A deep cycle battery is specifically engineered to be regularly discharged to 50-80% of its capacity and then recharged, hundreds or thousands of times. This is fundamentally different from starting batteries.
The Critical Difference from Starting Batteries
Starting batteries deliver brief, high-current bursts (hundreds of amps for 5-10 seconds). They are destroyed by repeated deep discharge.
Deep cycle batteries deliver sustained moderate current (at 0.1-0.3C rate) over hours. They are designed to tolerate this pattern.
Never use car/starting batteries for solar storage.
Deep Cycle Battery Technologies
- Flooded lead-acid: True deep cycle. Thick tubular plates. Best cycle life for lead-acid. Requires maintenance. Best value for most applications.
- AGM: Sealed deep cycle. Spill-proof. Good for enclosed spaces. Moderate cycle life.
- GEL: Sealed deep cycle. Excellent for solar. Better cycle life than AGM. Premium price.
- OPzV tubular GEL: Premium deep cycle. Longest lead-acid cycle life (1,200-1,500 @ 80% DoD). Best for industrial and mission-critical applications.
- LiFePO4: Best deep cycle energy density. Longest cycle life (3,000-6,000 cycles). Higher upfront cost.
DoD and Cycle Life Relationship
The depth of each discharge directly determines total cycle life:
- At 100% DoD: ~400 cycles (AGM)
- At 50% DoD: ~1,200 cycles (AGM)
- At 30% DoD: ~3,000 cycles (AGM)
- At 80% DoD: ~1,500 cycles (OPzV)