How low can I discharge the house battery bank on Grace before starting the generator?

If Grace has lead-acid house batteries, start the generator at around 50% state of charge, roughly 12.2V on a rested 12V bank or 24.4V on a rested 24V bank. Stay above the stated lower limit of 11.5V or 23V rather than using it as your routine charging trigger; a lithium conversion needs different settings.

Check battery type and voltage

Read the house battery labels and check whether the bank supplies 12V or 24V. Flooded lead-acid, AGM and gel batteries need a different discharge policy from lithium iron phosphate, usually marked LiFePO4.

Grace's name alone does not establish her battery specification. A previous owner may have changed the original installation. Take readings from the house bank rather than the engine or generator starting battery.

• Flooded, AGM or gel: aim to recharge at approximately 50% remaining capacity to help preserve battery life.
• LiFePO4: around 20% to 30% remaining capacity is a practical planning trigger, subject to the battery manufacturer's instructions and the reserve you need.
• Unknown batteries: use the conservative lead-acid approach until you identify them.

Keep tools clear of battery terminals and remove metal jewellery before inspecting connections.

Read voltage after resting

Battery voltage changes with load, temperature and recent charging. A kettle running through an inverter can pull the reading down sharply. Stop it and the voltage recovers. Immediately after charging, voltage can look reassuring while overstating the charge remaining.

For a useful lead-acid check, stop charging, switch off substantial loads and allow at least an hour of rest if practical. Several hours gives a better indication. The following readings are approximate and apply at moderate temperatures; the battery manufacturer's figures take precedence:

• 12.6V to 12.8V / 25.2V to 25.6V: near fully charged.
• About 12.4V / 24.8V: approximately 75% charged.
• About 12.2V / 24.4V: approximately 50% charged; arrange charging.
• 11.5V / 23V: below a sensible routine operating level; reduce loads and recharge promptly.

These estimates do not apply directly to a bank powering equipment. If voltage remains low after heavy loads stop, charge now. If it collapses under modest loads, investigate battery condition and cable connections rather than simply lowering the alarm setting.

Set your charging trigger

A correctly configured shunt monitor is more useful than voltage alone because it measures current entering and leaving the bank. Its percentage reading is trustworthy only when battery capacity, charging parameters and synchronisation are correct, and all house-bank current passes through the shunt.