Glossary

Shunt

Unit A / mV

What is Shunt?

A precision resistor in the negative cable across which the voltage drop is measured. From this a monitor calculates the current and counts amp-hours — the basis of every state-of-charge display.

What matters about Shunt in practice?

It must be positioned so that really every current flows through it: nothing may be connected between battery negative and the shunt. A single cable left there makes the state-of-charge display useless.

After installation, the device needs the rated capacity, battery type and shunt value — and one full charge as a reference point. Without this synchronisation the display drifts, even though the device measures perfectly.

How does a shunt measure current?

A shunt is a very small, precisely known resistor. It is rated as current to voltage, for example 500 A / 50 mV: at 500 A it drops 50 millivolts, at 50 A it drops 5 millivolts. The battery monitor measures this voltage, calculates the current from it and counts the amp-hours flowing into and out of the battery.

Where is the shunt installed?

Directly on the battery's negative terminal, as the only part on that terminal. All negative conductors, including those of charger, solar controller and inverter, go to the other side of the shunt, ideally via a negative busbar. Only then does it really capture every current.

Which shunt is right?

  • The rated current should be above the largest continuous current, such as that of the inverter.
  • How long the shunt tolerates short peaks such as starter current is stated in the data sheet.
  • The monitor must know the shunt value. If it is set wrong, every reading is off by the same factor.

The BTM2 battery and tank monitor and the BLS battery monitor, for example, work with a shunt.

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