This is the most expensive misdiagnosis in RV electrics. The converter produces the right voltage, the battery still goes flat, and the converter gets replaced. Usually the converter was fine and something in the short path between it and the battery was open. There is a documented way to prove which, it takes about ten minutes, and both major manufacturers publish the numbers you are checking against.
Last reviewed: Sep 21, 2026, against current WFCO and Progressive Dynamics service documentation. We update these guides when real-world data changes.
The converter is carrying the whole load and the battery is contributing nothing. Either the battery is finished, or it is not being charged.
Lights are bright on shore power and normal on battery, but the battery never seems to gain. Charging is partial or slow. Check the path, then check whether the converter is simply small for the bank.
Check the reverse polarity fuses first, before anything else. This is the single most common cause and both major manufacturers open their own procedures there.
Possible internal fault or an overheated unit. Progressive Dynamics lists thermal shutdown as a cause of no output and says to check airflow and let it cool.
Stop and check the reverse polarity fuses. WFCO is explicit that a reversed connection, even for one second, blows them.
That points at the converter or its AC supply rather than the battery, because the alternator and the converter are charging the same bank by different routes.
Charging travels from the converter along a short chain to the battery. Understanding this chain is worth more than any amount of testing at random, because each link has a specific symptom.
It produces around 13.6 volts DC. A failed converter gives either nothing or a low reading.
These deserve more attention than they get. Their stated job is to protect the converter if the battery is connected backwards, but the manuals reveal something more important: they are also the main connection between the converter and the DC fuse board. That means when they blow, charging stops completely rather than merely being protected. WFCO warns that a reversed battery connection, even if only for a second, will blow them, and publishes the ratings per model, from a single 40 amp fuse up to two 40 amp fuses depending on output.
The battery positive is a dedicated fused circuit on the panel. WFCO publishes these per model: in the 8700 series a WF-8712P uses a 15 amp fuse on circuit four, while the WF-8725P uses 30 amps on the same circuit, and the 8735P and 8740P use 30 amps on circuit six. Check your panel rather than assuming.
This is installed by the RV maker rather than the converter maker, and WFCO notes it is often an auto-resetting thermal type located within about eighteen inches of the battery positive terminal. When it is open, charging stops but your 12 volt loads still work, which is a distinctive combination. WFCO's own instruction for a converter reading zero at the battery is to check for exactly this.
This physically separates the battery from the coach 12 volt bus. WFCO names an open disconnect alongside an open fuse as a cause of no charge. One caveat that is worth knowing: whether the converter is wired on the battery side or the coach side of the disconnect varies by RV, so the only reliable test on your own unit is to measure battery voltage with the switch on and again with it off and see whether the reading changes.
And then the battery itself, plus the wiring and grounds that carry the current. Loose, corroded or undersized battery wiring all limit charging, and a battery with a shorted cell cannot be charged no matter how good the converter is.
Unusually, both major converter manufacturers publish pass tests rather than leaving you to guess. Here they are, in their own figures.
Disconnect the battery cables at the battery, put your meter across the converter output, and read with no load connected.
WFCO, 8900 and 9900 series: if the voltage reads 13.6 volts DC plus or minus a fifth with no load, the converter is functioning properly. On the 8700-AD series the window is wider, 13.6 to 14.4. On the lithium-capable 8900LiS, the lead-acid switch position should read 13.6 plus or minus a fifth, and the lithium position 13.6 to 14.6.
Progressive Dynamics, current guidance: loosen the screw and disconnect the negative battery wire, then read. The voltage should be 13.6 volts, or 14.6 for lithium models, plus or minus three tenths. If it is in that range, the converter is good. Their older PD4000 documents give normal as 13.6, boost as 14.4, and storage as 13.2, all plus or minus three tenths. The 9100 and 9200 series manuals describe the unit as working properly if the voltage is above 13 volts on 12 volt models.
Zero volts with AC present is a converter fault. Progressive Dynamics says to check the reverse battery protection fuses at that point, and if they are good, to move on to the AC input voltage. Their listed causes for no output are AC power not connected, blown external fuses usually through reverse polarity, a short circuit, a thermal shutdown, or an over-voltage shutdown which happens above 132 volts input. For low rather than absent output they list an excessive load, an AC input outside 105 to 130 volts, or a battery with a bad cell.
This is the whole guide in one procedure, and it is documented by the manufacturers rather than being trade folklore.
Disconnect the battery cables and measure across the converter output. You want the figure from the pass test above, around 13.6 volts.
Still with the cables disconnected from the battery, measure across the positive and negative leads themselves. You should see the same figure. This is the step that catches the fault, and Progressive Dynamics words it precisely: if the voltage is not there at the battery leads, there is a problem with the leads to the battery, so check the inline fuse in the positive lead near the battery.
