Powering a 5th wheel with a small Predator generator
I just need to charge the batteries and run a few lights…
Dear Mike,
I would like to leave a small Predator inverter generator in my pickup bed and use my fifth wheel pigtail to at least keep the batteries charged and power the lights. I’m worried because I have Lithium batteries and an 80-amp converter/charger. Is it possible to get 80-amps from a 2000-watt generator? Thanks. —Ron M.
Dear Ron,
That set should work well if you get the right kind of 120-volt adapter and extension cord. Let’s explore why…
Current needed at 12 volts vs 120 volts
Okay, looks like you have a converter that can charge your RV batteries at 80 amps. That’s pretty common nowadays with lithium batteries.
So does that mean that your generator needs to supply 80 amps of current? Nope… it’s more like 1/10th of that, which is 8 amps or so at 120 volts (maybe 10 amps max, adding in all the various conversion inefficiencies). That’s because there’s roughly a 10:1 ratio of voltage to current needed between the 120-volt AC and 12-volt DC systems in an RV.
It’s all about the Watts, baby!
A 12-volt battery that’s charging at 80 amps is using around 960 watts of power or so. That’s because 12 volts x 80 amps = 980 watts.
Note: At 14.5 volts charging for Lithium batteries it’s actually closer to 1,100 watts. But we’ll stick with 12-volt calculations for these basic math examples.
To figure out the current needed to be supplied by a 120-volt generator all we have to do is divide 980 watts / 120 volts = 8 amps.
Voila! Since there’s a 10:1 ratio between the 120-volts AC from the generator and the 12-volts DC needed by the batteries, there’s an inverse 1:10 ratio of amperage needed to convert from 120-volts AC to 12-volts DC.
Can a 2000-watt Predator supply 8 to 10 amps at 120 volts?
You bet your sweet bippy it can. Let’s look at 2,000-watt Predator (or similar) inverter generators.
Runtime: 12 hours @ 25% load
Quiet, only 66 dB
Electronic overload protection
79.7cc air-cooled OHV gas engine
2000 starting watts, 1600 running watts
So let’s convert 1,600 running watts to find how many amps of current would be available at 120-volts AC. We just have to divide watts by volts to find amps. That’s 1600 watts / 120 volts = 13.3 amps.
That means that a 2000-watt Predator inverter generator should be able to supply 13 amps of current at 120 volts continuously. And since your converter/charger only needs around 8 to 10 amps of current at 120 volts, there’s at least 3 amps at 120 volts left over for powering lights and such. And that 3 amps at 120-volts AC converts to around 30 amps at 12-volts DC. Easy peasy!
The hookup
This also suggests that you really don’t need to run a heavy 50-amp shore power cord over to the generator in the bed of your truck since it’s limited to 16 amps of output current. I would suggest you get a 15-to-50 amp dogbone adapter and a heavy-duty 12-gauge extension cord, maybe 25 feet in length.
With that setup you should be able to keep your batteries charged while powering your lights. And if you pull too much current from your generator, it will just shut itself down.
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…. Mike
i understand all the math in the article but the initial question mentioned using the fifth wheel pigtail cable. your answer is very appropriate but it uses the fifth wheels shore power connection. this is the best way as it will power the 80 amp converter. but you might want to explain why this should be used and not existing 5th wheel pigtail.
to try to use the pigtail you would have to get the generator output transformed down from 120 vac to 12 vdc. but the pigtail would not support anything near 80 dc amps. some generators have a 12 dc output but again you would have to get an adapter to plug the pigtrail into it and again, no way to get anything near 80 dc amps.