Generators - Poor Choice For Charging Batteries?

[Mollysdad]

If a tow vehicle has a 140 amp alternator, it doesn't mean that juice is available to charge the trailer batteries.
After all, it has to run the truck engine and all the electronics therein.
Otherwise no manufacturer would be putting a big alternator in a truck!

I just can't justify running the truck for hours just to charge the batteries.
Option three:
Generator to an outboard battery charger, such as the NoCo Genius. Then, everything is running in it's designed state.

[Airhead59]

I installed 340W of solar this spring with a Victron MPPT charge controller and battery monitor. Self-installed, thanks to a well thought-out package system, logical instructions, and technical support from AM Solar. I don't understand why everyone doesn't have solar on their trailer.

My two panels are installed flat, so not movable like a "suitcase" solar panel. However, they have my batteries (2x 90 AHr Interstate flooded lead acid) fully charged by noon each day. In cooler weather/locations (including Canada and Upstate NY) my furnace, 12V lights, music, etc., can run my batteries down to 50%-60% overnight (12.1V). The solar system alone will have my batteries back to 100% by 9:00 AM, or if overcast or camping in a forest, by 11:00 AM. Barring an all-day downpour, the solar panels and MPPT charge controller gather enough energy to meet my modest needs (no inverter, no CPAP, no hairdryer, no television, no AC). When it's really hot, I seek out a state campground with 30/50 amp service to run my AC (like this past June when I was visiting Carlsbad, NM where it was 109F!).

My storage location at home in NM is on the north side of a warehouse. Since mid-October my AS has been in the shade 24/7, but the solar system still keeps the batteries at 13.6V float charge state, almost continuously. I have no master power switch, so there are parasitic demands, and temps have been in the teens or 20s overnight, and 40s during the day. This will be the first winter I've left the batteries in the AS.

If I were full-timing or driving the Alaska Highway, I would want a generator. But for my modest power demands, the solar more than meets my needs for 3-season boondocking in the lower 48.

[ROBERT CROSS]

After 30+ years of using our Airstreams plugged into a portable generator to charge our batteries, I'm grateful to hear from the OP that it's the rong way to do it.👍
Now to get some 2ga and I'll be all set to go Stream'n. 🥴

Bob
🇺🇸

[uncle_bob]

Originally Posted by Adventure.AS

I use 2 gauge cables.

Hi

Ok, so assuming the jumper cables are 10 feet long, that's a 20 foot round trip. If they are copper (most jumper cables are not) then they come out at 0.15 miliohms per foot. Twenty feet gets you to 0.003 ohms. At 140A you have a 0.42 V drop just in the cables. It's a pretty good bet that the drop in each of the clamp sets is at least that large at this sort of current.

The truck alternator is trying to stay at a voltage that keeps the truck battery "happy". Taking the battery over voltage ... it's not going to do that. If the trailer battery. If your trailer battery is at 11V then yes, you will get current into it. If you let it go that far down, you probably have damaged it. If it's up around 12.2V or so and the truck is at 12.8V, you simply can't get 140A into the trailer battery.

Bob

[mkcurtiss]

It is always nice to have a totally separate source of power, to power your as, and if needed, to charge your batteries on your tv as well.....using your tv as a power source is not advisable.....not only is it noisy, but you do not want to be breathing in the exhaust fumes long term, burning the fuel, placing wear and tear on your tv......generators are cheap, compared to 50,000.00 trucks.

[jcl]

The OP’s question was why people didn’t just use their tow vehicle for charging. With all of the information and detail posted, it would be interesting to know if they are going to change their practices.

