Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. Today I discuss solar power backup to charge batteries when boondocking.
Dear Mike,
I have a new Rockwood trailer with a lithium battery and lots of solar panels. This has worked great for several boondocking trips already. However, last weekend there was very little sunlight, so we spent the entire day watching the battery State of Charge going lower and lower. Is there any way I can recharge the RV battery quicker than the 7-way plug from my truck? It doesn’t seem to provide much of a charge. —Tony
Dear Tony,
You’re in luck. I’ve been studying this issue for a while now, and there are at least two different ways to recharge your RV battery when boondocking with little sun. Note that neither of these methods is designed to run an air conditioner full time. But here are the possibilities.
The challenge
Lithium batteries can charge rapidly, but they are current hungry. For example, a 100 amp-hr lithium battery can easily accept up to 80 amperes of charging current. And I have a Rockwood GeoPro trailer with the Power Package option. That’s a 400 amp-hr battery that can be charged with up to 240 amperes of current.
Now, that doesn’t mean you need a 240-ampere electrical outlet for your 400 amp-hr battery, or even an 80-amp outlet to charge a 100 amp-hr battery. The most you’ll need will be about 1/10th of that current on the 120-volt side. So, a 100 or 200 amp-hr battery will typically require a maximum of 8 to 10 amperes of current at 120 volts AC. But a 400 amp-hr battery needs special consideration.
What about bigger batteries?
Your own Rockwood trailer with the optional Power Package includes a 3,000-watt Hybrid Inverter/Charger which has a maximum shore power current setting. That allows you to set the 30-amp shore power current to any maximum value less than 30 amps. That is what allows you to recharge the battery with something as small as a 1,000-watt generator.
It’s literally as easy as setting maximum current draw from the shore power line to something your AC power source can provide. So, if you have a 1,000-watt AC source, then set the maximum current at 120 volts down to 8 amperes and you’re in business.
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Option One: CarGenerator
This is a perfect application for a 1,000-watt CarGenerator. And it’s pretty much plug n’ play.
No gasoline generator is required. You simply connect it to your tow vehicle battery with the jumper clamps, or directly bolt the wiring connectors to your battery terminals (a more secure option). Plug in your 30- or 50-amp RV shore power cord to the CarGenerator with the appropriate 15-amp dogbone adapter, and you’re in business.
How long does this take?
If you have a fully depleted 100 amp-hr lithium battery (that’s 1,200 watt-hrs of storage), the 1,000-watt CarGenerator should be able to completely recharge it in a little over an hour of engine idle time. Or it could take a little more than 2 hours of idle time to recharge a 200 amp-hr lithium battery.
Don’t worry about running your vehicle engine for a few hours at idle. Any modern vehicle is rated for hundreds of hours of continuous idle time without any spark plug fouling or gasoline washing down into the oil.
That idle issue was a common problem when I was a motorhead in the ’70s. But nowadays all gasoline engines in vehicles have electronic ignition and fuel injection, so they carefully meter in the minimum amount of fuel to each cylinder. That’s one of the ways that vehicle manufacturers have reduced emissions while steadily improving miles per gallon.
Option Two: DC to DC charger
As you’ve discovered, the 7-way trailer plug from your vehicle can only provide a small amount of charging current, perhaps 4 or 5 amperes at most.
That limited amount of current would take 20 to 30 hours to recharge a 100 amp-hr battery. If you wanted to recharge a 400 amp-hr battery, it could take a week of charging time. So the stock 7-way plug in your tow vehicle is just not practical for anything except keeping your RV batteries topped-off while driving.
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How do you install it?
Now, this takes more installation work, but a DC to DC charger can provide 40 to 60 amps of 12-volt charging current to your RV battery while driving.
I’ve experimented with a REDARC® DC to DC charger. It allows you to combine charging current from your vehicle alternator along with solar panels on your RV.
It does require that you run heavy wiring (as large as 4 gauge) from the battery of your tow vehicle all the way back to your trailer battery, including a heavy-duty, 100-amp, quick-connect Anderson connector to plug into the trailer. This type of charger will create the correct charging voltage for your lithium batteries as well as limit current draw from your vehicle alternator so you don’t damage it by overheating.
I can cover this more in a future article. It would be a great option for anyone who wanted to do extended boondocking without a generator. You may not need it all the time, but if the sun ain’t shining, depending on where you’re boondocking, it could be a real game change.
