For those without extensive electrical experience, buying a battery can feel a bit overwhelming. With all the different styles, sizes, and brands, it can be easy to throw up your hands and buy the one suggested by someone else. But understanding the specifications of batteries can go a long way toward helping find the correct battery. One specification you may have seen is battery reserve capacity. Here’s why it’s so important to understand for your next battery search.
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What Is Battery Reserve Capacity?
Reserve capacity is simply the time in minutes that a 12V lead-acid battery can sustain a 25 amp load and remain above 10.5 volts.
When evaluating batteries, it’s essential to make sure buyers are comparing apples to apples. That’s where battery reserve capacity (also called RC) comes in. The longer a fully charged battery runs before dropping below a specific voltage, the higher the battery reserve capacity. Most types of traditional lead-acid batteries display this measurement.
At a high-level reserve, capacity is a more accurate measurement of how long a lead-acid battery will last under a sustained load than its amp-hour rating. This rating will be much lower than the actual capacity of the battery primarily due to the Peukert effect.
Need a refresher before we dive in? Check out Amps, Volts, and Watts: Differences Explained in Simple Terms
What Do CCA and RC Mean on a Battery?
RC stands for reserve capacity and is commonly seen on deep-cycle lead-acid batteries. It’s critical to note that reserve capacity measurement is done at 80 degrees Fahrenheit — optimal temperature conditions for a battery.
CCA is a completely different measurement that is specific for vehicle starting applications. CCA stands for “Cold Cranking Amps”. As opposed to battery reserve capacity, CCA is measured in amps, not minutes. It shows how many amps a battery will deliver over 30 seconds at 0 degrees Fahrenheit without dropping below 7.2 volts. This is critical information to know when starting a vehicle to make sure the engine gets enough power from the battery to turn over in less than ideal conditions.
It’s important to note that CCA has nothing to do with battery capacity and will not indicate if a battery is useful for deep discharges. A starting battery is constructed very differently than a deep cycle and is intended for short bursts of energy only followed by an immediate recharge.
How Is Reserve Capacity Calculated?
Battery reserve capacity measures time so you will see it depicted in minutes. To calculate the RC of a battery, the battery has to have a full charge first. Then, manufacturers draw 25 amps of power from the battery at 80 degrees F until it drops below 10.5 volts. The number of minutes it takes for this to occur is the battery’s reserve capacity.
What Is A High Reserve Battery?
Much like the name implies, high reserve batteries are batteries that can provide reserve capacities higher than average. These batteries often produce a lower but still usable charge, meaning the overall capacity will last longer.
These are useful for those who consistently use large amounts of battery capacity between charges. They’re also good for those who may leave their batteries unused for extended periods. High reserve batteries are more likely to retain some energy even after sitting idle for a while. This helps avoid unexpected dead batteries.
Is Reserve Capacity the Same as Amp Hours?
No, these are separate measurements that reflect different things. For one, reserve capacity is a simple measure of time, while amp-hours measures the number of amps a battery can provide over an hour-long period.
However, these two measurements are related, and you can convert one to the other. Divide the RC by 60, and then multiply this number by 25 to obtain the amp hours. If you have the amp hours, divide this number by 25, and then multiply that number by 60 to find the battery reserve capacity.
Keep in mind that this doesn’t quite mean equal energy, as the measurements and conversions don’t take voltage into account.
Do Lithium Batteries Have Reserve Capacities?
Lead-acid batteries will see a lower reserve capacity due to the 25-amp draw and the Peukert Effect. The Peukert Effect shows how traditional lead-acid batteries see decreased capacity as the rate of discharge increases. High-quality lithium like our Battle Born line does not suffer significantly from the Peukert effect and the amp hour rating of the battery is the actual amount of charge you can get from the battery under most conditions.
There’s another important thing to note about maxing out battery reserve capacity levels in lead-acid batteries. Reaching the full RC will draw the battery down to 10.5 volts, which is lower than 50% of charge. This is a level that will dramatically shorten lead-acid battery life if reached regularly. Therefore, consider RC more of an upper limit than a target range.
While it’s not a published number you could say our 100AH battery has a 240-minute reserve capacity even though the battery will never go below 10.5 volts. The internal BMS would shut it off before then. It’s important to keep in mind that a 240-minute drain on a Battle Born will not harm the battery as it would with a lead-acid battery.
Why Is It Important to Know About Battery Reserve Capacity?
RC is important only when looking at lead-acid deep cycle batteries intended for use in a long discharge situation. For engine starting applications, RC has no benefit and CCA is the measurement that is more important.
When choosing a battery, it’s critical to know whether it’ll meet day-to-day needs and how far it can push in emergencies. Battery reserve capacity is one of the most valuable ways to quantify this with lead-acid. While different manufacturers and types of batteries may each promise confusing or unclear benefits, RC makes it simple to make an apples-to-apples comparison.
While both CCA and RC numbers are just a small part of choosing the correct battery (along with type, brand, size, and other considerations), it’s vital to understand their roles.
Want To Learn More About Electrical Systems and Lithium Batteries?
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