Glossary

Isolating diode

Also Charge splitter

What is Isolating diode?

A component that allows two battery circuits to be charged together without discharging each other. Current can only flow in one direction.

What matters about Isolating diode in practice?

The price is a voltage loss across the diode, which is missing for the last few percent of the charge. Without a regulator that compensates for this loss, the battery bank behind the diode never becomes completely full.

That is why the isolating diode has mostly been replaced today by a split-charge relay or a DC-DC charger. Where one is still fitted, it is worth checking during a refit whether the regulator compensates for the loss.

How does an isolation diode work?

An isolation diode has one input from the alternator and two or three outputs to the batteries. Each battery sits behind its own diode. Current flows only from the alternator to the batteries, never between them. A flat service battery therefore cannot drain the starter battery.

How large is the voltage loss?

A silicon diode drops about 0.7 volts or more under load, Schottky diodes somewhat less. In a 12-volt system that means several percent of the charging voltage is missing. Without compensation, a lead-acid battery stays permanently undercharged and a lithium battery does not reach its end-of-charge voltage. Some regulators therefore sense behind the diode and raise the output voltage by the loss.

What replaces the isolation diode today?

  • Split charge relay: connects the batteries while the engine runs without significant voltage loss.
  • DC-DC charger: charges the service battery with its own charging profile, lithium included.
  • Electronic isolators with MOSFETs: work like a diode, but with very low loss.

Which solution fits depends on battery type and alternator. More on the page boat alternator.

Related terms

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