The MCB that looked like it was on

Some faults announce themselves. A burnt connector, a cracked case, a smell. You find those in the first five minutes. This one was a switch that was already off and looked like it was on.

The call

Customer with a Solis inverter and a Pylontech battery stack. The battery had simply stopped working, with no charge, no discharge, and the house back on full grid import.

What the portal said

Before going anywhere, we looked at the data. Two things stood out.

On the evening it failed, the system was entirely normal right up to 18:00: battery voltage 48.6 V, battery current 63 A, power flowing, status online. A working battery doing exactly what it should on a summer evening. Then it stopped.

With the system down, the same portal was reporting battery voltage 0.5 V. And here is the part that matters: at that same moment it was also reporting battery state of charge 78% and a maximum discharge current from the BMS of 80 A. An alarm had been raised against the system.

Stop and think about what that combination actually means.

• The BMS is alive. It is reporting a state of charge, and it is reporting its own discharge limit. Those numbers come from inside the battery. If the battery were dead, or disconnected from the inverter entirely, they would not be there.
• The pack is 78% full. Whatever has happened, it has not been drained.
• But the inverter is seeing effectively no voltage at its battery terminals.

A charged battery that is still talking to you, and no voltage where the voltage should be. That is not a flat battery and it is not a dead BMS. That is an open circuit somewhere in the DC power path.

Why comms and power tell you different things

This trips people up constantly, so it is worth being explicit.

The battery talks to the inverter over a separate communications cable, CAN or RS485, a thin data lead. The actual energy travels over big DC cables, through an isolator or a DC-rated MCB, into the inverter's battery terminals.

Two different paths. Break the power path and the comms path carries on regardless. The BMS keeps chatting away, cheerfully reporting a healthy 78% pack, while the inverter sits there seeing nothing at all.

So when you get a healthy battery and zero volts, the fault is almost always somewhere between the two: the DC cabling, an isolator, or a breaker. That is the same signature behind a lot of batteries that will not discharge, and it is worth separating from a genuine battery communication fault, where the data stops arriving too.

The breaker

Which brings us to a small grey enclosure on the wall, marked in marker pen: BATTERY BREAKER.

We looked at it. The toggle was up. Up means on.

Except it was not.

The enclosure lid was fouling the breaker's toggle and physically preventing it from dropping. The MCB had tripped internally, the mechanism had done its job and broken the circuit, but the arm could not travel far enough down to show it. From the outside it sat in the on position, looking for all the world like a healthy breaker.

We opened the enclosure and the toggle dropped down on its own the moment it was free.

That is the whole fault. A tripped breaker that had no way of telling anyone it had tripped.

The fix

Reset the MCB, power cycle the inverter, and the system came straight back up. Battery charging and discharging normally, customer back to saving money the same afternoon.

No parts. No return visit. A fault that would have looked, to almost anybody, like a dead battery.

What we told the customer

A tripped breaker is a symptom, not a cause. Something made it trip, and we do not yet know what. So the advice was:

• Check the breaker periodically. Now you know the toggle cannot be trusted from the outside, open the enclosure to look properly.
• If it trips again, tell us. Once is an event. Twice is a fault, and the next step is to find out what is drawing enough current, or what is degraded enough, to keep operating it.
• Do not just keep resetting it. A breaker that trips repeatedly is doing its job. The answer is to find out why, not to keep putting it back up.

The lesson

If your battery has stopped working, check the DC breaker before you conclude anything expensive, and open the enclosure to check it. Do not trust the position of the toggle through a lid that might be holding it. If you need to isolate the system properly first, our shutdown guide covers the order to do it in.

More generally: when the monitoring shows you a battery that is clearly alive and clearly charged, but the inverter cannot see any voltage, resist the urge to condemn the battery. The energy and the information travel by different roads. Follow the one that is broken.

Solar Tech Support is an independent solar and battery repair company. We are not affiliated with Solis, Pylontech, or any manufacturer. Most faults we diagnose remotely, free, no fix no fee.