Glossary

Deep discharge

Unit V

What is Deep discharge?

Discharge below the permitted end-of-discharge voltage. With lead-acid, damage begins immediately; with lithium, the battery management system usually disconnects before it gets that far.

What matters about Deep discharge in practice?

A single deep discharge can permanently cost a lead-acid battery a large part of its capacity. The most common case is not operation but lay-up: standby current over weeks without charging.

Protection comes from load shedding, which disconnects loads when a threshold is undershot — and from a state-of-charge display that makes the condition visible in the first place.

When is a battery deeply discharged?

For a 12 V lead-acid battery, a rest voltage below about 11.8 V counts as deeply discharged, and below 10.5 V under load the end voltage is reached. With LiFePO4 the battery management system usually disconnects at around 10 V, before individual cells are damaged.

What happens?

With lead-acid, sulphation sets in and the capacity drops permanently. If the battery stays discharged, the damage grows every day. With lithium the BMS protects the cells, but the battery is then switched off and has to be woken up again with a suitable charger.

How do you prevent deep discharge?

  • Install a low-voltage disconnect that cuts loads in time
  • Open the battery main switch during downtime
  • Know the standby currents of alarm systems, electronics and chargers
  • Watch the state of charge with a battery monitor, not just the voltage

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