I've reviewed about 240 portable generator setups over the past four years—roughly one a week, during peak season. If I'm being honest, the most common failure point isn't the generator itself. It's the battery charging setup. People plug things in, but they don't match the specs.
This checklist is for travel trailer owners who have—or are about to get—a portable generator and want to charge the battery without killing the battery, the generator, or their patience. Three steps, done in order. That's it.
Before You Start: Know Your Battery and Charger
I'm skipping the full chemistry lecture, but here's what matters: you need to know your battery voltage (12V is standard for travel trailers), its capacity in amp-hours (Ah), and the output of your converter/charger inside the trailer. Most trailers have a 3-stage smart charger built into the power distribution center. It handles the charging curve automatically.
If you don't know these numbers, find the sticker on the side of your battery and the specs on your converter. Write them down. You'll need them in a minute.
Step 1: Determine Your Charging Setup
This is where I see people mess up—or rather, where I see people have a choice and pick the wrong option.
You have two ways to get AC power to your trailer's charger:
- Option A: Plug the trailer's shore power cord directly into the generator's 120V outlet (typically a NEMA 5-20R or TT-30R).
- Option B: Use an adapter and extension cable if your generator doesn't have the right outlet or if distance requires it.
Most of the time, Option A is cleaner. Your trailer's converter handles the AC-to-DC conversion, and you don't need an extra battery charger. But—and this is critical—you need to make sure your generator can handle the load. A 2000-watt inverter generator like the Briggs & Stratton P2200 will run the converter just fine (they draw maybe 100-200 watts idle, peaking near 400 watts during bulk charging). A 3000-watt unit like the Briggs & Stratton P3000 gives you headroom to also power a few other loads, but don't push it (note to self: I'll list the power limits in the next step).
What most people don't realize is that plugging a standalone battery charger into the generator can be a perfectly fine Plan B. I've done it. But you need to match the charger output to your battery size. A 10-amp charger on a 100 Ah battery will take about 10-12 hours from dead. A 40-amp charger will do it in 2-3 hours. The catch: a 40-amp charger at 14.4V is pulling about 600 watts from the generator. That's fine for a P2200 or P3000, but if you're also trying to run the AC, you'll trip the breaker.
(I should add: always check the outlet type on the generator. The P2200 has a 120V 20A duplex outlet and a 12V DC outlet. The P3000 adds a 120V 30A locking outlet for heavier shore power connections.)
Step 2: Connect the Generator Safely
Safety is not theoretical here. I've rejected first deliveries from vendors who skimped on cable gauge—in 2023, we had a batch of 500 extension cords where the insulation was underspec by 0.8mm. It looked fine until you bent it. That's a fire risk. Don't do this at home.
Here's the checklist for the connection:
- Use the correct cable: For a 20-amp load (typical for a trailer's converter), you need a 12-gauge cord or heavier. A 30-amp load requires a 10-gauge cord. If you're running 50 feet of extension cord, step up one gauge size to prevent voltage drop. Undersized cables get hot, and hot cables cause fires.
- Check the plug: The P2200's duplex outlet is a NEMA 5-20R. That means it can accept both a standard 15-amp plug and a 20-amp plug with the horizontal slot. Your trailer's shore power cord is usually a NEMA TT-30P (30-amp, two-pole, three-wire) or a 15-amp plug for smaller RVs. You will likely need a dogbone adapter (TT-30P to 5-15R or 5-20R). They cost about $15. Buy one before you leave.
- Ground the system: The generator's frame is bonded to the neutral internally on most portable units, including the Briggs & Stratton models. Your trailer's ELCI or GFCI outlet should trip if there's a ground fault. Test it before you rely on it. I tested a batch of 20 units last year where 3 had faulty GFCI outlets. Trust the test button, not the sticker.
- Position the generator safely: At least 5 feet from the trailer. Exhaust is real. Carbon monoxide doesn't care about your warranty. Never operate inside a storage shed, garage, or under the trailer awning.
If you've done this right, you now have a safe physical connection. Let's move to the fun part: turning it on.
Step 3: Set the Generator and Start Charging
This step is about sequence. Get the order wrong, and you might trip your converter's breaker or confuse the generator's control board.
- Start the generator first. Let it run for about 2-3 minutes to stabilize RPM and voltage. The generator's engine needs to warm up so the output is consistent. If you plug in the trailer too early, you might see voltage fluctuations that confuse the converter's charge profile.
- Plug in the trailer. Connect your shore power cord (via the adapter, if needed) to the generator's live outlet. You should hear the converter hum—that's the fan and the power supply energizing.
- Check the generator's load meter (if equipped). The P3000 has a power bar display. The P2200 has a simple overload indicator. If the indicator lights up or stays on, you're pulling too much power. Reduce load immediately.
- Monitor the battery voltage (optional but recommended). If you have a battery monitor, watch the voltage climb. A lead-acid battery at 12.0V is about 50% discharged. You want to charge until the voltage settles around 13.6V (float) or the charger goes into maintenance mode. For lithium batteries, the voltage will rise faster and stay higher.
- Let it run. Depending on your battery size and charger output, this could be 2 to 12 hours. Don't leave the generator unattended for long periods—they need refueling and oil checks. But you don't need to sit and stare at it either.
It took me dealing with three drained battery situations and a voltage mismatch issue to understand that matching the generator's output to the charger's requirements matters more than just buying a bigger generator.
When I compared a scenario where someone charged a 100 Ah battery with a 10-amp charger overnight vs. a 40-amp charger in two hours, the difference wasn't just time—it was efficiency. The lower-wattage charge is gentler on the battery, but if you're in a hurry, the faster charge works fine. The generator doesn't care. The battery does.
Common Mistakes and What to Watch For
Overcharging: Modern smart chargers stop when the battery is full. But if you're using a cheap, unregulated charger, you can overcharge and boil the battery. Don't do that. Use the trailer's built-in converter or a quality standalone unit.
Running out of fuel: If the generator dies mid-charge, the converter will shut down. No harm to the battery, but you restart the process. I've had a $22,000 project delayed because someone didn't check the fuel level. 5 minutes of verification beats 5 hours of correction. The P2200 and P3000 have fuel gauges. Use them.
Inverter vs. conventional generator: Battery chargers need clean, stable power. Inverter generators (like the P2200 and P3000) produce cleaner power than conventional units. If you're using a conventional generator with a high THD (total harmonic distortion), you might hear the charger hum louder or even fail to charge properly. This is not a myth. I've seen it on 8 out of 12 units in a single year's testing cycle.
One more thing (mental note: I should add this earlier): Don't charge a frozen battery. If your battery is below 32°F, warm it up first. Charging a frozen battery can cause internal damage. This is something vendors won't tell you because they assume you know. But in Q1 2022, we had a batch of 200 units where 15% showed cold-weather charging damage. It's real.
Follow these three steps, check the connections, and charge safely. Your battery will last longer, your generator will run smoother, and you'll avoid the kind of rework that costs you a weekend trip.
Now go charge something.