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Old 12-08-2018, 09:05 AM   #1
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Generators - Poor Choice For Charging Batteries?

Why do so many people on this forum continue to think that a generator is the best way for charging batteries when off-grid? (A generator does have value if you need it to power your air conditioner, for example.)

I believe it is more effective to use the charging output from your tow vehicle by using "jumper cables” or a DC-DC charger, hooked directly to the trailer batteries than a generator when boondocking, using solar on a cloudy day or in the shade, and your batteries are becoming depleted.

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.

It is important to understand that these charging conditions are present whether on shore power or using a generator if you are in “Use” mode, which of course on pre-2018 model airstreams was the only way the converter/charger connects to the batteries.

To paraphrase the Parallax Power FAQ the main issue with the Airstream Converter/Charger lies more with the application of the technology than the technology itself.

"In a typical RV 12-volt electrical system, the converter/charger and the battery or battery bank is connected in parallel with the rest of the 12-volt distribution system. In 'parallel' means where the charging source (converter/charger) positive and negative output is electrically connected to both the battery bank positive and negative, and the 12-volt distribution system positive and negative, at the same time.

The voltage output of the converter, even with a 'smart charger', is based on a 'detected' battery voltage. (The converter with a 'smart charger' feature claims to 'monitor' and respond to battery condition while connected to the entire 12-volt system.) Unless the charging source (converter) has a separate or isolated charging output, a converter or charger can’t effectively differentiate requirements of the batteries from other 12 volt loads while simultaneously connected to the rest of the 12-volt distribution system in the RV.

Detected use or voltage change on the RV 12-volt load system (i.e. lights, pumps, etc. being turned on) generally keeps these devices at a 13.6 -13.8 'float' voltage.

This raises the question whether any real additional benefit (i.e. full charging) is being provided to the battery bank by using the converter/charger with a generator."
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Old 12-08-2018, 09:34 AM   #2
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Charging Ratios

If I understand your question properly, all the methods can boost a charge pending situation. What the battery wants to see repetitively maximizing its life is another matter.

However most gens have unregulated “brute force” direct 12v +- charging capacity. This is not recommended as the only charge method by many as it cannot deliver a calculated smart charge to a battery/bank especially when wet, agm and lithium all prefer different charging voltages and patterns.. But ok in emergency.

Depending Converter model and battery technology combo can greatly vary the charge current able to be safely delivered to the battery and for longer life.

For example the PD4655 converter has the ability to deliver 55 amps total. If this converter is charging traditional wet cells,
the recommended safe/long term life current will be “far less”than say a superior rated AGM battery (or lithium) battery technology which can safely charge at far greater current levels and optimum required voltages per battery.

This means 3-5x safer, faster charging from gen using smart charger or converter. Far Less fuel or solar charge times too. Again, considering the longest capable life of battery. (Sample chart for example)

Randy at Best Converters can simplify this.
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Old 12-08-2018, 09:43 AM   #3
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Well, on my truck, I am not going to get 140 amps at idle. There is also the question of: do I want to high idle my 5 liter diesel prime mover for X hours a year or should I plug in my $900 Honda and let it do what it is designed to do?
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Old 12-08-2018, 10:33 AM   #4
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I don't know whether the "smart" converter/chargers are designed this way or not, but I suspect they could slightly increase the the charging voltage and measure the current increase taken by the combined load. A discharged battery will (I think) take more current for a given voltage increase than a resistive load like a light bulb or an inductive load like the furnace motor. The charger could then adjust its output voltage or current as required for the estimated state of charge of the battery.

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Old 12-08-2018, 10:48 AM   #5
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Generators - Poor Choice For Charging Batteries?

Yes, but there are 12vdc appliances and lamps in that circuit so varying the voltage is not ideal.
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Old 12-08-2018, 07:05 PM   #6
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Maybe people in general can overthink things at times.

My TV will charge my trailer batteries when I’m towing, and can be used in a pinch if my batteries become discharged, but if I need a charge and I have my generator with me, I’m going to use it because it’s easy to use, and it wont use much gas while topping off my cells.

1.6 gallons might last me twelve hours when not running my air conditioner, a vehicle at idle will use somewhere in the neighborhood of a gallon an hour.
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Old 12-08-2018, 07:15 PM   #7
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My Yamaha 2000i kept me charged for 14 days while boondocking in Nevada a couple years ago, mean while I had the use of my truck to do other things. To each their own, but I like the generator as a backup for off the grid power.
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Old 12-08-2018, 07:50 PM   #8
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"Why do so many people on this forum continue to think that a generator is the best way for charging batteries when off-grid?"

