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.
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
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.
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
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
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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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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