Tow vehicle charging RV batteries

a 2nd alternator works well--but best bang for the buck is a dc to DC device. If you are doing flooded--set it to 14.8 volts.

Old wizard 13.2 to 13.4 is measured at the trailer batteries truck idling. After a 6 hour run home stop for lunch in camper Refrigerator on gas lights on for lunch a few other parasitic loads. I back in barn use battery disconnect 24 hours later I can check battery voltage and it will be 13.1 volts check it many times . The AGM do carry a higher voltage though.
What I’m getting at is my truck has no trouble keeping my batteries up traveling even after a night of Wally World. Where you run into trouble is when you try to charge a big battery bank. I have 2 AGM capable of supply 80 amps to 50 % state of discharge

valhalla360 wrote:

theoldwizard1 wrote:

Seon wrote:
As a backup I carry a 12v charger powered by a 2K generator.

The 12VDC output on most of today's generators does a poor job of recharging a lead acid battery. You are better off plugging in a AC battery charger.

I presume he meant a generator putting out 120v AC power a dedicated charger outputting 12v.

Yes, I plug a 12v a/c charger/maintainer into the 2k generator putting out 120v ac to charge the batteries.

BarabooBob wrote:
I added a 6 gauge wire from the alternator to the TT battery. I did install a circuit breaker on each end to avoid problems. My voltage meter in the TT definitely shows and improvement over the charge line on the 7 pin TT plug.

Try this test. Run your TT battery down to about 75% SOC (12.4V with no load). Start your tow vehicle and immediately check the battery voltage using a meter. As quickly as possible, check the voltage at TT battery. Drive for about 10-20 minutes and repeat the two measurement with the engine running. Check also at high idle.

You will be surprised how low the voltage at the TT battery is !

JCK wrote:
I’ve seen as high as 18 amps going to trailer batteries on start up of truck. And hour later or so still seeing 6 amps. Even if voltage is 13.2 to 13.4 amps is what gets the job done. Sure it won’t fully charge but on a 4 to 6 hour run I’m usually close to full charge.

You will NEVER get a "full" charge on any type of lead acid battery if the voltage is only 13.2V. "Close" is very vague. 80% ? 90% ?

I added a 6 gauge wire from the alternator to the TT battery. I did install a circuit breaker on each end to avoid problems. My voltage meter in the TT definitely shows and improvement over the charge line on the 7 pin TT plug.

When I owned my Class C, I had to replace my battery isolator and I ran 6 gauge wire from the isolator to the battery, about 8 feet. I improved my house battery charging immensely.

I added a 6 gauge wire from the alternator to the TT battery. I did install a circuit breaker on each end to avoid problems. My voltage meter in the TT definitely shows and improvement over the charge line on the 7 pin TT plug.

When I owned my Class C, I had to replace my battery isolator and I ran 6 gauge wire from the isolator to the battery, about 8 feet. I improved my house battery charging immensely.

jdc1 wrote:
Dual alternator. Best you could ever do. Some super duties come equipped that way.

time2roll wrote:
Single alternator has plenty of amps. The trick is getting the proper voltage on the battery that needs charging. A DC-DC charger will work excellent for this purpose. Still takes 3 to 5 hours to get a flooded battery from low on charge to 95%+.

I agree with t2r ! The time it takes to recharge depends on the size of the battery bank and how far it is depleted.

One 12V "dual purpose" battery will recharge much faster than four 6V golf cart batteries if both are at 50% state of charge.

I’ve seen as high as 18 amps going to trailer batteries on start up of truck. And hour later or so still seeing 6 amps. Even if voltage is 13.2 to 13.4 amps is what gets the job done. Sure it won’t fully charge but on a 4 to 6 hour run I’m usually close to full charge.
I’m using AGM batteries with low charging resistance. The charge line on my truck carries a 40 amp fuse so manufacturer has to size charge wire to carry load.

theoldwizard1 wrote:

Seon wrote:
As a backup I carry a 12v charger powered by a 2K generator.

The 12VDC output on most of today's generators does a poor job of recharging a lead acid battery. You are better off plugging in a AC battery charger.

I presume he meant a generator putting out 120v AC power a dedicated charger outputting 12v.

Charging with the truck has a few major limitations:
- Wiring is too small and voltage drop limits both the voltage and amperage that gets to the battery. You can run what are effectively battery cables back to the batteries but 30-40ft of good quality battery cable ain't cheap.
- Multiple connections are prone to corrosion or just general poor connection, so they often don't work well.
- Standard alternators are designed to put out their rated amperage for a few minutes to replace the short but high amp draw from starting the motor. After that they tend to drop way back. You can buy specialty alternators that put out more amps but again, not cheap.

A couple hundred watts of solar is a much cheaper solution and if you are stationary, it still feeds the batteries.