Since batteries have been able to report their own values to the P-Bus, there are two ways to determine the state of charge: calculated from the current in the negative line or reported by the battery management system. Both are correct — the question is which batteries are installed in the system.
The short answer
| Situation | The right way |
|---|---|
| BullTron battery with P-Bus support | direct integration, no shunt required |
| Lead-acid, AGM, or gel battery | battery management shunt |
| Lithium battery without bus connection | battery management shunt |
| Second battery bank with its own monitoring | second shunt, even alongside an integrated battery |
| Solar yield, consumption of a device | energy shunt — it sits alongside the battery measurement, it does not replace it |
What the shunt does
A battery management shunt is located in the negative line and measures the current entering or leaving the battery. This results in a continuous ampere-hour balance, and from that balance, the state of charge. This works regardless of the manufacturer, cell chemistry, and battery electronics — making it the only option for a large portion of all systems.
For lead-acid, AGM, or gel batteries, there is simply no alternative: these batteries have no BMS that could report anything. The same applies to lithium batteries from other manufacturers without bus support. Which shunt belongs in which system is explained in the article SHX 348 or SHX 648; what to consider during installation is detailed in the article Installing a shunt correctly.
Where this measurement principle reaches its limits
The limitations of a shunt lie not in its accuracy, but in the principle itself. Three points frequently come up in practice.
The measurement path must be complete
A shunt only sees what passes through it. If a ground connection remains at the battery terminal, this consumer is missing from the balance — and the display does not jump, but rather drifts away slowly. This is the most common installation error and the one that is noticed last. With a BMS, this question does not arise: it is inside the battery, behind all outputs.
The state of charge remains a calculation
Every measured value enters a continuous balance, and deviations accumulate over days instead of balancing out. Therefore, the calculation regularly needs to be synchronized with a known state — usually full charge. How to set this is explained in the article Calibrating a battery monitor. If the system rarely reaches a full charge, the synchronization does not occur and the display drifts.
The battery remains a black box from the outside
Cell voltages, cell temperatures, and the status of protection circuits cannot be measured with a shunt. For LiFePO4, this is the most interesting part: a cell drifting out of balance shows up in the individual cell voltages much earlier than in the state of charge.
NoteNone of this is an argument against the shunt as a device. A battery management shunt measures accurately from a few milliamperes across the entire range — the three points above relate to the measurement principle and not its quality.
What direct integration achieves
A BullTron battery with P-Bus support reports its own values: state of charge, current, voltage, cell data, temperatures, and the status of protection functions. Additionally, it provides the state of charge history for 30 days. The shunt for this battery is eliminated, along with approximately €120 in costs and one component in the negative line.
The conversion is a one-time process handled via two apps, not tools — the procedure is in the step-by-step guide. What the integration specifically provides is explained in the article Integrating a BullTron battery directly into the P-Bus.
The point overlooked during the switch
A battery management shunt usually also measures the starter battery voltage. If the shunt is removed, this measurement point is also lost — and you typically only notice this when the system is already set up.
The solution is unspectacular: the starter battery is connected directly to the control unit (PCU) and remains displayed there. The measurement point moves, but the information remains. Only the wiring to that point needs to be included in the planning, not in remedial work.
WarningCalculate the line from the PCU to the starter battery before you decide to save on the shunt. Retroactively, this path is regularly the most time-consuming part of the conversion — and it is not included in any cost savings calculation.
Mixed systems
The two paths are not mutually exclusive. A BullTron battery can be integrated via the P-Bus while a shunt simultaneously monitors a second bank — for example, a lead-acid bank as a buffer for a winch or a separate emergency battery. How such a system is sensibly configured depends on how the banks are wired together and where charging takes place; the basics for this are in the article Separating starter and service batteries.
Typical errors
- Operating both sources in parallel. The shunt and BMS provide two numbers for the same battery that never quite match — this creates inquiries without any gain in insight.
- Forgetting the starter battery when removing the shunt. The measurement point must be moved to the PCU, otherwise, it will be missing without replacement.
- Assuming the shunt applies to all batteries. Direct integration applies to the integrated battery, not the entire system.
- Never setting the synchronization. A shunt without synchronization will show a value after weeks that no one trusts anymore.
- Confusing energy measurement with battery measurement. Solar yield and individual consumers belong on an energy shunt, not the battery management shunt.
