The charger stops, and you find out next spring

The converter’s DC fuse let go on the Friday, quietly, with the rig plugged in and everything apparently normal. Nothing changed that anybody could see. By Monday the bank was dead, and it had spent three days sitting discharged, which is the part that costs money — not the evening the lights went out, but the battery you buy next spring.

“Not charging” is three completely different failures wearing the same face. The converter can die while you are on shore power. The solar can die while the sun is out. The alternator or DC-DC charger can die while you are driving. The battery reading looks the same for all three, and each one wants a different afternoon of your time.

The rule everybody writes first, and why it fails

The obvious rule is “alert when current goes negative”. Do not write it. It will fire every night, on every rig, forever, because a battery that is not being charged is being discharged and that is simply what night is.

The rule that works is a voltage floor while a charger ought to be running. A healthy converter holds a lead-acid bank at 13.2 to 13.6 volts on float, so below 12.9 volts means nothing is going in. On lithium, which rests at 13.2 to 13.3, use 13.0. Any voltage source will do it: a SmartShunt at 60 seconds, a Lancol at 120, the voltage line off a SeeLevel board you already have.

Telling the three apart

a. The converter, on shore power

This is the one a second device settles instantly. If a Power Watchdog says the pedestal is live at 120 volts and the bank is still falling, the campground is fine and the fault is on your side: the converter, its breaker, or its DC fuse. If the Watchdog is dark, you have an outage and this article is the wrong one.

Do not try to build the alert out of the Watchdog’s own current. A converter charging hard is a real AC load — a 55 amp unit is roughly 700 to 800 watts, six or seven amps at 120 — but that number moves with the kettle, the water heater and everything else in the rig, so a threshold on it is noise. Its job here is only to prove the pedestal is live so the DC side can be blamed.

b. The solar, in daylight

A Renogy BT-1 bridges a Renogy charge controller and reports whatever that controller reports, which is what its own app shows — the exact set depends on which controller is behind the module, so check what appears on your dashboard before writing a rule against it. It is read-only by design; we never write settings to a charge controller.

Do not alert on solar output. It goes to zero on every cloudy afternoon and every single evening, and a rule that is right twice a year and wrong two hundred times is a rule you will mute. Use the controller for the dashboard and the comparison, and leave the alerting to the battery floor. What the controller genuinely gives you is the other half of a pair: it sees what it puts in, and it knows nothing about what the rig takes out.

One thing we cannot do here, said plainly: Victron’s SmartSolar and Orion units broadcast, and we recognize them, but their record layouts are not decoded. They are not a source for this. Only the battery monitors decode fully.

c. The alternator, on the road

Turn the key and the house voltage should climb to a charging level, 13.8 to 14.4 on lead-acid, and stay there for the drive. If it sits at 12-something through four hours of highway, the DC-DC charger, the isolator or its fuse has gone.

This one is a dashboard read rather than an alert, because no rule can know you are driving. Look at the voltage chart for the drive when you park — it is a shape you learn to recognize in about two trips, and its absence is unmistakable once you have seen it there.

d. The one that is not a fault at all

Most LiFePO4 batteries refuse to accept charge at or below freezing; that is the BMS protecting the cells, and it is normal. So on a lithium bank, a bay thermometer is a charging sensor. Warn below 35°F and you get the warning while the bay is heading down rather than after the charge has stopped. An Inkbird IBS-P01B has a wired probe you can tape to the battery case, which is the right way to do it, because a sensor on the bay wall is reading air.

That difference matters more than it sounds. Air cools and warms far faster than sixty pounds of battery, so a wall sensor says “blocked” before the battery is and “clear” before it is. Batteries with built-in heaters hold on lower. Lead-acid does not do this at all — it simply charges slowly in the cold.

The combinations

As throughout the cookbook, the pairs are where the diagnosis lives.

• Pedestal at 120 volts, bank falling for an hour in the daytime with nothing big running — the converter, its breaker or its DC fuse. Not the campground.
• Array producing, bank not gaining at midday — either the loads are eating everything the panels make, which you check by watching the current with the inverter off, or the controller’s output is not reaching the bank at all: a blown fuse or a bad connection between the two.
• Array making nothing on a clear day, converter or alternator charging normally — the solar side alone is dead, and the bank is not in danger yet. Panel, wiring or controller, in that order of likelihood.
• Bay below freezing, no charge going in on shore power or in full sun, everything still working — a lithium charge block. It clears itself when the bay warms. The same reading at 60°F is a real fault.
• Voltage flat at 12-something through a long drive — the DC-DC charger or isolator. Nothing else produces that particular shape.

What none of this tells you

Which stage failed. Voltage and current say that nothing is going in; they cannot say whether it is the panel, the controller, the converter board, a transfer switch or a fuse. Everything above narrows it to a system, and then somebody opens the bay.

What to do about it

On a shore-powered rig: a voltage floor at 12.9 and a pedestal monitor. That pair is the whole article — one says the bank is not gaining, the other says whose fault it is, and between them there is nothing left to guess.

On a solar rig: the shunt’s state-of-charge floor as the backstop, because it is the only reading that survives a cloudy week without lying to you. Add the controller if you already own the module, and put a probe on the battery case before the first frost if the bank is lithium.

And when the alert does arrive, check the pedestal before you check anything else. Most of the time you unplugged an hour ago and had forgotten — which is not a false alarm so much as the alert doing exactly what it was asked, on a question it was never given enough information to answer.

Where to get one

The shunt is the one purchase that changes this from guessing to reading, and the battery monitor guide is the longer argument for it. It needs its Instant Readout key pasted in before anything decodes, which takes about two minutes. The compatibility database has the rest.