TV Charging Options

grassgd

I am currently in the process of selecting a new AGM battery between 100 and 130 AH’s to replace the original Interstate battery.  I have never felt very confident with the WFCO’s capability to recharge the battery, therefore I just installed a 120 volt Victron Blue Smart Charger rated for 25 amps which is used to charge the trailer battery instead of the WFCO unit. The Victron can be set to limit the max amps to 10 if needed or desired based on battery AmpHours rating and battery manufactures recommendation - yet to be determined. I installed a 120 Volt relay that disconnects the battery from the trailer upon sensing 120 volt power, thus the Victron charger only sees the battery when charging and the WFCO then only supplies 12 volt for the trailer while connected to 120 volt power and does not provide any charging to the battery at all.  I hope this will provide a charge profile to ensure the longevity and 100% charge of the battery when on 120 volt power.  I can now monitor the battery status on the Victron BMV-712 Smart Battery Monitor also recently installed.

The question is when towing. I currently rely on my 2022 Toyota 4Runner to provide recharging via the 7 pin trailer connector/wiring.  I understand the 4Runner’s trailer wiring circuit is provided with a 30 amp fuse which should be sufficient for recharging the battery during towing.  Since I do off gird camping quite often and after a night of camping hit the road with the battery at some state of discharge, I am concerned with the charge profile and how well the battery will be recharged by the 4Runner. 30 amps would be more than adequate, but I don’t know how the 4Runner’s alternator will interact with the trailer electrical needs during towing which consist of battery recharging and running the 2-way 120vac/12 vdc compressor refrigerator also recently installed.  Next time I pull the trailer I can monitor the performance of the 4Runner’s charging with the BMV-712, but in the meantime, considering options to address the issue if needed.

Option 1 would be to install a Victron DC-DC charger.  More $$$$$ to spend and I understand these chargers run very warm/hot and ventilation may be needed.  This option seems like the one most people would suggest and should work fine if the 4Runner can really supply up to 30 amps but if the tow vehicle power is limited how can a DC-DC converter make up power – it can’t, it can only change the voltage to meet the charger profile required and the current should be limited to the power coming from the alternator.

Option 2 use a 700 watt inverter I currently  have (no new $$$$) powered by the 4Runner/7 pin connector with the 120 volt output powering the trailer during towing only  (ie Victron charger likely set at 10 amps max and 120vac/12vdc refrigerator load).  This should be well under the 30amp 4Runner ability and output of the 700 watt inverter. This would require manually plugging the Victron battery charger into the inverter when towing or installing a transfer switch.

Looking for any comments or suggestions for those with knowledge of these matters.

Grumpy_G

My semi-educated guess is that the 4 runner can not supply 30A at any voltage that is useable for charging. The fuse is there to protect against the worst case scenario of a dead short to ground. As has been harped on here many times, the wiring in the vehicle causes a voltage drop that is proportional to the current. I look at this as a self-limiting design: As the trailer battery is trying to pull higher charge current the voltage drops, which in turn reduces the charging voltage and thus the current. Under normal circumstances the fuse should never blow regardless of how discharged the trailer battery is. 

DC-DC converters are somewhat similar to the Hughes autotransformers that many RV park dwellers use. They increase the current draw on the source to buck the voltage up to a higher level. Both make the load demands on the source worse causing even more voltage drop, which is why quite a few RV parks don't allow autotransformers (not that they would be able to tell if the device is installed inside). DC-DC converters basically do the same to the 12V feed from the tow vehicle. 

With a lead acid/AGM battery I would not worry about boosting charging voltage. Incremental gains for exponential cost. 

grassgd

Grumpy_G, thanks for your reply.  You confirm my suspicions about DC-DC converters.  If the power is limited from the tow vehicle, the DC-DC converter/charger is not much if any better than the standard tow vehicle connected in charging a low battery while towing.

Even my other options of using an inverter would fail if the power from the tow vehicle is limited, especially with the loss in the inverter.

When moving from one off grid campsite to another off-grid camp site, that leaves upgrading the tow vehicle alternator and wiring which would be costly, to using a solar panel while towing. I do have a solar panel but it's not mounted on the trailer to enable setting it up to get the best sun exposure when camping.

One off the wall option would be a wind type generator specifically for near highway speeds that could put out enough power. That would be ideal since it would not rely on sun direction or availability.  I doubt anyone makes such a wind generator.

I still don't have the new battery yet.  The temporary battery is an old Optima blue top which is on it's last leg.  When I tow the trailer, I can at least verify how much power the 4Runner puts out with the Victron BMV-712 and the Optima battery.  

AnOldUR

When my son was young, he had a project for school where his idea was to put a fan on a sailboat to push it through the water. Your wind generator idea is similar to this @grassgd. I doubt that the power generated would be worth the loss of gas mileage.

Putting a flexible panel on your trailer might be a better solution. Along with our 100W rooftop solar, I recently added 100W of panels to the top of our Wrangler. Those, along with a 100W suitcase have kept our batteries topped off. We do carry a little 700W propane generator for emergency charging, but have only used it a few times (more just to play than out of necessity).