Battery Reserve Capacity Explained: What It Is & Why It Matters
When there are endless battery options in the market, it can be daunting to choose the right kind. However, the process can be made less tiring by understanding battery specifications such as battery reserve capacity. These metrics help you determine whether a battery is enough to fulfill your power needs.
The concept of reserve capacity may seem complicated with so many things involved like amps, voltage, time, etc. But, no worries. I made sure to simplify the concept, so you can get the hang of it in no time. So, let's see what reserve capacity is and how you can estimate your required RC.
What is battery reserve capacity?
Battery reserve capacity (RC) is one of the many specifications to consider when buying a battery. So, what exactly is RC on a battery? Simply put, it is the measurement (in minutes) of how long a 12V battery will supply 25 amps before dropping to 10.5V. If a battery has a reserve capacity of 200, it means that it can provide 25 amps for 200 minutes before dropping to 10.5V.
When talking about battery specifications, another important concept comes up, which is Cold Cranking Amps. It measures a battery's capacity to ignite an engine in cold weather. Cold Cranking Amps tells you how many amps a 12V battery may supply for 30 seconds at -17.8 °C before going down to at least 7.2V per cell.
Low temperatures can impact your battery's performance, so learning about CCA can help you pick the right battery that won't leave you stranded when the temperature falls below freezing point.
With that said, the higher the CCA rating on a battery, the more power it has to start an engine in cold weather.
How is battery reserve capacity affected by Peukert's Law?
Peukert's law mathematically explains that a battery's capacity decreases as its discharge rate increases. When you operate your battery on a high discharge rate, its internal resistance produces voltage sag that shortens the time it will last. Peukert's formula is given as:
t = H ( C⁄IH )k
where,
- t is the actual time in hours to discharge the battery.
- H (in hours) is the rated discharge time.
- C (amp-hours) rated capacity at the given rate of discharge.
- I is the actual discharge current in amperes.
- K is the Peukert's constant. It's different for every battery type. For instance, it is 1.1-1.23 for gel batteries and 1.05-1.15 for AGM.
Peukert's law applies to the lead-acid, AGM, and gel batteries but Lithium-ion batteries hardly experience this effect. But why is that? Lithium-ion batteries can discharge 98% of their energy while other batteries like lead-acid cannot. Lead-acid can only have around 80 to 85% of disposable energy.
How is the capacity of a battery reserve measured?
Measuring a car battery reserve capacity is pretty straightforward. Here's how a 12V lead-acid battery's RC is calculated.
- Initially, charge the battery to 100 percent.
- Start a clock and begin drawing 25 amps of energy from the battery at 80°F.
- As soon as the voltage goes below 10.5V, stop the clock.
Now the total minutes for which your battery could supply 25 amps is your RC rating.
Various factors affect battery RC. Let's see what they are and what impact they have.
- Temperature: Lower temperatures decrease RC while hotter climates increase it.
- Usage habits: Deep discharging your battery frequently can lower its reserve capacity while shallow discharge can maintain it.
- Maintenance: Fulfilling your battery's maintenance requirements also helps prevent lowering its RC over time.
- Age: As your battery gets older, its reserve capacity reduces because of wear and tear.
- Discharge rate: A higher discharge rate can also be one of the reasons for your battery's lowered RC.
Another thing to keep in mind is that reserve capacity varies for different types of batteries.
Battery reserve capacity vs. amp hours
Amp hour is another metric to consider when buying a battery. While reserve capacity tells you the time for which your battery will consistently supply at 25 amps, amp-hours give you insight into how many amps a battery will provide in 1 hour.
Hence, Ah rating is crucial when it comes to determining what load you can connect to the battery and how long your battery will last. So, if your battery is labeled 100 Ah, it can provide 100 amps for one hour, 50 amps for two, and so on.
Now, let's say you're going on a trip in your RV and need to determine what size battery you need for RV. For this, you can use amp-hours. You would consider the amperage of the appliances in your vehicle and roughly how long you'll be powering them.