Please let me know if you would like me to do full RVelectricity articles and videos on both of these options. While portable generators are great for many of you, if you’re trying to camp using solar and battery power most of the time, a CarGenerator or DC to DC charger could be the perfect solution for boondocking.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Send your questions to me at my new RVelectricity forum here.
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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I would like to see more information on DC to DC charging. I have been thinking about this for a long time but have not been able to find good information on how to set it up. My RV is a 5th wheel and can only boondock for one night when traveling before I need a campground or generator to recharge the batteries.
Perhaps something like this 60-amp Renogy DC to DC charger would do the trick. Let me see what I can figure out. I’ll ask for a sample unit. https://www.renogy.com/12v-60a-dc-to-dc-battery-charger/
Is CarGenerator bad for your alternator? Lets dispel this “old wives tale” once and for all, and prove it to yourself using a DC clamp amp meter on your own vehicle.
Automakers spend millions of dollars to engineer alternator capacity so that with any modern vehicle you can start the engine and switch on all accessories and run it for 1 hour or 5 hours or 24 hours, and everything is powered with no harm.
CARGENERATOR is safe and simple, instead of using alternator power for accessories like fan and headlights for example we pull this power out and convert to 110 volts for camping or backup power needs.
PROVE IT to yourself, take a simple DC clamp amp meter on your alternator wire and measure power used idle with all accessories off, then switch on all items in your vehicle and calculate the difference. This amount is the power that you can draw all day long at idle, as engineered by your vehicle manufacturer.
See an example here. https://youtu.be/Js4K6N5rMCg
As an owner of a Redarc 50 amp DC/DC charger, a full article would be welcome! I have used mine for the last year and a bit with great success! I no longer carry or even need a generator, I just plug in to my truck at idle when boondocking and it charges just as fast as a 3000 w inverter generator. It also acts as a solar charge controller, AND it is build Australian solid, not Chinese crap!
3) When the engine idles, the fan inside the alternator also spins slowly, providing very little airflow through the alternator (relative to turning at normal engine speed) making overheating of alternators at low speed, virtually guaranteed.
4) Because the vehicle is not moving, air exchange in the engine compartment is relatively low and the interior temperature is high. The radiator fan is not designed to cool the radiator or engine without moving. The movement of the car actually produces most of the cooling airflow through the radiator/engine compartment.
2)Typically a passenger car alternator is ‘rated’ at around 100A. 100A at @13V (which is what a smart alternator puts out at idle) is 1300W AT 6000RPM. Since the vehicle is idling, the alternator is running about 2000RPM and will have a reduced capacity of around 60% (60A = 780W). If this 60A is produced, the engine will need 20A to operate (fuel injection, fuel pump, ignition system, etc.) leaving 40A to send to the ‘car generator’….Power in, minus conversion efficiency = power out. 40A x 13V = 520W. Inverters operate ~90% efficiency at half power so 520W input is (520x.9) 468W. Not the 1000W at which the inverter in the ‘car genertor’ is rated. Again, IF the alternator produces this much, it will almost certainly overheat. Alternators do not have protection from overheating and, if heavily loaded, just continue to heat till they fail.
I’m a retired mechanical engineer with a DIY adventure vehicle with 17.5kWh/24V of LMN Leaf batteries, 950W/24V solar, a 220A/24V second alternator….I have a lot of experinece with alternators and think that the so-called ‘Car Generator’ is more likely to be a problem for battery charging than a solution. Since I’m limited in comment length, Ill post issues separately.
1) Automotive alternators are not designed to generate a large amount of electricity, beyond operating the automobile in which they are installed. Trying to draw a lot of energy, at low engine RPM and especially when not moving, is virtually guaranteed to overheat the alternator. This device MAY work for a short time (10-15 minutes) but the potential to damage your expensive vehicle and leave you stranded is high.
Is CarGenerator bad for your alternator? Lets dispel this “old wives tale” once and for all, and prove it to yourself using a DC clamp amp meter on your own vehicle.
Automakers spend millions of dollars to engineer alternator capacity so that with any modern vehicle you can start the engine and switch on all accessories and run it for 1 hour or 5 hours or 24 hours, and everything is powered with no harm.
CARGENERATOR is safe and simple, instead of using alternator power for accessories like fan and headlights for example we pull this power out and convert to 110 volts for camping or backup power needs.
PROVE IT to yourself, take a simple DC clamp amp meter on your alternator wire and measure power used idle with all accessories off, then switch on all items in your vehicle and calculate the difference. This amount is the power that you can draw all day long at idle, as engineered by your vehicle manufacturer.
