Solar batteries lose charge over time, necessitating consistent recharging to power electrical systems and devices. But how long can solar energy remain in a battery?
Solar energy can be powered in a new battery for months or years, slowly self-discharging while not in use. The precise amount of time it takes for a battery to lose charge varies depending on its age, type, temperature, and how often it’s charged.
This article will discuss how long a solar energy battery retains its charge and the factors impacting its ability to retain a charge. Read on to discover how long it takes for a battery to self-discharge solar energy.
How Long Does a Fully Powered Solar Energy Battery Last?
A solar energy battery at 100% can retain a charge for between 20 and 66 months. Charge retention time depends on battery type, condition, and the temperature of its storage environment. Lithium-ion batteries stored in cool, dry environments retain solar energy longer than any other battery.
A fully powered solar energy battery will retain its energy for longer if it’s not in use because the self-discharge rate is always lower than the active discharge rate.
So, to determine how long a fully powered solar battery will last, you must determine whether the battery will remain inactive (only losing power via self-discharge) or be used to provide power to electrical systems and devices.
Solar Energy Battery Duration: When Not in Use
The amount of time it takes for a stored (not in use) fully powered solar energy battery to reach a zero charge depends on several factors, including its type, condition, and the temperature of its surrounding environment.
That said, every battery type has an average self-discharge rate. We can use these averages to estimate how long a fully-charged solar energy battery will last while not in use.
The chart below summarises how the self-discharge rate impacts the lifespan of stored solar energy inside a battery.
|Battery Type||Average Discharge Rate (Per Month)||Potential Maximum Stored Energy Lifespan (Full Charge to Full Discharge)|
As you can see, lithium-ion batteries are more likely to retain a charge (while not in use) for the longest period. In contrast, lead-acid batteries self-discharge more quickly, resulting in a stored energy lifespan.
That said, it’s crucial to note that most lithium-ion batteries self-discharge about 5% of their stored energy within the first 24 hours after being charged. Only after this point, their self-discharge decreases to 1.5%, decreasing more rapidly while in use.
It’s also important to note that the maximum stored energy lifespan is rarely ever reached. Various factors, such as heat, age, and internal damage, impact self-discharge rate and often prevent batteries from reaching the theoretical store energy lifespan high point.
But how long does a fully-charged solar battery’s energy stores last when you’re actively using it to power devices?
Solar Energy Battery Duration: When in Use
When actively using the stored energy in a battery, energy consumption is the greatest factor impacting the time it takes to reach full discharge (aka 0% battery level).
For example, a fully-charged solar energy battery with a capacity of 100,000 mAH can instantly reach a fully discharged state if connected to devices with a power consumption equal to or greater than that charge.
But the same battery could take hours or days to reach a fully discharged state if it’s connected to devices with a much lower power consumption rate.
So, if you connected a fully-charged 100,000 mAh solar energy battery to the average laptop, which consumes 60 watts per hour, it would take about 200 hours for the battery to reach a power level of 0%.
In short, the time it takes for the stored power inside a solar battery to deplete varies significantly depending on its total stored energy and the energy consumption rate.
Why Do Solar Batteries Lose Charge When They’re Not Being Used?
Regardless of the type of energy they store, all batteries lose charge over time, even when they’re not actively powering anything.
That’s because batteries constantly release energy via heat through self-discharging, which results from a chemical reaction inside a battery.
There’s no way to prevent a battery from self-discharging over time, as this is a natural process resulting from the battery’s internal components.
However, several factors can either increase or decrease the rate of a battery’s self-discharge.
Factors That Impact Solar Battery Self-Discharge Rates
Before calculating the length of time solar energy can remain stored in a battery, it’s vital to consider the factors that can impact that battery’s self-discharge rate.
The most influential of these factors include:
- Battery age
- Battery condition
- Battery type
- Ambient temperature
Let’s explore these factors in more detail to discover how they impact the self-discharge rate.
New Batteries Self-Discharge More Slowly
No battery lasts forever; new batteries tend to self-discharge more slowly than older ones. That’s because every battery, regardless of type, has limited charging cycles.
For example, most lithium-ion batteries can last up to 500 charge cycles, but high-quality ones can withstand up to 7,000 cycles. Like lithium-ion batteries, GEL batteries have highly variable cycle lifespans of between 1,000 and 5,500 cycles.
In contrast, the standard lead-acid battery can only last up to 1,000 cycles.
Essentially, the estimated number of cycles a battery has represents the number of times you fully discharge a battery or consume its total power capacity. The more cycles a battery completes, the more its self-discharge rate increases.
As a result, older batteries can self-discharge quickly, retaining almost no solar energy after only a few days, weeks, or months.
On the other hand, new batteries can retain a charge for several months or potentially years.
Consistently Charged Batteries Retain Charge Better
Allowing a battery to discharge fully can significantly decrease its lifespan while increasing its self-discharge rate.
Lead-acid and lithium-ion batteries are most prone to suffering internal damage upon reaching a full discharge state. Some may refuse to accept a charge upon reaching full or deep discharge.
GEL batteries aren’t nearly as sensitive to deep discharging, but fully consuming a GEL battery’s stored energy will result in a decreased charge capacity. This decreased capacity means that you won’t be able to store as much solar energy in the battery after a full discharge.
