The two readings that diagnose it
Reading one: battery voltage with shore power connected. A working converter holds the battery bank at charging voltage — roughly 13.2 to 14.4V depending on charge stage. If you read 12.6V or less at the batteries while plugged in, the converter's output isn't reaching them.
Reading two: voltage at the converter's DC output terminals. Around 13.6V there but not at the battery means the problem is between them — fuses, connections, or a battery disconnect switch. Nothing at the output means the converter itself (or its AC supply) is down. Check that its breaker in the AC panel hasn't tripped and that the outlet the converter plugs into (many are just plugged in behind the panel) is live.
The reverse-polarity fuses everyone misses
On the face of most converters sit two or three 30–40A blade fuses labeled reverse polarity. They blow instantly if battery cables were ever touched to the wrong terminals — even briefly, even by the previous owner — and a blown set means the converter can be perfectly healthy yet charge nothing.
They're the RV equivalent of checking whether it's plugged in. Pull and inspect each one (or meter them for continuity) before condemning anything more expensive.
Listen for the fan, feel for the heat
Converters cool themselves with a small fan that cycles under load. A converter that's charging hard in a warm bay with a fan that never runs is cooking itself — and a dead fan is a common failure that kills the unit slowly through heat. Conversely, a fan that screams constantly can signal an overworked converter feeding a big parasitic load or failing batteries that never come up.
Corroded battery terminals and loose lugs do the same quiet damage: the converter pushes, the connection resists, voltage drops, and the batteries stay hungry. Clean and tighten before blaming electronics.
Old single-stage converters murder batteries
If your rig is older, it may carry a single-stage converter that holds one voltage (around 13.5V) forever. That's too low to fully charge a depleted bank quickly and — worse — slowly boils the water out of flooded batteries left plugged in all season. If you're replacing batteries every year or two, the converter is the likely killer.
Modern multi-stage converters (bulk/absorption/float) charge faster and then drop to a safe maintenance float. They're a direct swap in most rigs and one of the best-value electrical upgrades there is.
Lithium changes the rules
Swapping to LiFePO4 batteries? Your existing converter may never fully charge them — lithium banks want a higher, flatter charge profile (around 14.4–14.6V) and no long float. Most converter brands sell lithium-capable replacements or have a jumper/switch to select a lithium profile.
Mismatched charging is the #1 cause of "my new lithium batteries only charge to 80%" complaints. Check the converter's profile before you blame the batteries — or ask the Genie with your converter model number and battery specs.
Stuck on this one? Put Major in your pocket.
Tell RV Journey Genie your make, model, and symptom and get a step-by-step fix — by text, voice, or photo — before you ever call the shop.
Frequently asked questions
Why aren't my RV batteries charging when plugged into shore power?
Check three things in order: battery voltage while plugged in (below ~12.7V means no charge is arriving), the converter's reverse-polarity fuses (30–40A blades on its face that blow if cables were ever crossed), and the converter's own breaker and supply outlet. A healthy converter holds batteries at 13.2–14.4V on shore power.
How do I know if my RV converter is bad?
Meter its DC output with shore power on: roughly 13.6V at the output terminals means the converter works and the fault is downstream (fuses, disconnect switch, corroded terminals). No output with confirmed AC supply and good fuses means the converter has failed. A dead cooling fan is a common root cause worth checking too.
Will my RV converter charge lithium batteries?
Maybe not fully. Lithium (LiFePO4) banks need a higher charge voltage (14.4–14.6V) and no extended float — many older converters top out too low and leave lithium at partial charge. Use a converter with a lithium profile, either a replacement or one with a selectable jumper, to get full capacity from a lithium upgrade.