See an example here. https://youtu.be/Js4K6N5rMCg
I would appreciate more in-depth information about DC to DC chargers, please!
Thank goodness for Anderson Power Poles. Amateur Radio has been using them for years to help Hams connect to power without having to worry too much with reverse polarity. Installed properly it is impossible to mis-connect during an emergency and a diverse team/radio mix.
Watch the videos, get the proper tools, and get connected, power wise.
Somehow running your vehicle to charge your batteries is better than running a little generator? I’ll stick to my Honda 2000i. By the way on the diesel trucks they don’t recommend idling for hours with them check your manual.
The diesel issue is due to the particulate matter trap that must be burned off occasionally. Gasoline engines don’t have that trap and don’t care how long they idle.
And remember that a gasoline generator or lawn mower produces more emissions than a modern vehicle driving down the highway. Catalytic converters really do work.
Why would you run your vehicle to charge your batteries, instead of running a little generator? Lots of reasons…
It’s cleaner and quieter. just quietly idle your car engine instead of a noisy primitive gas lawnmower type engine with no emissions management
dragging a 50-100 pound extra motor and smelly gas cans around.
Uses about the same amount of fuel https://youtu.be/_g888eWJl5o
Lightweight and convenient. Just 11 pounds, instead of dragging around a 50-100 pound extra motor and storing dangerous gas cans and messy filling.
Reliable and Zero extra maintenance ever. No extra maintenance or small engine oil changes, and ultra reliable. your vehicle starts, voila – you have power….
Thanks Mike. Does the car generator require a dc to dc charger as well and/or extra fuse? Or is it set up to work ‘as is’? (with stock alternator, in my case 2016 GMC Canyon). Thanks!
It works as is. Just hook it up to your battery and start powering your RV.
Yes, there are cheaper ways to DIY this, but you better know what you’re doing…
Thanks!
You can also take a set of battery cables, cut them in half and connect each half to a DC to DC charger. Just clamp it to the tow vehicle battery and the other end to the Lithium batteries, like you are jump starting a car, and run the tow vehicle until they are charged. Renogy makes decent, inexpensive units of varying sizes that will do the job well at about 1/4 the price of a CARGENERATOR and you can move them from vehicle to vehicle to charge other vehicles. They can also be permanently mounted.
That does work as long as you’re careful to observe polarity. But the advantage of an installed DC/DC charger is that it can provide sufficient current to operate a residential refrigerator in a trailer while towing it. And your house batteries will always be fully charged when you arrive, no matter what you had on in your trailer.
Incidentally, an F-150 PowerBoost generator operates while driving, so you can actually run a pair of air conditioners in your RV while towing it. I’ve tried this myself…😁
The more solar panels, the better. I’ve got a little over 700 watts on our roof and I’ve almost never had a problem recharging on a cloudy day. However, during a typical Houston downpour/street flooder a couple years ago, I learned that nothing is infallible . . .
More solar panels are indeed better, but some days the clouds and rain get in the way…
“It does require that you run heavy wiring (as large as 4 gauge) from the battery of your tow vehicle all the way back to your trailer battery”
It might be more economical to buy a generator!
This is not about economics. There are some state and federal campground wilderness areas that don’t allow generators at all. This is a backup plan for those of you who can’t or don’t want to carry a portable generator.
You could buy a Ford F150 hybrid with the Pro Power On Board Generator.
I had a loaner F-150 PowerBoost last July with the onboard 7.2kW generator and it would easily do this.
I am so looking forward to my F150 Powerboost. Ordered last October and was told I should be in the next batch of production slots.
My, my, my, it seems so unusual that solar power would be so low when the sun is obscured buy overcast sky. The way everyone talks solar and wind are never ending solutions to our energy needs, just ask CA and TX. Now if we could stop the earth from rotating with the sun shining bright on our side of the world, or we could somehow put an ocean next to each state so there would always be a wind blowing inland we’d have it made. Lol
Better yet, convince the public of the long term benefits that renewable energy sources provide. If we don’t do something to cut global warming, there WILL be oceans next to a lot more states than now.
I just report my basic experiments and the math. But I’ve not been able to get any support from energy producers to go deeper into it.
That’s your OPINION….mine is you are WRONG…
Texas now gets up to 40% of its electricity from wind, and apparently it bailed them out of a bunch of brown/blackout conditions during high heat a few weeks ago. But I need to study this more to find out exactly what happened.
Someday we will be able to educate everyone on how solar and wind generated electricity is stored for ‘anytime’ use by batteries and capacitors. This technology is great now and will only get better