Leaving a solar energy battery in storage for extended periods without recharging can result in a full discharge, so it’s crucial to charge your batteries consistently. Doing so can prevent the battery’s self-discharge rate from accelerating prematurely.
Lithium-Ion and GEL Batteries Lose Energy More Slowly Than Lead-Acid Batteries
The type of battery you use can greatly impact solar energy storage duration. Lead-acid batteries self-discharge faster than lithium-ion and GEL batteries, meaning they lose stored solar energy far faster.
So, let’s say you had two batteries connected to a solar panel, a lead-acid battery, and a lithium-ion battery. Now let’s say that you charge these batteries to their maximum capacities.
Even if these batteries featured the same charge capacity, they’d lose stored energy at different rates, with the lead-acid battery likely self-discharging energy more quickly than the lithium-ion one.
In summation, if you’d like to retain solar energy for the longest possible lengths of time, you’ll want to invest in lithium-ion or GEL batteries instead of lead-acid ones.
Heat and Sunlight Can Increase the Rate of Self-Discharge
Low temperatures can make it challenging for your solar energy batteries to function at peak efficiency, essentially slowing the rate at which they release energy. However, high temperatures can have a much more damaging effect on solar energy batteries.
When exposed to high-temperature environments or direct sunlight, the internal reactions inside a battery increase in intensity and regularity. These reactions are responsible for a battery’s self-discharge rate.
Increasing the rate of these internal reactions is equivalent to increasing a battery’s self-discharge rate.
Heat can also lead to thermal runaway, which is when a lithium-ion battery self-heats to the point of destruction. For this reason, you’ll want to keep solar energy batteries stored in cool, not freezing, areas and away from sunlight.
How Long Can Solar Energy Remain in a Solar Power Bank Battery?
Solar power banks are essentially compact solar batteries that feature built-in solar panels.
Most solar power banks contain some type of lithium-ion battery. Although these batteries generally have a reduced charge capacity compared to standard solar power lithium-ion batteries, their self-discharge rate is similar.
As we’ve established, lithium-ion batteries lose about 5% of their charge within the first 24 hours of receiving a charge, then an additional 1.5% monthly.
Consequently, a solar power bank can theoretically retain energy for about the same amount of time as a standard solar energy lithium-ion battery, approximately 63 months at most.
However, because most solar power banks feature solar cells, which absorb heat energy via sunlight, the actual amount of time a solar power bank can retain a charge might be much shorter. After all, heat and light can significantly increase a lithium-ion battery’s self-discharge rate.
For this reason, storing solar power banks in cool, dry spaces away from sunlight is crucial when they’re not actively absorbing solar energy.
How To Extend the Storage Time of a Solar Energy Battery
Maintaining your solar energy batteries is key to increasing the maximum storage time for any solar energy you acquire.
To extend this storage time as much as possible, you’ll want to do the following.
Invest in Batteries With Long Cycle Lives
Investing in batteries capable of lasting thousands of charge cycles is one of the best ways to ensure your solar energy batteries retain solar power for the maximum amount of time.
High-quality lithium-ion batteries and GEL batteries are the best choices when it comes to maximum cycle life. Some of these batteries can withstand thousands of charge cycles, resulting in a much slower self-discharge rate and greater solar energy retention.
If you’re unsure what the estimated charge cycle number for your preferred solar energy battery is, don’t hesitate to contact the manufacturer for clarification.
Consistently Recharge Your Batteries
Remembering to recharge your batteries is crucial to avoiding full discharge.
Because full discharge, also called deep discharge, can decrease storage capacity and increase self-discharge rate, you won’t want to wait until a solar energy battery has 0% stored energy to connect it to a power source.
Store Batteries Away From Heat
While solar energy batteries store energy absorbed from sunlight, they’re not designed for long-term exposure to heat or light.
Keeping your batteries in a dark, cool space, such as a finished basement, can extend their lifespans and ensure that their stored solar energy is retained for longer periods. You also want to ensure you aren’t exposing your batteries to freezing temperatures.
Temperatures of 32 °F (0 °C) or below can cause the internal components inside a battery to freeze up, making it difficult for the battery to release or absorb energy. Extremely low temperatures can also damage a battery’s internal components, leading to decreased capacity and an increased self-discharge rate.
The ideal environment for solar power batteries is indoors and temperate, with a temperature ranging between 50 °F and 70 °F (10 °C and 21 °C).
All types of solar energy batteries, such as lithium-ion, lead-acid, and GEL, self-discharge while not in use. Lithium-ion batteries self-discharge more slowly than GEL batteries, and GEL batteries self-discharge more slowly than lead-acid batteries.
But a battery’s type isn’t the only factor affecting its ability to retain a charge. The rate at which a solar battery loses power varies depending on its age, type, temperature, and how often it’s recharged.
Still, a lithium-ion battery has a self-discharge rate of about 1.5% per month, while a GEL battery has a rate of about 3% per month.
- Panasonic Eneloop: What is battery self-discharge? And how do you counter it?
- Battery University: BU-802b| What does Elevated Self-discharge Do?
- EVreporter: Understanding self-discharge of a Lithium-ion battery
- UniEnergy Technologies: How Long Will A 100 AMP Hour Battery Last? (Calculation Method & Influencing Factors)
- PowerTech Systems: Lead Acid battery downsides
- ScienceDirect: Gel Battery
- UL Research Institutes: What is Thermal Runaway?
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