How Long Will My Battery Last? A Practical Guide to Battery Runtime 

One of the most common questions we hear at Battery Warehouse is: 

“How long will my battery actually last?” 

Whether you’re heading away for a weekend camping trip, spending a week in your caravan, powering a boat, or setting up an off-grid cabin, knowing how long your battery will run your equipment is essential. 

Unfortunately, there isn’t a simple answer. 

Two people can own exactly the same battery and get completely different runtimes depending on what they’re powering, the type of battery they’re using, the weather, and even how old the battery is. 

The good news is that once you understand a few simple principles, estimating battery runtime becomes surprisingly easy. 

In this guide, we’ll explain what affects battery runtime, show you how to estimate how long your battery will last, and highlight some of the most common mistakes people make when planning their power requirements. 

If you’re new to battery terminology, we recommend reading What Are Amp Hours? A Practical Guide to Choosing the Right Battery Capacity first. If you’re still deciding what size battery you need, our guide to How to Calculate the Right Battery Size for Your Needs is the perfect place to start. 

What Does “Battery Runtime” Actually Mean? 

Battery runtime simply refers to how long a battery can power your equipment before it needs recharging. 

For example: 

• A camping fridge. 

• LED lights. 

• A CPAP machine. 

• A fish finder. 

• A caravan water pump. 

• A laptop. 

Every appliance draws power from the battery. 

The more power you use, the faster the battery runs flat. 

Think of your battery like the fuel tank in your vehicle. 

A larger fuel tank doesn’t make your car faster—it simply lets you drive further before needing to refuel. 

A battery works in much the same way. 

A larger battery doesn’t make your fridge colder or your lights brighter. 

It simply allows them to run for longer. 

What Affects Battery Runtime? 

Many people assume that battery capacity is the only thing that determines runtime. 

In reality, several factors all play an important role. 

These include: 

• Battery capacity (Amp Hours) 

• Battery chemistry (AGM or Lithium) 

• Appliance power consumption 

• Ambient temperature 

• Battery age 

• Charging efficiency 

• How deeply you discharge the battery 

Understanding each of these factors will help you get a much more realistic estimate of how long your battery will last. 

The Four Biggest Factors 

1. Battery Capacity

The simplest rule is: 

The more usable capacity your battery has, the longer it can power your equipment. 

For example: 

Remember that battery capacity should always be considered alongside usable capacity. 

We’ll cover that shortly. 

2. How Much Power Your Appliances Use

Every appliance consumes electricity. 

Some consume very little. 

Others consume a surprising amount. 

The table below shows typical power consumption for common camping and caravan equipment. 

The more equipment you run at the same time, the faster your battery will discharge. 

3. Battery Chemistry Makes a Big Difference

This is where many people get caught out. 

Two batteries might both be labelled 120Ah. 

However, they won’t necessarily provide the same amount of usable power. 

For example: 

This means: 

A 120Ah AGM battery generally provides around 60Ah of usable capacity. 

A 120Ah Lithium battery may provide close to 100Ah of usable capacity. 

This is one of the main reasons lithium batteries often deliver significantly longer runtime than an equivalent-sized AGM battery. 

If you’d like to learn more, see our guide to AGM vs Lithium Batteries. 

4. Temperature

Temperature has a surprisingly large effect on battery performance. 

Cold weather slows the chemical reactions inside the battery. 

Very hot weather accelerates battery ageing. 

Neither is ideal. 

For campers, this means: 

• Winter camping usually reduces battery runtime. 

• Extremely hot summer conditions can reduce battery lifespan. 

• Fridges often work harder during hot weather, increasing power consumption. 

It’s one of the reasons battery runtime can vary significantly from one trip to the next. 

A Simple Battery Runtime Formula 

If you know two things: 

• Your usable battery capacity 

• Your average power consumption 

Estimating runtime becomes straightforward. 

