Written By Michael Howard

When purchasing a new battery, it’s likely that you’ll be comparing different batteries from different manufacturers to decide which is the best quality for the amount of money that you want to spend.  In doing so it might be tempting to rely solely on the battery ratings listed on the package, and more specifically, the number of amp-hours the battery can support.  However, this rating system can be somewhat misleading, so it is important to fully understand how battery manufacturers come up with the numbers for their rating system.

The number of amp-hours that a battery can support is derived by taking the current flow and multiplying it by the number of hours the battery takes to discharge.  The current flow is measured in amperes, therefore, a battery that sustains five amperes steadily over 20 hours will have a 100 amp-hour rating.  Some battery rating systems may also include the charge rate.  This clarifies the number of hours it takes for the battery to discharge.  You may see on some labels something along the lines of “C/10” which would actually indicate that the 100 amp hours listed on the package are based on a discharge rate of ten hours.

Unless the manufacturer specifies on the package the number of hours that the battery takes to discharge, using the amp-hour system to compare batteries is not a good idea.  Consider this – a 100 amp-hour rating could be a current flow of ten amperes over ten hours, or it could be four amperes over twenty-five hours.  By comparing amp-hour ratings, you might actually be comparing apples and oranges.

There are other rating systems that can be used to make battery selection somewhat easier.  The cold-cranking amperage (CCA) is a rating system for starting batteries (used to start an engine) and this number refers to the maximum number of amperes that the battery can power for thirty seconds before it becomes unusable.  For instance, if the CCA is 500 then the battery is capable of delivering 500 amps for thirty seconds.  CCA is always measured at zero degrees Fahrenheit.  Be aware that this is different from marine cranking amperage (MCA) and cranking amperage (CA) which are measured on a different system.

For deep-cycle batteries, it is important to look at the reserve capacity of the battery.  The reserve capacity indicates the number of minutes that a deep-cycle battery can sustain a twenty-five ampere discharge.  Higher numbers will indicate a higher capacity for running more devices over a longer period of time.  This is the best way to compare deep-cycle batteries as it gives a much more realistic picture of what the battery can handle.  You might also find information on how many cycles the battery can sustain.  This refers to how many times the battery can be fully discharged and then recharged again, and will give some idea as to how long you can expect the battery to remain useful.

Just like any other product, batteries are packaged to make each one look like the best on the market.  Knowing what to look for and how to read the manufacturers’ claims about their products is a must if you are to make an informed battery choice.

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