AGM Battery Useable Capacity?


Hi Jeff, a quick question regarding the batteries you installed earlier this year. I believe you said the usable range was between 35% and 85% charged or roughly -110Ahr and -500Ahr for the 780Ahr batteries I have. Is this correct?


Hi Ernest,

Your numbers are close for AGM batteries.

The top end of the range reflects the fact that under the absorption rate of charge the time to charge the last 15% of battery capacity is time-intensive and unproductive. As such, you are better to stay within bulk charging which is between 85% and 30%.

  • The top end of the range, while cruising will (780 AHrs * -15% = -117 AHrs).
  • The bottom end of the range should never be below 30% (780 AHr * -70% = -546 AHrs)

Basically your range is -117 AHrs to – 546 AHrs.