How Long Will a 12V Battery Last? Runtime Guide for Campervan & Leisure Batteries

How long will a 12V battery last?

The runtime of a 12V battery depends on two main things: how much usable energy the battery has and how much power your appliances are using. A small load can run for many hours. A large load can drain a battery surprisingly quickly.

That is why there is no single answer to the question. A 100Ah battery running only LED lights can last much longer than the same battery running a fridge, heater fan and inverter-powered devices.

• bigger battery capacity usually means longer runtime
• higher appliance load means shorter runtime
• AGM and lithium batteries do not provide the same usable capacity
• inverter use can reduce runtime much faster than many people expect

The simple rule for battery runtime

A simple way to estimate battery runtime is:

Battery runtime = usable battery capacity ÷ current draw

For example, if you have 50Ah of usable battery capacity and your devices draw 5A continuously, the estimated runtime is about 10 hours.

This is a simplified calculation, but it is a useful starting point for campervan electrics, leisure battery systems and off-grid 12V setups.

What battery capacity actually means

Battery capacity is usually shown in amp-hours, written as Ah. This tells you how much energy the battery can store, but not all of that energy is always usable in practice.

For example, a 100Ah battery does not always give you a full 100Ah of usable power. The actual usable energy depends on the battery chemistry and how deeply you are prepared to discharge it.

• 100Ah AGM battery often gives around 50Ah usable capacity
• 100Ah lithium battery often gives around 90Ah usable capacity
• older batteries usually provide less real runtime than new ones
• cold weather can reduce effective battery performance

Visual example: battery size, load and runtime

The runtime of a 12V battery changes quickly depending on what you connect to it. A low-load setup may last all day, while a higher load can shorten runtime dramatically.

12V battery runtime examples

The examples below show why battery runtime varies so much depending on load and usable battery capacity.

Why AGM and lithium batteries last for different lengths of time

Two batteries with the same rated Ah capacity do not always last the same amount of time. Lithium batteries usually provide more usable energy than AGM batteries because they can be discharged more deeply without the same performance penalty.

This means a 100Ah lithium battery often feels much larger in real use than a 100Ah AGM battery.

• AGM batteries are often used to about 50% depth of discharge
• lithium batteries are often used to about 90% depth of discharge
• lithium voltage tends to stay more stable under load
• AGM batteries may feel weaker sooner as voltage drops

How appliance type changes runtime

Not all appliances use power in the same way. Some draw a small continuous current, while others cycle on and off or create short bursts of heavier demand.

Low-draw devices

LED lights, USB chargers and small electronics often use relatively little power. These usually allow long battery runtime.

Medium everyday loads

Water pumps, heater fans and routers may not seem large individually, but together they can add up quickly over a full day.

High-demand devices

Fridges, inverters and mains-powered equipment can reduce runtime much faster. These are often the loads that determine whether a leisure battery system feels large enough in real life.

Why inverter use drains a battery faster

Inverters can reduce battery runtime dramatically because they allow mains appliances to run from a 12V battery. That conversion process is not perfectly efficient, so the battery sees more demand than the appliance rating alone might suggest.

Even relatively short use of an inverter can consume a large amount of battery capacity, especially with appliances such as kettles, coffee machines, chargers, TVs or mains-powered tools.

• inverters add conversion losses
• high-power appliances create very high battery current draw
• short appliance runtime does not always mean low battery use
• inverter-heavy systems often need much larger batteries

Visual comparison: low load vs high load battery runtime

The same 12V battery can last very different lengths of time depending on the appliance load. This is why understanding real current draw matters more than guessing.

How long will a 100Ah 12V battery last?

A 100Ah 12V battery is one of the most common battery sizes in campervan and leisure setups, but runtime depends heavily on battery type and load.

As a rough guide:

• 100Ah AGM battery often provides around 50Ah of practical usable energy
• 100Ah lithium battery often provides around 90Ah of practical usable energy

That means:

• at 5A load, a 100Ah AGM battery may last around 10 hours
• at 5A load, a 100Ah lithium battery may last around 18 hours
• at 10A load, runtime drops to around 5 hours AGM or 9 hours lithium

How long will a battery last overnight in a campervan?

For many campervan owners, the real question is whether the battery will comfortably last overnight. The answer depends on what is running while parked.

A simple overnight setup might include:

• LED lighting
• phone charging
• fridge cycling on and off
• heater fan during colder weather

A modest battery may manage this for one night, but repeated overnight use without recharging can drain the system faster than expected. Fridge use and heater use are often the biggest overnight factors.

Why real battery runtime is often shorter than the estimate

Runtime calculations are useful, but real systems are never perfectly efficient. In practice, batteries often last less time than the simple calculation suggests.

• battery age reduces usable capacity
• cold weather affects performance
• voltage drop in wiring can reduce system efficiency
• inverters create conversion losses
• compressor fridges and motors do not draw perfectly steady loads
• charging habits affect battery health over time

Use the battery runtime calculator

The easiest way to estimate runtime properly is to use a calculator based on your real appliance load and battery size. This gives a much better result than guessing from the battery label alone.

How to make a 12V battery last longer

• reduce unnecessary appliance use
• replace higher-draw devices with more efficient ones
• limit inverter use where possible
• increase battery capacity if runtime is consistently too short
• add solar charging or improve daily charging opportunities
• avoid repeatedly over-discharging AGM batteries

In many cases, improving efficiency is just as valuable as installing a larger battery.

Related battery system guides

Electrical installation safety notice

This guide is provided as a general reference for estimating 12V battery runtime in campervans, motorhomes, leisure battery systems and off-grid electrical setups.

Real battery runtime depends on battery chemistry, battery age, depth of discharge, temperature, wiring losses, inverter efficiency and the actual electrical load connected to the system.

Battery estimates should be used as a planning guide rather than a guaranteed runtime value, especially where high-demand appliances or inverter-powered loads are involved.

Always confirm manufacturer specifications before selecting or operating AGM or lithium battery systems.