A current status display tells you how full the battery is right now. It does not tell you whether the system will return to the same level over the course of the week — for that, you need the history. The PCU Control Unit shows the state of charge over the last 30 days, and this curve contains most of what can be said about a system remotely.
Note The 30-day history is available if a BullTron battery is connected directly via the P-Bus. The values are then sourced from the battery's BMS. Information on how to set up this connection can be found in the article Connecting a BullTron battery directly to the P-Bus.
Why the history says more than the instantaneous value
The state of charge fluctuates in a daily rhythm: lowest in the morning, highest in the afternoon. Every single glance at the display hits some point in this rhythm — and therefore says nothing about whether the system is in balance. Two systems that both show "73 percent" may have arrived there in completely different ways.
The history places the low points and high points side by side. Only then does it become visible whether the battery recovers to the same level every day or is trending downward over weeks. This is a question that simply cannot be answered by an instantaneous value.
Four curve shapes
Sawtooth with constant peaks
The state of charge falls overnight and reaches almost the same peak value again the following day. The system is in balance: what is consumed is returned. This is the normal case with sufficient charging power and the state you want to see.
Sawtooth with falling peaks
The daily fluctuation remains, but each peak value is lower than that of the previous day. The charging balance is negative. Typical causes include insufficient solar yield during the darker months, a consumer that runs continuously, or a travel time that is too short for the charging booster. If this pattern persists for weeks, the battery will eventually reach the BMS shutdown threshold — on a day you cannot choose.
Curve that never reaches the upper range
The state of charge oscillates in a middle band over several days and does not go to the top. Charging is regularly interrupted before the battery is full — because the solar yield drops off in the afternoon or the trip ends before the absorption phase is complete.
For the battery itself, this is not critical. For planning purposes, it means the usable reserve is smaller than the nominal capacity, and for a shunt, it additionally means that calibration to full charge does not occur and the calculation drifts. With directly connected batteries, this second effect does not occur.
Slow decline during downtime
The vehicle or boat is stationary, yet the state of charge is steadily decreasing. A consumer is running that is not on the radar: a control unit in standby, a refrigerator, a gas alarm, an inverter in idle mode. The history shows how quickly the system is draining and thus provides the order of magnitude for what to look for.
Investigating a drop
- Determine the period. Over how many days did the state of charge drop by how much? This gives you the average current you are looking for.
- Check current consumption. What is being drawn right now? If the value deviates significantly from the average, the consumer is only running intermittently.
- Switch off consumers individually. Observe the current after each step. Halving the circuit works just as well here as it does when troubleshooting on the bus.
- Compare again after a few days. Only the history shows whether the measure was effective — the instantaneous value does not.
Observation and classification
| Visible in the history | Classification |
|---|---|
| Daily fluctuation, peaks remain the same | System is in balance |
| Peaks drop over several days | Negative charging balance — check charging source or continuous consumers |
| Upper range is never reached | Charging interrupted too early; usable reserve smaller than expected |
| Drop while vehicle is stationary | Look for quiescent current, starting with control units and cooling |
| Jump upwards without charging | The BMS has recalibrated the state of charge — not an error |
| Flat history despite usage | Battery is connected to a charging source that provides continuous support |
The history is a diagnostic tool, not an error message. It shows where to look; the cause is found by taking measurements on the system.
What the history does not show
- No individual consumer. It shows the total. For individual sources and consumers, use the energy shunt.
- No information on wiring. A voltage drop under load is not in the curve — for that, see the article Limiting voltage drop.
- No state of health. For that, you need the cell data, which the PCU additionally displays for directly connected batteries.
- No reason, only a pattern. Two different causes can produce the same curve.
Which other values the PCU displays for the battery and how to classify them can be found in the article Battery values on the PCU. Whether the state of charge comes via a shunt or directly from the BMS is covered in the article Shunt or BMS data.
Typical mistakes
- Concluding the state of the system from a single day. A single day says nothing; the conclusion lies in the sequence of the peaks.
- Mistaking a calibration jump for an error. A jump upwards without charging is a normal BMS process.
- Judging immediately after switching off a consumer. The effect can be seen in the history, not in the minute that follows.
- Not noting the state of charge before a period of downtime. Without a starting value, nothing can be classified upon return.
- Interpreting a never-reached full charge as a defect. Usually, it is a charging phase that is too short, not a weak battery.