Formula 

Battery Runtime (Hours) = Usable Battery Capacity (Ah) ÷ Average Current Draw (A) 

Example 

Battery: 

120Ah AGM 

Usable capacity: 

Approximately 60Ah 

Average fridge current: 

3A 

Calculation: 

60Ah ÷ 3A = 20 hours 

This gives you a useful estimate of runtime. 

Remember that every camping setup is different, so think of this as a guide rather than an exact prediction. 

Worked Example 

Let’s imagine you’re camping for the weekend. 

Your setup includes: 

Total average current draw: 

4.5A 

Using a 120Ah AGM battery with approximately 60Ah usable capacity: 

60Ah ÷ 4.5A = 13.3 hours 

Switching to a 120Ah Lithium battery with around 100Ah usable capacity: 

100Ah ÷ 4.5A = 22.2 hours 

Even though both batteries are labelled 120Ah, the lithium battery provides considerably longer runtime because more of its stored energy is usable. 

Why Two People Can Get Completely Different Results 

This is probably the biggest source of confusion. 

One camper might say: 

“My 120Ah battery lasted all weekend.” 

Another might say: 

“Mine barely lasted overnight.” 

Both people could be correct. 

Their setups might be completely different. 

Factors include: 

• Different fridges. 

• Different temperatures. 

• Different battery ages. 

• Different charging systems. 

• Different lighting. 

• Different appliance usage. 

This is why battery runtime calculators can only ever provide estimates rather than guarantees. 

Real-World Battery Runtime Examples 

Knowing the formula is helpful, but what most people really want to know is: 

“Will my battery last for the weekend?” 

The examples below are based on typical power consumption and are intended as practical guides rather than exact predictions. Actual runtime will vary depending on your battery’s condition, charging system, temperature and the appliances you’re using. 

How Long Will a 100Ah Battery Last? 

A 100Ah deep cycle battery is a popular choice for smaller camping setups and light marine applications. 

If you’re using an AGM battery, it’s generally recommended that you only use around 50Ah of its capacity to maximise battery life. 

A lithium battery of the same size can typically provide 80–90Ah of usable capacity. 

Typical Applications 

• Small camping setup 

• Fish finder 

• LED lighting 

• Phone charging 

• Small portable fridge 

Estimated Runtime 

*Approximate usable runtime based on around 50Ah usable capacity. 

How Long Will a 120Ah Battery Last? 

For many New Zealand campers, 120Ah is the sweet spot. 

It offers an excellent balance between capacity, cost and weight, making it one of the most popular sizes for camping, caravans and boats. 

One of our most popular AGM batteries is the Synergy 120Ah AGM Deep Cycle Battery, which is ideal for weekend camping and general recreational use. 

Typical uses include: 

• Compressor fridge 

• LED lighting 

• Water pump 

• Device charging 

• Small inverter 

• CPAP machine 

Estimated Runtime 

*Approximate usable runtime based on around 60Ah usable capacity. 

How Long Will a 200Ah Battery Last? 

If you’re spending extended periods off-grid or running multiple appliances, a 200Ah battery can provide a significant increase in runtime. 

For larger systems, Battery Warehouse often recommends either: 

• Remco RM12-200LFP Lithium Battery 

• Predator ED122200 AGM Deep Cycle Battery 

These batteries are commonly used in: 

• Caravans 

• Motorhomes 

• Marine applications 

• Off-grid cabins 

• Solar storage systems 

Estimated Runtime 

*Approximate usable runtime based on around 100Ah usable capacity. 

Lithium batteries of the same capacity generally provide considerably longer runtime because more of their stored energy is usable. 

Weekend Camping Example 

Let’s put everything together. 

Imagine you’re heading away for two nights. 

You’ll be using: 

Total average current draw: 

5 amps 

With a 120Ah AGM battery providing around 60Ah usable capacity: 

60Ah ÷ 5A = 12 hours 

At first glance this doesn’t sound like enough. 

However, in the real world many appliances don’t run continuously. 

For example: 

• Compressor fridges cycle on and off. 

