Tow vehicle charging RV batteries

StirCrazy wrote:

theoldwizard1 wrote:

Sometime around 2000, the entire auto industry switched to "smart charging". Smart charging is when the PCM controls the output voltage to the alternator to limit the voltage to "just enough" to replenish the battery after a start and not left the voltage go so low as to discharge the starting battery.

This means that about 5-10 minutes after starting you vehicle the output of the alternator is about 13.2V-13.4V and there is not much you can do about that !

USING HUGE WIRING FROM THE VEHICLE BATTERY TO THE RV BATTERY DOES NOT SOLVE THIS PROBLEM !

The only solution is a DC-DC battery charger mounted close to the RV battery. Most can accept voltages as low as 10V-11V and boost them up to the proper voltage (>14V) to charge a battery.

if your rv batteries are down this should put a load on the system that the PCM will see and it should increase the output accordingly which would mean the larger size wire should work.

Steve

Hi Steve,

No. Even with wire intended to carry 80 amps, I do not get "more charging" from the alternator--unless I put a 150 amp load on the house batteries. As soon as I remove the load charging goes back to being minimal.

01tundra wrote:
I ran a dedicated 4AWG charge circuit from one of our truck batteries back to a 150A Anderson plug mounted on the hitch. Used a Blue Sea 50A battery post fuse on the truck.

Installed a 4AWG pigtail in the trailer that powers a Victron 12/12-18 DC-DC charger.

We now get 25A charge current to our trailer batteries.

This is great, thanks for the images.

theoldwizard1 wrote:

Sometime around 2000, the entire auto industry switched to "smart charging". Smart charging is when the PCM controls the output voltage to the alternator to limit the voltage to "just enough" to replenish the battery after a start and not left the voltage go so low as to discharge the starting battery.

This means that about 5-10 minutes after starting you vehicle the output of the alternator is about 13.2V-13.4V and there is not much you can do about that !

USING HUGE WIRING FROM THE VEHICLE BATTERY TO THE RV BATTERY DOES NOT SOLVE THIS PROBLEM !

The only solution is a DC-DC battery charger mounted close to the RV battery. Most can accept voltages as low as 10V-11V and boost them up to the proper voltage (>14V) to charge a battery.

if your rv batteries are down this should put a load on the system that the PCM will see and it should increase the output accordingly which would mean the larger size wire should work. where your describing would be when the rv batteries are charged and that wouldnt matter much at that point.

it does still leave the questions on weather the voltage will be the ideal voltage for your batteries where a good dc to dc will let you chose charging profiles, instead of just being in bulk the whole time

Steve

I ran a dedicated 4AWG charge circuit from one of our truck batteries back to a 150A Anderson plug mounted on the hitch. Used a Blue Sea 50A battery post fuse on the truck.

Installed a 4AWG pigtail in the trailer that powers a Victron 12/12-18 DC-DC charger.

We now get 25A charge current to our trailer batteries.

Mine are automatic circuit breakers--intended to protect the alternator.

Open breaker has the same effect as an open fuse..... No charging.

Or add a circuit breaker before the dc to DC device.

Most oem charge circuits are fused at 40 amps so I would just get the 20 amp Renogy (or similar) so as not to open the fuse.
I assume the 20 amp rated output can require 30 amps input.

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DC-DC battery charger with correctly sized wire, breakers, and connector to the tow vehicle.

A 40 amp output Renogy charger will draw up to 60 amps.

This equipment works really well in overcast, rain, snow, and at night.