Rick's Free Auto Repair Advice

Car battery reserve capacity

What is car battery reserve capacity?

Automotive batteries are rated in three categories: cold cranking amps, cranking amps and reserve capacity. I’ll cover cold cranking amps, cranking amp in this post and concentrate on just reserve capacity here.

Automotive battery reserve capacity is a rating to battery reserve capacitygive you an idea of how long the battery can be discharged at a rate of 25-amps while still maintaining at least 1.75-volts per cell, or 10.5 total volts. A 12-volt battery is capable of outputting 2-volts for each of the six cells. The reserve capacity is measured at 80°F. So a battery with 100 RC rating is capable of providing 25-amps for 100 minutes. Since a lead acid battery works by way of a chemical reaction and chemical reactions slow in colder temperatures, you’ll get less reserve capacity time than the stated reserve capacity in cold weather.

Why do you care about reserve capacity?

Reserve capacity is more important for deep cycle applications like a battery used for an electric trolling motor. But it’s has some importance reserve capacity for cell phone chargerin automotive applications in cases where you’ve had an engine failure, are stranded and must rely on the battery to provide power for lights or to power your cell phone.

Reserve capacity is temperature and discharge dependent

As you can see in this chart, reserve capacity falls as the temperature goes down. But reserve capacity is also affected by the rate of discharge. As I mentioned above, a battery’s reserve capacity is based on discharging at a rate of 25-amps. But Peukert’s law states that a battery’s reserve capacity falls faster as the rate of discharge increases. So it’s not a linear rate. Draw more amps and you’ll run the battery down at a faster rate. To use the example above, if you use the same battery at the same 80°F temp, but draw 50-amps instead of 25-amps, you won’t get 50 minutes of power. The battery will drain at a faster rate due to the higher current draw.

battery reserve capacity
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