There's no single number for how many times you can drain a car battery before it fails. It depends heavily on the battery type and the depth of each discharge. A standard flooded lead-acid battery, common in most vehicles, can typically handle only a handful of deep discharge cycles (draining it to near-zero) before suffering permanent damage. In contrast, an Absorbent Glass Mat (AGM) battery, often found in cars with start-stop technology, is designed to withstand many more cycles.
Each deep discharge causes irreversible damage to the lead plates inside the battery through a process called sulfation, where sulfate crystals build up and reduce the battery's ability to hold a charge. The more severe and frequent the discharges, the quicker the battery's capacity degrades. The best practice is to avoid a full drain whenever possible. Leaving your headlights on overnight is far more damaging than the cumulative effect of many small discharges from normal use.
For reference, here’s a general comparison of cycle life based on depth of discharge (DoD) for common automotive battery types:
| Battery Type | Typical Deep Discharge Cycles (to 80% DoD) | Typical Cycles for Shallow Discharge (to 20% DoD) |
|---|
| Standard Flooded Lead-Acid | 10 - 50 | 100 - 500 |
| Enhanced Flooded (EFB) | 100 - 200 | 500 - 800 |
| AGM (Absorbent Glass Mat) | 200 - 400 | 800 - 1500 |
| Lithium-Ion (in EVs/Hybrids) | 1000 - 3000+ | 3000 - 8000+ |
To maximize your battery's life, if you have a parasitic drain or don't drive often, using a battery maintainer (trickle charger) is the most effective solution. It keeps the battery at an optimal charge level without overcharging.
