RV converter not charging the battery

Everything 12-volt dies as soon as you unplug

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.

The converter is completely dead, no output at all

Check the reverse polarity fuses first, before anything else. WFCO's own troubleshooting procedure opens exactly there: "If there is no DC output coming from the WFCO ... first check the reverse polarity fuse(s)." Progressive Dynamics starts one step earlier, with the converter's own output voltage, and only reaches the fuse when that reads zero. Same fault, two different first moves, so start with whichever your converter's manual puts first.

The converter hums or the lights flicker

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.

It worked before you changed the battery

Stop and check the reverse polarity fuses. WFCO is explicit that a reversed connection, even for one second, blows them.

The battery charges on a long drive but not on shore power

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.

First, the converter output itself

It produces around 13.6 volts DC. One caveat belongs here: 13.6 volts is the figure for a lead-acid bank. A lithium bank runs a different profile entirely, 13.6 to 14.6 in two stages on the WFCO auto-detecting units, so a 13.6 volt reading on a lithium setup is not the same clean bill of health it is on a flooded or AGM one. A failed converter gives either nothing or a low reading.

Second, the reverse polarity fuses on the converter

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.

Third, the battery branch fuse on the DC board

The battery positive is a dedicated fused circuit on the panel. WFCO publishes these per model. On the 8700-AD series the battery circuit is 30 amps: circuit four on the WF-8725-AD, and circuit six on the WF-8735-AD and the WF-8740-AD. Check your panel rather than assuming.

Fourth, an inline fuse or breaker in the battery positive lead

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.

Fifth, the battery disconnect switch

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.

The method is the same for all of them

Disconnect the battery cables at the battery, put your meter across the converter output, and read with no load connected.

WFCO, 8900 series: if the voltage reads 13.6 volts DC plus or minus 0.2 with no load, the converter is functioning properly. The 8700-AD series window is wider, 13.6 to 14.4, and the 9900 series shares the WF-9800 deckmount manual, which gives the same 13.6 to 14.4. On the lithium-capable 8900LiS, the lead-acid switch position should read 13.6 plus or minus 0.2, 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, described as approximate rather than with a stated tolerance. The 9100 and 9200 series manuals describe the unit as working properly if the voltage is above 13 volts on 12-volt models.

What the failure looks like

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.

Two readings, and both come from the manufacturers' own troubleshooting procedures.

Reading one, at the converter

Disconnect the battery cables and measure across the converter output. You want the figure from the pass test, around 13.6 volts.

Reading two, at the battery cables

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.

How to read the result

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 break points on the diagram is open. Low at the converter, below the pass figure but not zero, points at the converter itself rather than at the path, because everything between it and the battery can only subtract voltage, not add it. Either way, and this is the point, the converter is not the problem and should not be replaced.

Then check the AC side if both readings were dead

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.

The disconnect-switch reading

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.

Before you buy anything, it is worth understanding what a healthy converter does.

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 when the output current reaches its maximum, which is what a deeply discharged battery asks for, and it holds bulk until the current falls to about five amps or four hours pass, whichever comes first. It reaches float, 13.2 volts, only after a long stretch at absorption with almost no load.

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, which can look like a converter that is not working.

How to tell the difference between slow and broken

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. The current is the easier of the two to read: a clamp meter goes around the positive cable at the battery, so nothing is disconnected and nothing is interrupted. Expect a fraction of the converter's rating rather than all of it, because the stages taper as the bank fills, and read the two together: a bank that is well down and taking almost nothing while the voltage at the converter is right is the wiring, not the charger. 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.

Lithium changes the numbers

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.

The manufacturer's threshold

WFCO states that any battery reading below 12 volts while disconnected is a possible indication of battery trouble.

Two things the makers tell you to check

Progressive Dynamics lists bad battery cells in its low-output table, with the action simply being to replace the battery, and WFCO's own troubleshooting checklist asks whether the battery has been tested for shorted cells before you call their support line. A battery that will not hold a charge after a proper rest is the one to test.

The resting voltage test

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 against the maker's own table. Trojan's runs in ten percent steps: 12.73 volts at full charge, 12.10 at half, 11.66 at a fifth, down to 11.51 at ten percent. A battery that reads below 12 after a full charge and a proper rest has a problem of its own. The rest period is what makes the reading mean anything.

The hydrometer, for flooded batteries

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.

Decide with the resting voltage

Converter passes its test and the cable reading matches, but the battery rests below about 12.1 volts, which is half charge on Trojan's table, or it 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.

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.

The recharge times

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.

The wiring matters as much as the rating

The wire can be the limit rather than the rating. 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.

Documented failure points, in a workable order (your own manual may start elsewhere):

One, blown reverse polarity fuses, caused by a battery connected backwards even momentarily. Two, the battery branch fuse on the DC board, which the panel's own labelling identifies per model. Three, an open inline fuse or breaker in the battery positive lead. Four, the battery disconnect switch open, or the charger wired on the wrong side of it. Five, a bad or shorted battery cell. Six, loose, corroded or undersized battery wiring and grounds. Seven, a failed converter with no output while AC is present. Eight, a converter that is undersized or loaded out. Nine, a problem with the AC supply, including a tripped breaker or an over-voltage shutdown, where your particular model has one and names its own threshold. Ten, converter overheating with poor ventilation. Eleven, storage mode being mistaken for a fault.

What drives the bill is labour and access rather than parts. A reverse polarity fuse costs a few dollars, an inline fuse is often less, and a tripped breaker costs nothing. The converter itself is the one real part in the chain, and the hardest of them to reach, which is why a shop quote for replacing it is worth testing against the two readings first.

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.

Start with the 12-volt diagnostic guide if you have not already, because it lays out the whole system and the voltage-drop method.

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.