The converter has been holding the bank at 14.8 volts since some time in July. Nothing has tripped, nothing has complained, and every reading on the dashboard looks like a battery being charged. In September you top up the cells and two of them take far more water than they should, which is the first anyone knew about it.
Too much charge is a fault, and it is the quiet one. A charger that stops announces itself within a day, because things go dead. A charger that never stops takes a season and costs the bank.
It is also the only battery fault on this site that produces heat, which is why it is the one where a five dollar thermometer stops being a convenience and becomes the point.
The ceiling, by chemistry
Normal bulk and absorption on lead-acid sits around 14.4 to 14.8 volts, so the ceiling goes above that rather than at it. Examples: flooded above 15.0, AGM above 14.8, LiFePO4 above 14.6 — the lithium figure being roughly where a cell hits 3.65 volts and most BMSs step in and do the job for you.
Any voltage source can carry this rule. A SmartShunt reads every 60 seconds, a Lancol every 120 with its clips on the terminal, which is exactly where you want to measure this. A Bluetooth SeeLevel board reports a voltage alongside its tanks and will do the job for nothing if the coach has one.
Equalization will trip this and is not a fault. Plenty of converters and solar controllers deliberately push a flooded bank to 15.5 volts or beyond for a few hours, on a schedule or monthly, because that is what equalizing is. If your alert fires on the same day of the month every month, you have found your charger’s calendar rather than a failure — and if equalization is enabled on a bank that is AGM or lithium rather than flooded, you have found something much worse.
The current says which kind of overcharge it is
This is the part a voltage-only monitor cannot do, and it splits one alert into two completely different afternoons.
High voltage with the current near zero is a float set-point that is simply set too high. The bank is full, nothing much is going in, and the charger is holding it at a voltage it should have stepped down from. It is a configuration problem, it is slowly cooking the bank, and it is not an emergency.
High voltage with a large positive current while the bank reads full is the converter stuck in bulk — still pushing amps into a battery that has nowhere to put them. That is the classic failed converter, it is where the heat comes from, and it is the one to act on today.
A probe on the case
Warn above about 110°F on the battery case, or more simply at 20 to 30 degrees above whatever the rest of the rig is reading. An Inkbird IBS-P01B taped to the case is the right tool because it reads the battery; a puck on the bay wall reads bay air and lags the battery by about an hour.
The comparison matters more than the absolute number. A bay next to the generator, next to an exhaust, or behind a sun-facing door reads hot in August for reasons that have nothing to do with charging, so a second sensor elsewhere in the rig is what turns a hot reading into a hot battery. And lithium does not really do this — a mild overcharge does not warm a LiFePO4 bank much, because the BMS disconnects long before the heat arrives.
The combinations
Voltage alone raises the question. As usual in the cookbook, the second reading answers it.
- Above 15.0 volts and the battery temperature climbing — stop the charging now. Pull the converter breaker, or cover or disconnect the panels. If the temperature is flat and this is a scheduled equalize, let it finish.
- Above the ceiling with shore power present — the converter is the source. Above the ceiling with the pedestal dark and the sun up is the solar controller instead, and the usual cause is a flooded charge profile left enabled on a bank that is not flooded.
- High voltage, current near zero — the float set-point. Fix the profile when convenient.
- High voltage, large positive current, bank already full — stuck in bulk. This is the one that costs a battery bank in a season.
- Above 14.6 volts, and then the next poll shows near zero or every 12-volt sensor at once goes quiet — the lithium BMS disconnected on over-voltage. Fix the charger before you reconnect anything.
The version that passes every threshold on this page. A charger that holds the bank at a perfectly normal 14.4 volts and never drops to float is overcharging it continuously, and no ceiling you can set will ever fire. There is no rule shape for “has been above 13.6 for six weeks”. You find it by looking at a month of the voltage chart once a season and asking whether the line ever comes down. On a healthy system it does, every day.
What none of this tells you
Water level in flooded cells, which is the thing overcharging actually consumes and the reason the fault is usually discovered with a torch and a hydrometer rather than with an app.
And nothing on the AC side is evidence here. A pedestal reading 130 volts is a shore power problem and not an overcharge signal, because the converter regulates: a high line in does not become a high line out. If you own a Renogy BT-1 it will show whatever charge stage your particular controller reports — the reading set depends on which controller sits behind the module — which is useful for confirming the solar side is the culprit and is not something to build a rule on.
What to do about it
Set a voltage ceiling on whatever monitor you already own — a Lancol is perfectly adequate for this one job — and put a probe on the battery case. The probe is what turns this from a maintenance question into a safety one, and it costs less than the water you will lose finding out the slow way.
Then look at a month of voltage once a season, for the one thing no threshold catches: whether the line ever comes back down.
Where to get one
A voltage source and a probe. Both are things half the readers here already own for other reasons, which is the usual shape of this cookbook. The compatibility database lists everything that reports a house-bank voltage.
Victron SmartShunt and BMV
Victron's Instant Readout broadcast. The battery monitors decode fully. The rest of the range is recognized, and that is all we will claim for now.
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