How much battery charging from alternator?

[ewarnerusa]

Having spent a few years here I often read about the concept that while a typical trailer wiring connection from the vehicle to the camper will supply 12V, it is of low amperage and as such the camper batteries are just being trickle charged. The reasoning being very long wire length of inadequate diameter between the alternator and trailer wiring harness. However, this wiring can be upgraded which results in better charging capacity.

I've recently begun reading a solar panel forum and sticking to the RV section. I've noticed that every time a discussion comes up, one of the frequent posters immediately chimes in to first install a battery isolator to allow for alternator charging and that the alternator will provide more power in an hour than solar panels can in days. No mention of wiring upgrades, just this battery isolator concept. This poster clearly knows his solar but I don't get the impression of actual RV knowledge beyond carry over solar concepts.

I recognize that alternators are rated to deliver a healthy supply of amps, but this concept from the solar panel forum doesn't mesh well with what seems to be the common thought here on rv.net. So can anyone enlighten me on how many amps one can expect for battery charging from an OEM trailer wiring arrangement and from an upgraded wiring arrangement, and what this battery isolator may do? Thank you.

[pianotuna]

beemerphile1 wrote:

pianotuna wrote:
The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.

I think a better choice of words would be 'prior' to any disconnect.

Good point bemmerphile1. English can be so slippery.

I think "around" any disconnect is even better.

[beemerphile1]

pianotuna wrote:
The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.

I think a better choice of words would be 'prior' to any disconnect.

[ewarnerusa]

Thanks for confirming what I thought to be the case.

[westend]

The trailer brakes, while underway, use the hauling vehicle's battery through the brake controller. The emergency brake-away power through the break-away switch is delivered by a battery on the trailer.

[pianotuna]

The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.

[ewarnerusa]

OP revisit.
So I have backed off from regularly visiting that RV Solar forum because I really can't believe the advice I read on it based on my own experience and what I've learned on these forums. The latest eye roll was a recommendation of 500 watts of solar for someone's 2x6V GC batteries and a follow up post of needing more like 900 watts due to the poster's latitude in NY.
But one topic that I contributed to but am now uncertain of is the battery power supply for trailer brakes when break away is engaged. I said the camper battery supplies this power and is one of the reasons that a battery is required in an RV trailer. A response was that trailer brake system needs to be completely isolated from rest of camper system. I know that the wiring for the braking system is not part of the RV power distribution panel but it does use the same battery, right? There isn't another small battery for this purpose hidden away somewhere? I know that cargo trailers need a small battery for this purpose, but I've been under the impression that in a camper it just uses the main 12V camper battery.

[time2roll]

pnichols wrote:
Why is an isolator needed? How about just a manual switch on the dash

isolator never forgets, chassis battery stays charged.

[PaulJ2]

My older truck had a factory installed (camper package) isolation relay and #10 charge wire back to the 7 pin trailer connector. I had istalled an analog amp gauge in the circuit. The most I ever saw with a nearly dead trailer battery was 15 amp.
My newer vehicle has no isolation relay, power there all the time. I think I like that better as I can rob power from either battery if one is dead, (truck or trailer). Also when camping with hook ups converter keeps trailer battery and truck battery topped up. If you don't want that simply unplug.

[pnichols]

Why is an isolator needed? How about just a manual switch on the dash (and heavy cabling between the towable and the alternator) that just directly connects the engine starting battery to the coach battery bank? You turn this switch ON whenever the engine is running and turn this switch OFF whenever the engine is not running.

My Class C motorhome came set up this way from the builder - except that the "switch" is a heavy duty solenoid that is held ON automatically whenever the engine is running. My wet cell engine starting battery is direct parallel connected to the two deep cycle 12V coach AGM batteries whenever the engine is running. This arrangement has worked fine for over 10 years using two 12 volt deep cycle AGM batteries wired in parallel in the coach. This means that whenever the motorhome's engine is running, I have three 12V batteries directed connected in parallel - one wet cell and two AGM.

This three battery combination does for short periods show the coach batteries as drawing 55-60 amps from the alternator going down the road after camping and running down the coach batteries.

[pianotuna]

Hi Mex,

The inverter route does work in a pinch. Honestly I'd rather beef up my solar but that means starting over because of one stupid mistake (mea culpa).

[westend]

Nice work and good info for us Ford guys. I'll have to follow you along on that front tool box/hitch work. I haven't had a front hitch for many years and really want one.

[NRALIFR]

I just finished a camper battery charging upgrade project on my truck. Not quite to the level described by MEX, but it is much improved over OEM.

I have a topic over in the Truck Camper forum where I posted details of what I did, and the materials used. I just finished it yesterday, and the initial indications look pretty good. I need to upgrade the wires in the TC umbilical cord next, but that will have to wait a few weeks.

Buying a New TC Hauler

:):)

[MEXICOWANDERER]

Inverter / converter? Yeah 50 amps versus possibly 150. Exactly WHICH plug and socket would a person select for 120 vac? How ya gonna gain access to the AC? Send 2000 watts pure sine by inverter? Hmmmmmm.....

[SoundGuy]

Golden_HVAC wrote:
A typical RV plug to the truck is going to have 7 wires. They are typically either #12 or #14, rated at 20 and 15 amps, with some wires being smaller, say #16 for back up lights, or turn signals, rated at 10 amps. The "Ground" wire or return path is only #12 wire.

Other sources would indicate 10 gauge for both ground and charge line as "typical". I know my own local RV dealer uses this convention when adding brake controller wiring and a charge line to any vehicle not yet so equipped.