For instance, you have a 5 amp AC, 5 amp RV fridge, 1 amp light bulb, 6 amp electric kettle, 0.5 amps phone charger, and an 8 amp toaster. Adding amperage for all the appliances you'd get 25.5 amps. Multiply the result by running time, eg. 10 hours, to get the required amp-hours. For this example, the needed amp hours are 255.
While you'd use Ah to determine the battery size you need, reserve capacity will give you an idea about how long you can use your battery at its initial voltage (eg. 12V for 12V batteries).
How are Ah and RC related?
Even though both are different, amp hours and reserve capacity are somehow related. Let's see that relationship using a mathematical formula.
- Ah = (RC / 60) x 25
Using this formula, you can also convert Ah into RC and vice versa. For instance, if I'm given 150 minutes RC and need to convert it to amp hours, I would simply divide it by 60 and multiply by 25.
- Ah = (150 / 60) x 25
- Ah = 62.5
How do you estimate the appropriate battery reserve capacity for your needs?
To estimate the appropriate battery reserve capacity for your needs, simply calculate for how long you need 25 amps of current. First, find the amperage of your devices. For instance, you have a 0.5 amps light bulb, 1 amp ceiling fan, 6 amps fridge, and a 5 amps microwave. That comes out to a total of 12.5 amps.
Now, how long will you need these devices? Let's say 2 continuous hours during a power outage. So, you would need a total of 25 amps for 2 hours or 120 minutes. With that said, my required reserve capacity is 120 minutes.
Do lithium batteries have reserve capacities?
Yes, lithium batteries have reserve capacities and they're higher than traditional lead-acid ones. This is because the Peukert effect has a lesser impact on lithium batteries as compared to lead-acid.
For instance, a 12V 100Ah lithium battery has a reserve capacity of 240 minutes while a 12V 100Ah lead-acid battery will last about 170 to 190 minutes.
A good quality lithium-ion battery, like the ones offered by Renogy, is a great alternative to traditional lead acid batteries due to its efficiency and portability. It's basically more power while occupying less space. Renogy offers a wide range of lithium batteries of varying sizes and numerous features.
Importance of battery reserve capacity
Understanding battery reserve capacity is important as it relates to dropping voltage, and voltage is directly related to power. Therefore, when voltage decreases so does power affecting the energy supply to your devices.
Also, if you use less voltage, the required appliances will draw more current producing extra heat that may lead to the motor of your devices burning out. Hence, knowledge of battery RC can indirectly save you from spending hundreds of dollars on appliance repairs.
Further, learning about reserve capacity can save your RV trips and have your back in power outages. For example, you can consider your power needs for your trip and then go for a battery that has enough reserve capacity to fulfill your energy needs throughout the whole journey.
Conclusion
Battery reserve capacity tells you for how long your battery can supply 25 amps before its initial voltage drops. Numerous factors like maintenance, temperature, discharge rate, and aging can impact RC.
Reserve capacity is a crucial metric that helps you make informed decisions when purchasing a battery. It also gives you an idea of what appliances you can power using your battery and for how long. This saves you from running out of energy while on camping trips or when there's a power outage.
Also, when you choose the battery with the appropriate RC according to your needs, it provides the required voltage consistently saving your appliances from burning out. Therefore, learn the concept of reserve capacity to choose the right battery for a hassle-free journey in your RV.
Frequently asked questions
What is a good reserve capacity in a battery?
A good reserve capacity depends on your needs. Whatever fulfills your power needs will be an adequate RC for you. For example, if you need 25 amps for 150 minutes, a reserve capacity rating of 150 would be ideal for you. However, one aspect to consider is how many amps a battery supplies for a dip in the voltage. Usually, an ideal amount is 25 amps.
What does poor reserve capacity mean for a car battery?
A poor reserve capacity means the rating is lesser than what you need, resulting in voltage dropping rapidly. And if the voltage goes down quickly, you won't be able to provide enough energy to your car since voltage is directly related to power. This could lead to inefficient use of energy or your car may not function as expected.
Why do lithium batteries differ in reserve capacity?
Lithium batteries differ in reserve capacity from other batteries like lead-acid, due to their experience with the Peukert effect. Since lithium batteries experience much less Peukert effect, their reserve capacity is not much reduced from higher discharge rates.
