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Understanding Battery Ratings

by Chris - March 23rd, 2011.
Filed under: Solar Education. Tagged as: , , , , , , .

A battery rating is a measure of how much energy is stored in the battery. Measuring the capacity of a battery is a difficult thing to do because batteries are usually built for a specific application. Also, the amount of charge they hold varies drastically with temperature – a batteries capacity decreases as it gets colder and increases as it gets warmer.

Battery Rating Basics – Basic Amp Hour Calculation

The most basic way of describing a batteries capacity is with the following equation:

$Capacity / Cycle Time = Battery Rating$

Where Battery Rating is in amps, Capacity is in amp-hours, and Cycle Time is in hours. Here is an example:

A battery with 100 amp-hours of Capacity can deliver 1 amp (Battery Rating) for 100 hours (Cycle Time). This would also be known as the C/100 rate. Likewise, the same battery should be able to deliver 100 amps (Battery Rating) for 1 hour (Cycle Time), which could be described as the C/1 rate.

While this works out to be a nice mathematical formula, it’s not entirely accurate. Batteries are able deliver current for longer if they are discharged at a lower rate (less amperage). Likewise, they deliver less current than the formula calculates for high discharge rates. To model this ‘non-linearity’ engineers use Peukert’s equation. However, the above equation is a good-enough rule of thumb for every-day use.

Understanding Amp Hours

Almost all batteries will be labeld with a rated ‘Amp-Hour’ capacity. This is closely related with the above equation, but is not exactly the same thing. The Amp-Hour (AH) rating labeled on batteries is actually its C/20 rate or 20-hour discharge rate. As long as the battery is discharged over a 20 hour (or longer) period, it will delivery 100% of it’s rated capacity. If it is discharged in less time (at higher currents), it will deliver less capacity.

Continuing our example above, a battery rated for 100 amp-hours can deliver 5 amps over a 20-hour period.

Cold-Crank Amps – Car Battery Ratings

Another very common rating, especially for starting batteries, is the Cold-Crank Amps or CCA rating. I feel obligated to point out that starting batteries are not the type of battery that you want to use for a solar system, but I’ll cover this rating to be thorough. Please note that car battery amp hours are a significantly different measurement than cold crank amps.

The CCA auto battery rating is the amount of current that a starting battery can discharge for 30-seconds at a voltage greater than 7.2 volts. Note that this assumes the battery is fully charged and at 0°F (-17.8°C).

There are other forms of this rating such as Cranking Amps (CA), Marine Cranking Amps (MCA), and Hot Cranking Amps (HCA). These are all slightly different variations of the CCA rating. Here is the conversion:

 Rating Amp Rating at °F (°C) Conversion to CCA Cranking Amps (CA) 32 (0) CCA = CA * 0.8 Marine Cranking Amps (MCA) 32 (0) CCA = MCA * 0.8 Hot Cranking Amps (HCA) 80 (26.7) CCA = HCA * 0.69

Reserve Capacity

Most deep-cycle batteries are rated at a reserve capacity, measured in minutes. Deep-cycle batteries are the best for solar applications and this rating is the most informative for solar powered home application. The reserve capacity is the amount of time that a fully charged battery at 80°F (26.7°C) can deliver 25 amps until the voltage falls below 10.5 volts.

How To Convert Reserve Capacity To Amp Hours

When looking for a deep cycle battery for your solar application, sometimes a manufacturer will rate the batteries in amp-hours and sometimes they’ll rate them in reserve capacity. Here is an example to help you convert between the two:

Using the equation at the top of the page, we know that a 100 AH rated discharged at 25 amps should last 4 hours or 240 minutes:

$100 AH/25 A = 4 H = 240 min$

Thus, it’s reserve capacity is 240 minutes. However, since 25 amps is quite a bit higher than the 20-hour rating (of 5 amps), we know that we’ll actually get less than 4 hours.

Amp-hour, Cold Cranking Amps, and Reserve Capacity are the most common battery rating that you’ll see. There is a plethora of other standardized ratings out there for special applications, but for solar applications, you’ll want to pay the most attention to the amp-hour and reserve capacity ratings. These will give you the best estimate as to how long you’ll be able to power a load with a battery.

1 Response to Understanding Battery Ratings

1. A succinct and comprehensible battery rating discussion