Article reviewed by Jesús Andrade, an electronics engineer with an MBA from ESIC. He combines 20 years’ experience developing emergency lighting and risk-prevention devices with a strong background in business management. Andrade is currently CTO at SVE Worldwide, a company specialising in industrial risk-prevention solutions.
One of the main differences between a rechargeable lamp and a traditional lamp is its battery. Thanks to it, the lamp can provide light without being connected to the mains, as long as it has enough charge. However, not all batteries used in portable lamps are the same. Some have a higher capacity than others, which can translate into longer battery life, although this also depends on how much power the lamp consumes.
Choosing a long-lasting rechargeable lamp is not simply a matter of finding the model that claims the highest number of operating hours. So, how many hours do you actually need? And how can you roughly calculate rechargeable lamp battery life?
Understanding these factors can help you choose a cordless lamp that suits the way you intend to use it. If you are mainly going to use a battery-powered lamp at home, you may not need exceptionally long battery life, and other features may be more important, as you will usually be able to recharge it easily. If, on the other hand, you plan to use it for long periods without access to a mains socket, battery life becomes much more important.
Contents
How to calculate rechargeable lamp battery life
Factors that affect the battery life of a cordless lamp
How much battery life do I really need?
LILAK: balancing battery life, efficiency and practicality
Frequently asked questions about rechargeable lamp battery life
How to calculate rechargeable lamp battery life
Many product specifications for devices with batteries include an approximate runtime in hours. This gives us an idea of how long the product can be used when it is running on battery power rather than being connected to the mains.
But how can that runtime be estimated? To understand this, we first need to know two battery specifications: its nominal voltage and its capacity.
A simple relationship allows us to estimate its nominal energy:
Energy (Wh) ≈ nominal voltage (V) × capacity (Ah)
These are the main elements of the calculation:
- Voltage (V): in this case, we mean the battery’s nominal voltage, a reference value associated with the voltage at which it operates. Depending on the battery configuration, values such as 3.2 V, 6.4 V or 12.8 V may be found.
- Ampere-hours (Ah): indicate the battery’s electrical charge capacity.
- Watt-hours (Wh): express an amount of energy. For a battery, they give us an approximate indication of the nominal energy it can store and are particularly useful when estimating battery life.
For example, if we have a battery with a nominal voltage of 3.2 V and a capacity of 5 Ah:
3.2 V × 5 Ah = 16 Wh
The battery would therefore have approximately 16 Wh of nominal energy.
Under ideal conditions and using a purely theoretical calculation, those 16 Wh could power:
- a 16 W device for 1 hour;
- an 8 W device for 2 hours;
- a 4 W device for 4 hours.
To estimate runtime, we therefore also need to know approximately how much power the lamp consumes:
Theoretical runtime (h) ≈ battery energy (Wh) ÷ power consumption (W)
In practice, these calculations are useful as a guide, but they do not necessarily represent real-world battery life. Not all of the battery’s nominal energy will necessarily reach the LED. There may be losses in the electronics and during energy conversion, as well as limits imposed by the battery management system itself. The condition of the battery, temperature, selected light level and other factors can also affect runtime.
Battery capacity and battery life are not the same thing
A higher-capacity battery can provide longer battery life, but Ah alone do not tell you how long a lamp will operate.
When comparing batteries that work at different voltages, it is more useful to look at their energy in Wh. To estimate operating time, we also need to consider how much power the lamp consumes in the lighting mode being used.
This is why two lamps with apparently similar batteries can provide very different runtimes.
Factors that affect the battery life of a cordless lamp
Once we know the approximate amount of energy available and the lamp’s power consumption, we can make an initial estimate of how long it may operate on its battery. However, this is still only an approximation.
Other factors relating to both the battery and the way the rechargeable lamp is used also need to be considered.
Battery charge level
Battery-life figures quoted by manufacturers are usually based on specific test conditions and, in most cases, on a fully charged battery.
If our 16 Wh battery is not at 100% charge, there will be less energy available and therefore less time during which it can power the lamp.
The exact conditions used to calculate battery life can also vary between manufacturers, so it is always worth checking the product specifications and, where stated, the light level at which the quoted runtime was measured.