Correct at the converter and correct at the cables means the charging side is fine and your fault is the battery or its terminal connections. Correct at the converter and absent at the cables means the converter is fine and one of the four break points on the diagram is open. Either way, and this is the point, the converter is not the problem and should not be replaced.
Progressive Dynamics gives the acceptable input window as 100 to 125 volts AC on the PD4135, and WFCO cite 105 to 130. Outside that range, or with no AC at all, the converter cannot work. A tripped breaker on the converter circuit is a cheap fault that looks like an expensive one.
Measure battery voltage with the disconnect switch on and again with it off. If nothing changes, the converter is on the coach side of the switch and the switch is not your problem. If the reading disappears when you switch off, you have found the interruption.
A large share of converter complaints are not faults at all, and this is worth understanding before you buy anything.
Both manufacturers publish the same three-stage behaviour. WFCO runs bulk at 14.4 volts, absorption at 13.6, and float at 13.2. Progressive Dynamics runs boost at 14.4, normal at 13.6, and storage at 13.2, adding a 15 minute burst at 14.4 every 21 hours to stir the batteries. WFCO enters bulk only when system voltage falls below about 13.2 volts, which is roughly half charge, and caps bulk at about four hours before dropping back to absorption. It descends to float after a long period without significant current change.
The honest consequence, in WFCO's own words: even in normal absorption mode at 13.6 volts, your batteries are being charged, just at a slower rate. That is not a defect, it is the design. A deeply discharged bank on a converter spends most of its time in the slow stage, and owners routinely misread this as a converter that is not working.
Put a load on the system, or run the bank down, then watch whether the converter climbs into bulk at 14.4 volts and whether charge current appears at the battery. If it goes into bulk under load, it is working. If it stays at 13.2 volts with a discharged battery and no load, it may be stuck in storage rather than broken.
WFCO's auto-detecting units recognise the chemistry and adjust the profile without switches, running 13.2 to 14.4 for lead-acid in three stages and 13.6 to 14.6 for lithium in two, with a bulk target of 14.4 for lead-acid and 14.6 for lithium. Progressive Dynamics handles it with a switch position that holds a constant 14.6 volts and disables the staging. If your charger is set for the wrong chemistry, the battery will be chronically undercharged or overcharged and it will look like a fault.
These two produce the same complaint and need opposite repairs, so it is worth having a way to separate them.
WFCO states that any battery reading below 12 volts while disconnected is a possible indication of battery trouble. That is a genuinely useful line to remember.
WFCO notes that if the converter cycles between absorption and bulk, there could be a shorted battery cell. Progressive Dynamics lists bad battery cells in its low-output table with the action simply being to replace the battery. A battery that never lets the charger settle is telling you something.
Charge the battery, then disconnect it and leave it idle with nothing charging or discharging for at least six hours and preferably twenty four. Then read it. Around 12.7 volts is full, 12.4 is about three quarters, 12.2 is half, and 12.0 is a quarter. A battery that reads below 12 after a full charge and a proper rest has a problem of its own. The rest period is the part people skip, and skipping it is why tired batteries get called fine.
A fully charged cell should read a specific gravity around 1.277 with a tolerance of about seven thousandths. Voltage tells you the average of six cells; specific gravity tells you about each one, which is how you catch a single failing cell that the voltmeter averages away.
Converter passes its test and the cable reading matches, but the battery rests below 12.0 to 12.2 volts or fails a load test: the battery is the problem. Converter fails its test, or the path is open: the charging system is the problem and the battery may be fine. Both can be true at once, and that happens more often than people expect.
This is a real documented cause rather than a theory, and it is worth checking before replacing a working converter with a bigger one.
Progressive Dynamics lists excessive load for the converter as a cause of low output, with the action being to reduce the load or install a larger converter. They also give a practical floor for sizing: battery size should not be less than the converter size in amps. Xantrex publishes recommended maximum bank sizes for its units, 220 amp hours for the 40 and 60 amp models, and 400 amp hours for the 80 amp.
Progressive Dynamics documents a 125 amp hour battery discharged to 10.5 volts reaching full charge in about 70 hours on a 60 amp converter running at 13.6 volts without boost. With their boost function that becomes 90 percent in two to three hours and full in roughly fifteen. Cummins Onan gives the same shape of answer from the generator side: a 100 amp charger bringing a 600 amp hour bank from 60 percent takes about 4.2 hours.