• Water pumps only operate occasionally. 

• Lights are only used in the evening. 

• Phones charge for a short period. 

Your actual battery runtime is often much longer than simple calculations suggest. 

Caravan Example 

A typical caravan may run: 

• Compressor fridge 

• Television 

• LED lighting 

• Water pump 

• Device charging 

• Diesel heater 

Estimated daily consumption: 

Estimated daily usage: 81Ah 

This is why many caravan owners choose larger battery banks or lithium batteries combined with solar charging. 

Marine Example 

Marine batteries often have a very different duty cycle. 

Typical equipment: 

• Fish finder 

• Navigation electronics 

• Bilge pump 

• Cabin lighting 

Most of these appliances use relatively little power compared with camping fridges or electric cooking appliances. 

A quality 100Ah or 120Ah deep cycle battery is often sufficient for recreational boats, although larger vessels may require multiple batteries. 

AGM vs Lithium Runtime 

Another common question we receive is: 

“Will a lithium battery last longer than an AGM battery?” 

The answer is generally yes, even when both batteries have the same advertised amp hour rating. 

That’s because lithium batteries usually allow much deeper discharge without affecting battery life. 

If you’d like to understand these differences in more detail, read AGM vs Lithium Batteries. 

Why Battery Age Matters 

No battery lasts forever. 

As batteries age: 

• Capacity gradually reduces. 

• Internal resistance increases. 

• Charging becomes less efficient. 

• Runtime becomes shorter. 

If your battery seems to be lasting much less time than it used to, age may simply be the cause. 

This is one reason regular battery testing is worthwhile. 

Does Temperature Affect Runtime? 

Absolutely. 

Cold weather reduces the chemical activity inside the battery. 

Hot weather increases appliance demand—particularly for camping fridges—and can accelerate battery ageing. 

For example: 

A fridge running during a 35°C summer day will usually consume much more power than the same fridge operating on a cool autumn weekend. 

Bigger Isn’t Always Better 

Many people assume buying the largest battery available is the safest option. 

However, larger batteries are: 

• More expensive. 

• Heavier. 

• Slower to recharge. 

• Harder to install. 

The goal isn’t to buy the biggest battery. 

It’s to buy the right battery for your expected power requirements while allowing a sensible safety margin. 

This is exactly why calculating your daily power consumption is so important. 

If you haven’t already done so, our guide to How to Calculate the Right Battery Size for Your Needs walks through the process step by step. 

Frequently Asked Questions About Battery Runtime 

How long will a deep cycle battery last? 

This depends on three main factors: 

• The battery’s usable capacity. 

• How much power your appliances are consuming. 

• The type of battery you’re using (AGM or lithium). 

For example, a quality 120Ah AGM battery used for a camping fridge, LED lights and charging a few devices may comfortably last a weekend when paired with sensible power management. A lithium battery of the same capacity will often provide significantly longer runtime because more of its stored energy is usable. 

How long will a camping fridge run on a battery? 

There isn’t a single answer because every fridge is different. 

A quality compressor camping fridge typically draws an average of 2–4 amps, depending on: 

• The ambient temperature. 

• How often the lid is opened. 

• The thermostat setting. 

• How well the fridge is ventilated. 

Rather than relying on estimates, it’s best to calculate your own expected power usage before choosing a battery. 

Does a bigger battery last longer? 

Yes. 

A battery with more usable capacity will generally power your equipment for longer before needing to be recharged. 

However, bigger isn’t always better. 

Larger batteries: 

• Cost more. 

• Weigh more. 

• Take longer to recharge. 

The goal is to choose a battery that comfortably meets your needs without paying for capacity you’ll never use.  

Is lithium better than AGM for battery runtime? 

In most applications, yes. 

Lithium batteries typically offer: 

• More usable capacity. 

• Faster charging. 

• Lower weight. 

• Longer cycle life. 

• Better performance during repeated deep discharges. 

