The PCU displays voltage, current, temperature, and state of charge (SoC). Three of these four values are measured, one is calculated — and that is the very one you read in everyday use. Understanding where the state of charge comes from also helps you understand why it sometimes drifts.
NoteIf a BullTron battery is connected directly via the P-Bus, cell data and the 30-day state of charge history are also included. Information on how to interpret this curve can be found in the article Reading the state of charge history on the PCU.
What is measured and what is calculated
Voltage, current, and temperature are measured values. They come directly from the shunt or the temperature sensor and are as accurate as the measuring point allows.
The state of charge is something else. It is generated from an ongoing balance: starting from a known full state, every discharged and every charged ampere-hour is counted. This is the only practical method — and it has two prerequisites. First, all current must flow through the shunt. Second, the bank must regularly reach a reliable reference point, i.e., it must truly become full.
If either of these two prerequisites is not met, the display will drift. Not because the device is measuring poorly, but because it lacks the necessary foundation.
Entering the nominal capacity correctly
You must enter the nominal capacity of the bank, not the usable capacity. The PCU calculates the state of charge as a percentage of the nominal capacity — the amount you actually intend to use is a question of depth of discharge and belongs in the alarm threshold, not in the capacity setting.
For batteries connected in parallel, you add: three 100 Ah batteries result in 300 Ah. For batteries connected in series, you do not: two 12 V 100 Ah batteries in series result in 24 V at 100 Ah. The mistake of adding both numbers is more common than you might think and leads to a display that is constantly twice as optimistic as reality.
NoteWith lead-acid batteries, the removable capacity depends heavily on the discharge current — the nominal capacity applies to a specific discharge duration, often 20 hours. If you discharge quickly, you get less out of it. With LiFePO4, this effect is significantly smaller.
Battery type and related settings
Specifying the battery type is not a formality. It determines the voltage thresholds at which the display considers a bank to be full or empty, and for lead-acid, it also provides the correction factor for the discharge current.
The practical difference is evident in the voltage curve. A lead-acid battery can be estimated quite well via its resting voltage — it drops noticeably across the discharge range. LiFePO4 does not do this: the voltage remains almost constant over large parts of the usable range. For lithium, amp-hour counting is therefore not just one of several methods, but practically the only one.
Monitoring the starter battery
The PCU also monitors the starter battery. The fundamental difference compared to the service bank is this: for the starter battery, a state of charge in percent is irrelevant; there is only one question — is it there when you need it?
Voltage monitoring is sufficient for this. A starter battery that remains below a certain threshold for weeks serves as an early warning that it is no longer being recharged — typically because the separator relay or the charging converter is no longer working. This is a fault you would otherwise only notice when attempting to start the engine.
If you want a true current balance for the starter battery as well, you need a second measuring point. This is rarely necessary — it becomes useful where the starter battery does more than just start the engine, such as acting as a buffer for a winch.
Why the state of charge drifts
Two causes cover almost all cases.
A load is bypassing the shunt. Anything connected directly to the battery negative terminal instead of behind the shunt is not counted. Even small loads add up: 30 milliamperes of quiescent current amount to about 0.7 ampere-hours per day and five per week — invisible to the counting system, but felt by the bank.
The bank never reaches full. Without a true full state, there is no reference point against which the counter is reset. Small counting errors then add up over weeks. If you spend weeks oscillating between 40 and 80 percent, you will need to bring the display back into line occasionally via a full charge.
Checklist after setup
- Is the charging current positive? When charging, the sign must be correct; otherwise, the battery and load sides at the shunt are swapped.
- Is the quiescent current plausible? With everything off: a small value, but not exactly zero.
- Is the capacity added correctly? In parallel, Ah adds up; in series, voltage adds up.
- Is the battery type set? All thresholds depend on this.
- Is the starter battery visible and provided with a sensible warning threshold?
Typical mistakes
- Entering usable capacity instead of nominal capacity. The depth of discharge belongs in the alarm threshold, not in the capacity setting.
- Adding in series connections. Two 12 V 100 Ah batteries in series are 24 V and 100 Ah, not 200 Ah.
- Ground connected to the battery terminal instead of behind the shunt. The most common reason for a drifting display.
- Expecting a state of charge for the starter battery. Without a dedicated measuring point, there is only voltage available — which is sufficient for the purpose.
- Never fully charged. Without a reference point, every ampere-hour counter will drift, regardless of the device.
