What should I look for when buying deep cycle batteries?

The most important consideration in buying a deep cycle battery is the Ampere-Hour or Reserve Capacity rating that will meet or exceed your requirements. Most deep cycle batteries are rated in discharge rates of 100 hours, 20 hours, 10 hours or 8 hours. The higher the discharge, the lower the capacity due to the Peukert Effect and the internal resistance of the battery. Reserve Capacity (RC) is the number of minutes a fully charged lead acid battery at 80˚ F (26.7˚ C) can be discharged at 25 amps before the voltage falls below 1.75 volts per cell (100% DOD). To convert RC to Ampere-Hours at the 25 amp rate, multiply RC by .4167. More ampere-hours (or RC) are better in every case. Within the same battery footprint or industry group size, the battery with higher ampere hours (or RC) will tend to deliver longer cycle lives.

The “Battery Council International” manual BCIS-05 Rev. Dec02 provides some guidance regarding comparing amp hour capacity relationships. In it they state that “for guidance in establishing rates, ampere hour capacity relationships are approximately”:

    * 20 hour 125%
    * 6 hour 100%
    * 5 hour 95%
    * 4 hour 89%
    * 3 hour 82%
    * 2 hour 72%
    * 1 hour 55%

Finally, the available space and any weight restrictions will have to be considered in determining the appropriate battery.