How Long Does a 12V Rechargeable Battery Last? | Runtime by Load

There’s no single answer to how long a 12V rechargeable battery lasts because runtime hinges on a few variables: the battery’s capacity in amp-hours (Ah), the load you connect, its chemistry, and how deeply you discharge it. Most people searching for this want a practical estimate, not a physics lecture.

Here’s the straightforward way to think about it. The exact figure depends on whether you drain a lead-acid battery to 50% or a LiFePO4 battery to 80%, along with system efficiency. That range covers most real-world setups.

How To Calculate 12V Battery Runtime Yourself

You can estimate any battery’s runtime with a simple formula once you know two things: usable watt-hours and load watts.

Start by reading the battery label. It shows either amp-hours (Ah) or watt-hours (Wh). If you only see Wh, divide by 12V to get Ah. Usable watt-hours equals capacity times voltage times depth of discharge times efficiency:

Runtime (hours) = (Battery Ah × 12V × DoD × Efficiency) ÷ Load Watts

Work through an example with a 100Ah battery, a 100W load, 50% depth of discharge, and 90% efficiency:

100 × 12 × 0.50 × 0.90 = 540 usable watt-hours. Divided by 100W gives 5.4 hours.

Push that same battery to 80% discharge (common for lithium) and it becomes 100 × 12 × 0.80 × 0.90 = 864 watt-hours, or 8.6 hours. That’s the full spread for a common scenario.

12V Battery Runtime By Capacity And Load

The table below shows realistic runtime estimates across common battery sizes and loads, using 50% depth of discharge and 90% efficiency—the safe assumption for lead-acid and AGM batteries.

If the load runs through an inverter instead of direct DC, runtime shrinks. Inverter efficiency typically sits around 0.85 for a good pure sine unit and 0.80 for a modified sine model.

Why Chemistry Changes The Runtime Number

Depth of discharge—how much of the rated capacity you safely use—differs by battery type. Lead-acid and AGM batteries tolerate only about 50% discharge in daily cycling before their lifespan shortens noticeably. LiFePO4 and other lithium chemistries handle 80% discharge without that penalty.

That’s why two identically rated batteries deliver different real-world runtimes. A 100Ah lead-acid battery offers roughly 540 usable watt-hours under the safe 50% rule. A 100Ah LiFePO4 offers about 864 usable watt-hours at 80% discharge. The lithium battery genuinely runs longer despite the same label.

If you’re comparing options for an off-grid setup, the usable capacity matters more than the rated one. A smaller lithium battery often matches a larger lead-acid unit in practical runtime while weighing far less.

What Affects Real Runtime Beyond The Math

Estimates assume a steady load, and real life isn’t steady. Cold temperatures reduce capacity, aging cells hold less charge, and drawing high current from a small battery inflates losses. Health Canada’s battery safety guidance notes that rechargeable lithium-ion batteries are generally safe when charging instructions are followed, which is worth remembering when you push a pack hard.

Safety also matters for lead-acid chemistry. Exide’s instructions for VRLA batteries warn that they emit hydrogen and oxygen during charging and pose an explosion hazard in poorly ventilated spaces. Keep charging areas ventilated regardless of chemistry.

Three mistakes skew people’s estimates more than anything else: treating all 12V batteries as identical, using full rated capacity on lead-acid instead of the safer 50% limit, and forgetting inverter losses. If you meter the load in watts and confirm the battery’s true usable capacity, the formula above lands close to reality. For a roundup of tested models with real specs, our picks for the best 12V rechargeable batteries compare capacities and chemistries side by side.