I still think that. All I have to do is start the generator and plug in the trailer. Why would I want to fool around with all the jumping and cabling required to hook it to the truck? Would my alternator actually find the trailer batter and charge it with two full Optimas hooked to it under the hood? Why would I want to idle a 300 hp diesel engine to charge 2 batteries? My truck does not charge my trailer battery when I am driving. I can drive all day and the trailer battery voltage does not go up. Why would it charge when it is sitting still and idling?

So here is why I still think that: $900 bucks for a generator. Have been using it 10 years so far. About 1/2 gallon a fuel an hour. Fairly quiet. Not much exhaust output. Backup power when 110 is needed. Can leave it run when gone in the truck. No hookup. Not much danger of a short or overcharge. I do not like to idle the truck.
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Old 12-08-2018, 08:28 PM   #9
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Quote:
Originally Posted by Adventure.AS View Post
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?
First, look up the alternator output at engine idle rpm. It won’t be 140 amps. Second, 140 amps may be at 6000 rpm for the alternator, so check the pulley ratios and see what engine rpm that relates to.

Then calculate watts, a measure of power. 140 amps when it is at 12 volts isn’t directly comparable to 30 amps at 120 volts.

Finally, it is not a good idea to idle a vehicle engine for extended periods at light loads as a matter of course. It is fine in an emergency. You will generally pay in increased maintenance and repair costs.
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Old 12-08-2018, 10:19 PM   #10
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There's two ways to use the generator to charge the batteries.
1. Hook the AC power cord to it and allow the converter to operate as if you were plugged into shore power.
2. Connect the 12 v. Charge Port on the generator directly to the batteries. (I've never tried this.)
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Old 12-09-2018, 10:57 AM   #11
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Both of our Airstreams have Magnum chargers/inverters. The AC power inlet(s) go directly to the Magnum. The solar panels provide power to the solar charge controller and then to the Magnum. AC shore power passes thru the Magnum relay to the AC main fuse of the trailer.

The Magnum takes care of keeping the lithium batteries topped up and, when off grid, powering every 120Vac outlet or appliance in the trailer.
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Old 12-09-2018, 11:12 AM   #12
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We use a 2000 Honda plugged in to our as to charge the batteries when in the boondocks...
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Old 12-09-2018, 12:11 PM   #13
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That generator puts out 8 amps at 12-volts DC and 13 amps 120-volts AC (at full tilt.). No where near what your TV alternator can put out at fast idle. The 25 amp DC-DC charger, linked above puts out 25 amps 12-volts and cost less than $400.
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Old 12-09-2018, 12:33 PM   #14
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Hi

When people talk about charging batteries with a generator, they mean plugging the trailer into the generator and using the internal converter / charger to top up the batteries. I don't know of anybody who uses the "charge" output on one of these generators.

As noted in multiple comments above, your truck alternator really isn't going to put out 120A or 140A at a normal idle speed. If you get 40A at the end of those cables, you are doing pretty well (yes, I've used jumper cables a lot of times over the years ...).

If you put a voltmeter on the far end of those jumper cables (running at normal idle), the voltage you are putting on the batteries really isn't as high as you would like. Getting to 100% charge will be problematic as a result. The truck charging system simply isn't set up to do this. That's not a knock on the truck, you simply are asking it to do something it never was intended to do.

If the batteries really *need* charging, you are in for a 4+ hour session. Even with a generator, the last few hours burn a lot of gas for few(er) amps into the battery. Comparatively, those last hours are fantastically efficient compared to a truck engine (at any point in the process) run to do the charging. That's also ignoring any wear and tear issues related to long idle periods on a gas or diesel truck engine. I can throw away a typical generator and buy a new one for way less than an engine rebuild will cost on my truck.

Next up - if the alternator *did* put out 140A into the batteries ( = you fast rev the engine for 4 hours) ... is this a good thing? According to most battery data sheets the answer is no. They don't want you to hit the typical RV battery with that level of charge current. The "stock" chargers are rated at what the battery guys think is a good idea. You can indeed get *much* larger chargers for not much more money if the batteries liked that sort of charge.

A converter / charger's voltage profile is far better adapted to charging RV batteries than the alternator on a truck. The whole "multistage" thing is indeed the issue here. The truck system simply is not set up the same way. That may not make a big difference for topping things up (or not) rolling down the road. For full cycle charging ... it matters.

Truck alternators do not last forever. Run them at full output for long periods of time ... hmmm.... good luck. Even an alternator rebuild costs more than the typical brand new converter / charger. Indeed, blowing alternator diodes jumping dead cars *is* a common thing. ( = been there / done that and have lots of friends who have as well ....).

So lots of reasons ....

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