Calibrate battery monitor: Capacity, Peukert, Synchronisation

A battery monitor does not measure the state of charge. It counts ampere-hours and calculates a percentage value from this. For this value to be accurate, four settings must match the installed battery bank, and the counter must regularly receive a known reference point. This article covers both in the correct order.

Why the display needs to be maintained

The monitor forms an integral: it adds every ampere-hour flowed in and subtracts every ampere-hour drawn out. An integral has no absolute reference — it only knows changes. Every small measurement error, every current that flows past the shunt, and every deviation in the calculation parameters therefore add up instead of balancing each other out.

The reference point is full charge. If the monitor detects it, it sets the counter to 100 percent and starts over. Everything that happens between two full charges is calculated.

Step 1: Capacity

The nominal capacity of the entire battery bank in ampere-hours, not that of a single battery, must be entered. Three parallel 100 Ah batteries result in 300 Ah. In contrast, for series connection to a higher voltage, the capacity remains that of a single battery.

Two points are often done incorrectly here. Firstly, the usable capacity is entered instead of the nominal capacity — but the monitor calculates with the nominal value and derives the depth of discharge itself. Secondly, the value remains unchanged after a battery replacement. An aged battery bank actually has less capacity; this is a reason to correct the value downwards after a few years based on a capacity test, but not a reason to estimate it arbitrarily.

Step 2: Peukert Exponent

Lead-acid batteries deliver less capacity at high discharge currents than at low ones. The Peukert exponent represents this relationship. The higher the value, the stronger the dependence on the current.

Guideline values for onboard applications. If the manufacturer states its own value or two capacity specifications at different discharge times, this information takes precedence — the exponent can also be calculated directly from two such value pairs.

If unsure, leave the default value of the device for lead-acid and enter a value close to 1.0 for LiFePO4. A Peukert exponent that is too high for a lithium battery bank will cause the display to drop under load and then seemingly recover — behavior that is regularly mistaken for a battery fault.

Step 3: Charge Efficiency

More charge goes in during charging than comes out later. The difference is converted into heat and, in the case of lead, into side reactions. The charge efficiency reflects this: for lead, it is in the order of 90 percent, for LiFePO4 close to 99.

If this value is set too high, the monitor reports the battery bank as full too early, and the display drifts upwards over the weeks. If it is set too low, the battery bank appears permanently emptier than it is.

Step 4: Synchronization Thresholds

For the monitor to recognize a full charge, three conditions must be met simultaneously: the voltage must be above a set charge termination voltage, the charging current must have fallen below a small residual value, and both must persist for a minimum period.

The charge termination voltage must be just below what the charger actually achieves — otherwise, it will never be exceeded, and no synchronization will ever occur. The residual current is usually specified as a percentage of capacity and is in the range of a few percent; for a 300 Ah battery bank, this is roughly a few amperes. The minimum time prevents a brief voltage peak from being interpreted as a full charge.

NoteWith LiFePO4, this detection is more delicate than with lead-acid, because the charging current remains high almost until the end and then drops very quickly. If the windows are set too narrowly here, no synchronization will occur for days. When in doubt, set the charge termination voltage a little lower and the minimum time rather short.

Operational Check

Calibration is only confirmed when the display behaves consistently over several cycles. Two observations are sufficient for this.

First: After a full charge, the monitor must automatically jump to 100 percent. If this does not happen, the synchronization thresholds are incorrect. Second: After several cycles, the display must not systematically drift in one direction. If it drifts upwards, the charge efficiency is set too high or a consumer is drawing current past the shunt; if it drifts downwards, the capacity is set too high.

What the monitor cannot do

It does not know the aging status of the battery bank. A battery that only delivers 70 percent of its nominal capacity will still show 100 percent when full – but the absolute amount behind it is smaller. If you want to know the actual condition, discharge the battery bank in a controlled manner with a known load and compare the withdrawn ampere-hours with the nominal value. This is the only reliable test, and it should be done every few years.