-2500 RPM.....Not towing it is around 1800 RPM

I only need this extra battery charge when towing.