Field reports describe a 70 amp converter delivering only about 20 amps during a boost charge over a long run of undersized wire. Before deciding you need a bigger converter, measure the actual charge current at the battery and see whether it matches what the unit is rated to deliver. If it does not, the fault is the wire, not the rating.
The informal trade guidance of charging at around ten percent of amp hours is sensible and widely used, but we could not find it published as such by any manufacturer, so treat it as trade practice rather than specification.
Manufacturer documented, in the order both makers check them:
One, blown reverse polarity fuses, caused by a battery connected backwards even momentarily. Two, an open inline fuse or breaker in the battery positive lead. Three, the battery disconnect switch open, or the charger wired on the wrong side of it. Four, a bad or shorted battery cell. Five, loose, corroded or undersized battery wiring and grounds. Six, a genuinely failed converter with no output while AC is present. Seven, a converter that is undersized or loaded out. Eight, a problem with the AC supply, including a tripped breaker or an over-voltage shutdown above 132 volts. Nine, converter overheating with poor ventilation. Ten, storage mode being mistaken for a fault.
Cooling fan wear contributing to a slow heat-related failure, and converters degrading over years from heat, vibration, moisture and surges. Both are widely reported by technicians and appear in no manufacturer failure table we could find, so treat them as reasonable suspicion rather than documented cause.
No manufacturer publishes repair pricing, so these are commercial sources and ranges.
A common 55 amp converter sells for roughly $287 to $381 depending on where you buy it. Replacement installed runs broadly $500 to $1,200 depending on size and access, with one service company quoting $500 to $520 installed for a typical 55 to 60 amp unit in a specific trailer model.
A reverse polarity fuse costs a few dollars. An inline fuse is often less. A tripped breaker costs nothing. If you are being quoted a converter replacement without a two-reading test having been done, ask for the reading at the battery cables before agreeing to anything. It is one number and it either confirms the diagnosis or replaces it with a two dollar fuse.
Professional diagnosis runs around $95 to $185 for a mobile diagnostic, with certified labour at roughly $125 to $185 an hour. Given how often the answer is a fuse, a connection or a switch, that is usually money well spent rather than a way to avoid buying a part.
Start with the 12 volt diagnostic guide if you have not already, because it lays out the whole system and the voltage-drop method this page assumes.
If the generator runs but the batteries do not charge, that is a different path to the same symptom, covered in generator not charging. If a fuse blows again as soon as you replace it, you have a short rather than a charging fault, and that is fuse keeps blowing.
Probably not. Take a second reading at the battery cables with them disconnected from the battery. If you see the same 13.6 volts there, the converter and the wiring are both fine and the problem is further along, at the battery or its connections. If you see zero at those cables, the converter is fine and something in the path between it and the battery is open. Either way the converter itself is not the fault.
The reverse polarity fuses. Both WFCO and Progressive Dynamics open their own troubleshooting procedures with them. They sit on the converter itself, usually two, and their job is to protect the unit if the battery is ever connected backwards. A reversed connection for even a second blows them, and that is a very common self-inflicted fault after battery replacement. They are also the main connection between the converter and the DC fuse board, so when they blow, charging stops entirely.
Because a converter spends most of its time in absorption mode at about 13.6 volts, and that is a maintenance voltage rather than a fast-charge one. WFCO says it plainly: even in absorption mode the batteries are being charged, just at a slower rate. Bulk mode at 14.4 volts only runs when the system drops below about 13.2 volts, and it is capped at around four hours. If your bank is deeply discharged, expect most of the charging to happen slowly.
Three things to look for. First, WFCO states that a battery reading below 12 volts while disconnected is a possible indication of battery trouble. Second, if the converter cycles repeatedly between bulk and absorption and never settles, WFCO lists a shorted battery cell as the cause. Third, charge the battery and let it rest with nothing connected for at least six hours, preferably twenty four, then read it: about 12.7 volts is full, 12.2 is half, and 12.0 is a quarter.
Run the manufacturer checks first, because Progressive Dynamics states directly not to replace the converter unless those checks have been performed. The parts are not dear, a common 55 amp unit runs a few hundred dollars, and installed costs land broadly between five hundred and twelve hundred depending on size and access. But a reverse polarity fuse is a few dollars and an open inline fuse is often even less, so proving which one you have before buying anything is worth the ten minutes.
Yes, and it is a real documented cause, listed by Progressive Dynamics as excessive load for the converter. There is a practical floor too: Progressive Dynamics advises that battery size should not be less than the converter size in amps, and Xantrex publishes maximum bank sizes for its units of 220 and 400 amp hours. A small converter on a large bank with the fridge running will charge very slowly or appear not to charge at all.
The manufacturer and agency documents this guide draws on, so you can check any figure for yourself.