Battery type and ageing
The chemistry of the battery is another important factor.
Rechargeable batteries are designed to withstand numerous charge and discharge cycles, but their available capacity may gradually decrease as a result of chemical ageing.
The number of cycles is not the only factor involved. Temperature, charging and discharging conditions, the passage of time and the way the battery is used and stored can also affect its lifespan.
As a result, a lamp may provide less battery life after several years of use than it did when its battery was new.
Light level and power consumption
Many cordless lamps offer several lighting modes that require different levels of power.
When you select a lower light level and the lamp consumes less energy, the battery can provide a longer runtime. If you use a higher setting and power consumption increases, battery life will be shorter.
You can think of it like running a race: if you use your energy at a faster rate, you will run out sooner; if you use it more slowly, you can keep going for longer.
A lamp works in a similar way. The more power it consumes in a particular mode, the faster it will use the energy stored in the battery.
How much battery life do I really need?
The amount of battery life you need from a cordless lamp will mainly depend on where and how you plan to use it.
| Use scenario | Battery life to prioritise | What to consider |
| 🏠 Everyday use at home | Moderate battery life | Ease of charging, adjustable light levels, size and ease of use |
| 📖 Reading or working | Depends on the light level used | Power consumption at medium and high settings, stable light output and easy adjustment |
| 🌙 Ambient lighting | Moderate battery life may be enough | Good performance at lower light levels and low power consumption |
| ⚡ Power cuts at home | Several hours of operation | Runtime at useful light levels and the option of alternative charging or power generation |
| 🌲 Extended use without mains power | Longer battery life | Battery capacity, power consumption and alternative charging or power-generation options |
| 🔋 Occasional portable use | Maximum runtime is not essential | Weight, size, portability and charging time |
If the lamp is mainly going to be used at home, where it can normally be recharged easily from a mains socket, you may not need to prioritise exceptionally long battery life.
It is also worth considering exactly how you plan to use a rechargeable table lamp. For reading or certain tasks, you may need more light on the working surface than you would for ambient lighting. If the lamp allows you to adjust the light level, using a lower setting will normally reduce power consumption and extend battery life.
If you plan to use the lamp for extended periods without access to mains electricity, longer battery life can become much more important. In these situations, having an alternative way to generate power or recharge the lamp can also be useful.
More hours of battery life therefore do not automatically make a lamp better. What matters is that the battery and the lamp’s power consumption provide enough runtime for the way you intend to use it.
LILAK: balancing battery life, efficiency and practicality
LILAK develops rechargeable portable lamps for domestic use, with different runtimes depending on the lighting mode selected.
Just as a runner adjusts their pace according to the distance they need to cover, users can adjust the light level according to how they want to use the lamp. A lower-power mode can extend operating time, while a higher setting provides more light at the cost of using the available energy more quickly.
To cover situations where users do not have access to mains electricity, including during a power cut at home, LILAK’s cordless lamp also includes a manual power-generation system using a dynamo.
The lamp is designed to strike a balance between battery life, efficiency and practicality across different use cases.
Frequently asked questions about rechargeable lamp battery life
Should a rechargeable battery always be kept at 100% charge?
It depends on the battery chemistry and the electronic system used. With some lithium batteries, remaining fully charged for long periods can contribute to faster ageing.
If the lamp is going to be stored for a long time, it is best to follow the specific instructions provided by the battery or product manufacturer.
Does a rechargeable battery lose capacity over time?
Yes. Rechargeable batteries undergo chemical ageing, and their available capacity may gradually decrease with time and use.
The rate at which this degradation occurs depends on factors including battery chemistry, charge and discharge cycles, temperature, and the conditions in which the battery is used and stored.
What should you do if a rechargeable battery stops charging?
If a battery has reached the end of its useful life or is behaving abnormally, you should not attempt to revive it using improvised methods.
Where possible, it should be replaced in accordance with the manufacturer’s instructions. If the battery is built into the product and cannot be replaced by the user, the appropriate course of action is to contact the relevant service centre and then dispose of the used battery through the appropriate recycling channels.