Battery Runtime Calculator 🔋
Calculate how long your boat's battery bank will last based on capacity, battery type, and electrical load. Supports lead-acid, AGM, and lithium batteries.
How to Use the Battery Runtime Calculator
Select battery type, enter capacity (Ah), system voltage, number of batteries wired in parallel, and total electrical load in watts. The calculator estimates how long your battery bank will last.
Depth of Discharge (DoD) varies by battery type: Lead-acid and AGM should not be discharged below 50%, while lithium (LiFePO4) can safely discharge to 80%.
For marine use, always maintain a reserve. The 'rule of thirds' applies: use 1/3 of capacity, keep 2/3 in reserve. Consider solar panels or alternator charging for extended trips.
FAQ
How long will my boat batteries last?
Runtime depends on total battery capacity (Ah), depth of discharge (DoD), and electrical load. Formula: Runtime = (Capacity × Batteries × DoD%) / (Load watts / Voltage). For example: 2× 100Ah AGM at 12V with 120W load = (200 × 50%) / (120/12) = 10 hours.
What is depth of discharge (DoD) and why does it matter?
DoD is how much of a battery's capacity you can safely use. Lead-acid and AGM batteries should not be discharged below 50% (50% DoD) to preserve lifespan. Lithium (LiFePO4) batteries can be safely discharged to 80% DoD, providing more usable capacity.
Should I choose lithium or AGM batteries for my boat?
Lithium (LiFePO4) batteries offer 80% usable capacity vs 50% for AGM, last 3-5x longer, weigh 60% less, and charge faster. However, they cost 3-4x more upfront and require a compatible charging system. For frequent use and long-term savings, lithium is better.
How do I calculate my boat's total electrical load?
List all electrical devices and their wattage: navigation lights (~25W), chart plotter (~20W), VHF radio (~25W), refrigerator (~50W), cabin lights (~20W), etc. Add up all devices that run simultaneously. Include a 10-20% buffer for efficiency losses.