[68 Overlander]

With regard to boondocking, this has been debated sooooo many times (both here and other forums including the marine guys) and the moderately priced generator powering an on-board 4 stage converter or inverter/charger wins every time. Solar helps of course.
It doesn't matter how large the wire is from the alternator, you will never realize 140 amps from an alternator to the average pair of house batteries. First, the cable won't carry 140 amps and the resistance of the batteries wouldn't let it in if it did.
Most with a pair of Interstates for example, you will rarely see the max output rating of the stock converter (especially the Parallax and WFCO) and if you did, it would be brief before it tapers as the resistance builds. It's true AGMs can take a little more up to about 20%.
Having the ability to boost the charge, say up to 14.4-14.7 volts (force) does help overcome some resistance but only for awhile and then it too is not going to deliver the juice constantly. That is why they drop the voltage back down, to prevent excessive gassing and over charge.
That said, there is nothing wrong with charging with an alternator while driving. They do pretty good since they are regulated but after a few 8 hour day drives at 14 volts, I would have a look at the water level if using flooded batteries.

[switz]

We explicitly do not charge the lithium battery from the tow vehicle. The output from the tow vehicle to the battery is not varied to the profile required for our lithium battery. However, our solar panels are charging through the solar charge controller into the Magnum inverter/charger that is programmed for the lithium batteries.

We converted both Airstream trailers several years ago to a Vitrifrogo refrigerator with a 12Vdc Dan Foss freon compressor that draws about 56 watts when running. We have yet to have a problem. The 23D has a 300 amp hour lithium and about 90% of those amp hours can be used without damage. The Classic has a 600 amp hour lithium battery with the same characteriatic. It can actually power one A/C unit during a stop for lunch.

So the combination of solar and generator(s) can keep the house battery topped up for the hours of darkness when off grid.

Lead acid batteries can not use more than 50% of their amp-hours without damage. So the user must ascertain the power loads of "every" device to determine the proper amount of storage battery capacity that is usable in a given time frame.

[uncle_bob]

Hi

A stock setup AS with a 7 pin cable hooked up normally *does* charge from the TV while you are rumbling down the road. As many have observed over the years, it does not do a really great job of it. Some of that is indeed from the wimpy wire involved. Unfortunately that's not the whole story ....

Switching to lithiums (since we seem to have drifted in that direction), you very much *can* put 140A into a pair of lithiums and do it for quite a while. They actually rate them to work in this fashion. Getting up to the 90% level at very high currents can be done. What's still true is that they will last longer if you don't go this crazy. Is longer 150 years vs "only" 70 years? .... who knows

Bob

[tjdonahoe]

Why would you want to run an engine that cost. $ 8-20,000 , when a 800$ generator will do the same job? A no brainer ....unless it is an emergency

[rastapoodle]

Our Yahama 1000 propane generator is much more quiet than our big-ass-diesel-Chevy. It’s also much more economical. It’s light, easy to start, fits in a generic tub in the back of the truck. It does not put soot in our DPF on the truck that has to be regened to clean. Don’t need it often.

[Russ H]

I charge my batteries with solar on sunny days, seems like an easy choice. I also get a boost anytime I run my generator for the microwave or air conditioner.

[Bill M.]

I do not agree with the premise of this thread. But I am glad I read the thread. The DC to DC charger does sound interesting for when the truck is actually pulling the trailer. I do not get any meaningful charge added to the batteries even with an all day interstate pull. However I do not think we camp without power enough to really need to add this feature. Looks like $400 plus heavier wires in the truck to make it work. Glad to know how to do it though.

[r carl]

Originally Posted by Adventure.AS

I use 2 gauge cables.

That's welding cable territory.

[vintageracer]

Buy a new battery for $150.00!

[KJJorgen]

Several points. I have tried to use my Honda 2000 generator both with the 12 volt output hooked directly to the batteries, as well as running it through the converters. I thinks it works a bit better running through the converter, but the biggest advantage of course is that you can run 120 AC in your camper while the generator is running. I would disagree that it is more efficient to charge via jumper cables off a running tow vehicle than it is through a generator. Basically, it has to be more inefficient running the large engine than the small generator. My experience in trying using the jumper cables off a running engine has not worked well. Maybe at the time I did not have a heavy duty generator and sufficient wiring through the harness to the trailer.

When you mention comparing 30 amps from the hookup post to the 140 amps from the 12 volt alternator, the 30 amps from the post is 120 volts AC or 3600 watts, and the 140 amps from the alternator is 12 volt, which is 1680 watts, but I don't think you will ever get the full amount of either to your battery.

