DC-DC charger as an alternative to generator?

[Martinjmpr]

UPDATE 6/5/2025: Field tested!

We took a trip to Flaming Gorge in Utah last weekend. No electrical hookups at all and we were camped in a large group campground, essentially 'boondocking' but with reservations.

As with our trip to Key West last year, I once again forgot to bring my external 100w solar panel so all I was left with was the 100w panel on the roof.

To be fair, the rooftop panel actually did a great job of keeping the batteries topped off. Temperatures were in the high 70's to low 80's so the refrigerator wasn't too stressed, and we were in a bit of a shady spot. Of course, with the only solar panel mounted on the roof, shade was a double edged sword: Yes, it kept the trailer cooler, but it also limited the amount of energy the solar panel could provide.

Still, when it was getting direct sun light, the on-board display for the solar controller showed a respectable 8 - 9 A going into the battery, which was actually more than I had expected.

However, by day 4 the battery state of charge was starting to get close to 50%. Even though I only had 2 more days before we got home, I decided to see how well my charging system would work.

So I hooked up the inverter to the SB175 connector and then plugged the 120v AC charger into the inverter and plugged it into the Anderson connector on the trailer.



It may look cumbersome but it actually took me less than 5 minutes to connect everything. It's basically all "plug and play."

Here is the display at the start of charging at 12:52:



And here it is exactly an hour later at 13:52:



If you're wondering why it is only showing 9.39 and then 11.38A of current, it's because there are actually two batteries so each battery is getting 15A max from the 30A charger. And the remainder of the discrepancy is the current shown is the NET current, and there is a draw from the 12v fridge, which (according to this display) draws anywhere between 7 - 9 A when the compressor is running.

So I'm definitely happy with the results, it's about what I had expected (and wanted.) I can put 30AH into the two batteries in an hour of idling the engine and I don't have to mess with a separate generator.

 

[burleyman]

Congratulations! My similar setup on a van with a 130 amp dumb alternator has provided 50-60 amps into two 100ah lifepo4 batteries at idle with no problems.

That same setup worked fine with my camper and tow Silverado's smart alternator until I stepped up from a 30 amp charger to 50 amps. After inverter DC amps looking great for a short time, they began to creep higher. A quick look at battery voltage (analog factory meter) showed it had dropped to about 12v. Inverter input amps increased due to lower input DC voltage in order to maintain 120vac out..

The alternator was getting hot and was up to about 200F. How hot is too hot at idle? Too low voltage how low? Paranoia took over and I stopped charging. I'm too dumb to outsmart my smart alternator system. For now I'm sticking to the 30 amp charging.

Behind the dam at Flaming Gorge headed downriver in 1989.

 

[DaveInDenver]

The alternator was getting hot and was up to about 200F. How hot is too hot at idle?

An alternator will have a continuous rating and a peak or absolute maximum. I don't know yours specifically but typically medium, heavy duty, marine type Denso, Balmar and similar fall around 80°C to 90°C for continuous and might tolerate 105°C or so. For a light duty I would expect 93°C/200°F is going to test it.

What ultimately determines this is the type of insulation they used on the windings. Doing what you did, got hot and shut it down quickly, is not likely to immediately ruin it but holding there for an extended length of time probably would.

Too low voltage how low?

The alternator doesn't care, it's a voltage-controlled current source so practically the voltage could go to zero and keep producing current. There's too low for the load, a battery will stop charging for example. And there's a voltage too low to force current into a load.

It's also RPM-dependent, so at low RPM it'll be power limited and will have to regulate voltage at the expense of current. They aren't rated to produce full current from zero RPM so until you get it spinning at it's rated RPM it won't produce it's full current at rated voltage.

Also putting a load impedance on the alternator such that the voltage drops excessively will result in high current and high heat. The real question is "How much current (and thus heat) can you safely draw from an alternator?"

The amount that is lower than what results in melting insulation off the windings and cooking the grease from the bearings.

 

[TwinStick]

I just can't understand why the manufacturers don't make a solution for our trucks ?

So many people work from their truck and also use truck for camping/overland.

People don't want to spend the money for a new truck and find out that in order to make their truck work for them, they will void their warranty. Very aggrivating and frustrating.

They used to have packages available as options. HD charging system, dual batteries, Camper Special, HD cooling, HD suspension, etc..

 

[gator70]

Just read about problem at idle motor speeds. The DC - to DC charger should be off at idle. Using a "wait speed regulator". A Winnebago solution.

 

[rruff]

^Do you have any idea what that actually is?

 

[gator70]

About alternator output control and overheating - Is NOT solved by dc - to dc product vendors.

For me, a dc to dc product is safe when connected to a AGM aux battery and not the alternator. Using a isolator. This solves the alternator problem, but you get less amps during your drive. The isolator protects the alternator. It only costs $70.

The alternator senses AGM chemistry and not lithium

 

[gator70]

This guy said no to the dc-dc charger. He runs the generator while he drives for 300% charge output as compared to a dc -dc charger.