While AGM batteries remain an excellent choice for many camping and marine applications, lithium batteries often provide better long-term value for people who regularly camp off-grid. 

Learn more in our AGM vs Lithium Batteries guide. 

What drains a battery the fastest? 

High-power appliances generally have the biggest impact on battery runtime. 

Common examples include: 

• Compressor fridges. 

• Coffee machines. 

• Inverters. 

• Electric blankets. 

• Diesel heaters. 

• Televisions. 

• Laptop charging. 

Using several of these appliances at once can dramatically reduce battery runtime.  

Why doesn’t my battery last as long as it used to? 

If your battery’s runtime has noticeably reduced, there could be several reasons: 

• The battery is ageing. 

• It isn’t being fully recharged. 

• The battery has been repeatedly over-discharged. 

• The weather is unusually hot or cold. 

• Your appliances are consuming more power than before. 

If you’re unsure whether your battery is still performing as it should, Battery Warehouse can help you assess its condition and recommend the most suitable replacement if required. 

Battery Runtime Tips 

Getting the most from your battery isn’t just about choosing the right size. 

These simple habits can significantly improve battery performance and lifespan. 

✔ Recharge your battery as soon as practical after use. 

✔ Avoid repeatedly flattening AGM batteries. 

✔ Keep battery terminals clean and secure. 

✔ Check charging voltages regularly. 

✔ Store batteries fully charged when not in use. 

✔ Match your battery size to your expected power usage. 

Looking after your battery properly can often add years to its service life. 

Common Battery Runtime Myths 

Myth: Bigger batteries are always better. 

Not necessarily. 

Choosing a battery that is far larger than your actual requirements adds unnecessary cost and weight without always providing meaningful benefits. 

Myth: All 120Ah batteries provide the same runtime. 

False. 

Different battery chemistries and testing standards can produce very different real-world results. 

Always compare batteries carefully rather than relying solely on the advertised amp hour rating. 

Myth: My battery should last exactly as long as the calculator says. 

Battery runtime calculators provide estimates. 

Real-world conditions such as temperature, battery age, charging efficiency and appliance usage all influence the final result. 

Battery Runtime Checklist 

Before purchasing your next battery, ask yourself: 

What appliances will I be using? 

How many hours each day will they run? 

How many days will I be away from charging? 

Am I using AGM or lithium? 

Have I allowed a sensible safety margin? 

If you can answer these questions, you’re already well on your way to choosing the right battery. 

Battery Runtime Decision Flowchart

Continue Learning in the Battery Hub 

If you’d like to learn more about choosing and caring for your battery, these guides are a great next step: 

• What Are Amp Hours? A Practical Guide to Choosing the Right Battery Capacity 

• How to Calculate the Right Battery Size for Your Needs 

• AGM vs Lithium Batteries 

Together, these guides form a practical learning path that takes you from understanding battery basics through to choosing, using and maintaining the right battery for your application. 

Why Choose Battery Warehouse? 

At Battery Warehouse, we believe choosing the right battery shouldn’t be confusing. 

Whether you’re powering a camping fridge, caravan, boat or off-grid power system, our experienced team can help you select a battery that suits your application, budget and expected power usage. 

We stock a wide range of trusted AGM and lithium deep cycle batteries, backed by practical advice and nationwide delivery throughout New Zealand. 

If you’re unsure which battery is right for your setup, we’re always happy to help. 

Browse our range of Deep Cycle Batteries, explore our Lithium Batteries, or contact the Battery Warehouse team for personalised advice. 

Understanding battery runtime isn’t about remembering formulas. 

It’s about knowing: 

• How much power your equipment uses. 

• How much usable capacity your battery provides. 

• How different battery technologies perform. 

• How to plan with a sensible safety margin. 

Once you understand these basics, choosing the right battery becomes much easier. 

Instead of guessing how long your battery might last, you’ll be able to estimate your runtime with confidence and choose a battery that suits the way you actually use it. 

If you’re still unsure, the Battery Warehouse team is here to help.