I do think what you said about about the "smart charger" sensing the voltage in the "system" is quite true, and if the battery is fully charged, the load from the battery gets quite low to the charging system and the amperage into the battery goes way down. It is not clear to me that the smart charging system is that much of an advantage, UNLESS you leave the unit plugged in for weeks at a time, or UNLESS the ambient temp is high, which would promote losing fluid from the batteries.

Just my thought--I am NOT an electrical engineer, so don't take my word for the absolute truth.

[Scoutx]

Originally Posted by Adventure.AS

Most vehicles used for towing with a ‘tow package’ have an alternator capable of putting out more than 140 amps vs 30 amps max from shore power or generator through the trailer converter/charger? (The DC-DC charger linked above can charge at 25 amps.) If you have a gasoline-powered TV it is much quieter than any portable generator although some models of diesel trucks are quite too, but most that I have heard are louder than the portable generators, but if boondocking this might not be an issue.

Couple if problems with this:
1) That 140 amp alternator is only going to produce that at high RPM. When at idle you might be lucky to get 20 amps out of it.
2) Even at high RPMs you are only going to get a fraction of that 140 once the battery has some charge on it and the resistances that will occur. More so if you're trying to charge through the trailer connector instead of heavy gauge jumper cables.
3) The risk of accidently shorting out the jumper cables as to constantly connect and disconnect the as well as the possible risk of setting setting off the hydrogen gas that can be produced by an unsealed wet cell.
4) You are also going to dump the charge from your TV battery into the trailer battery thus risking leaving you TV battery nearly flat and risk being unable to restart the TV in you have any issues.
5) you will burn far more fuel using your TV and combined with increased wear make it car more expensive to recharge with the TV.
6) Your alternator doesn't care what liads are hooked to it either.
7) 12V devices have a wide voltage tolerance typically at least 11-15v, as such whether the converter is supplying a trickle voltage of 13.8 or your TV is supplyt a high charge voltage at 14.4 your device isn't going to care.
8) Finally, if you should inadvertently forget which would you rather allow to run dry? The generator or your TV?

No there are plenty of reasons to use a generator to charge and no real good ones for using your TV unless that's your only option.

[vito98103]

Idling a diesel engine (any diesel engine) to charge batteries is a very bad (and very expensive) idea.

[John McHugh]

The initial post reflects a fundamental misunderstanding of electricity. 140A at 12V is 1680 watts. 30A at 120V is 3600 watts. The generator makes more than twice as much energy available for charging than the alternator. In brute energy terms, the generator is equivalent to a 300A alternator. As others have noted, many other factors other than brute energy are involved in determining a fast, effective, charge rate, i.e. one that does not reduce battery life. These include battery chemistry, the physical construction of the battery, etc. Good chargers can be built as either AC-DC or DC-DC devices. All chargers will have some conversion inefficiencies, but 80%+ is not uncommon these days. The wattage of the charging source sets an upper bound on the charge rate but, for example, the 45A AC/DC converter common as the factory default in many units will not charge at a rate higher than 45A or so. It will draw about 5.6A at 120V if it operates at 80% conversion efficiency. If you want a faster charge from this type of unit, a higher capacity charger is needed. Since the charging itself is not 100% efficient, some of the charging current will appear as heat and this may limit the practical maximum charge rate.

There is a huge amount of information on batteries and their care and feeding available. The material at https://batteryuniversity.com/learn/ is a good starting point.

[amirm]

Not that this applies to most people here with trailers but the Airstream Interstate van happily pulls 100+ amps from the high-current Bosch alternator of the sprinter diesel engine. There is a pretty decent cable (1/0 ?) between the chassis and house batteries to enable this.

In my lithium conversion, I did have to put in a special lithium charger to enable the same.

Here are the measurements of the Bosch alternator:



So even at idle you get 100+ amps.

In the interstate, the gas generator is also awfully loud. The engine in contrast, is dead quiet so we routinely use it to top up the batteries. Just a few minutes can restore what was lost overnight.

As to wear and tear, I bought the thing to use it, not preserve it.