AGM batteries going down or need more panels?

Re: AGM batteries going down or need more panels?

Welcome to the forum Aguilq59,

They sound OK so far... It would not hurt them to try and cycle them to 50% capacity, measure the resting voltage (remove loads/charging current and measure resting voltage after 3+ hours of no current flow), and recharge them again. Note that batteries which are "maintained" in float may not last as long as batteries that cycle down 10-20% or more every so often (it also depends on the battery design/construction--The are Lead Acid batteries designed for floating, and others for cycling).

A 200 AH string (24 volts, two 200 AH 12 volt batteries in series) should support moderate loads to 100AH (50% of capacity) pretty easily.

100AH / 6.5 amps = 15.3 Hours of run time to 50% of capacity

So--If you measure your operating battery voltage and get below ~23.0 volts (11.5 volts @ 12 volt bank) under your relatively light 6.5 amp load before you hit ~15+ hours--Then I would worry that your batteries are not long for this world. At ~15 hours, turning of the loads, the batteries should bounce back to ~24-24.2 volts (~50% of capacity at 77F/25C).

Also--to a degree--Battery life is based on what is acceptable to you. One battery mfg. calls their batteries end of life if they cannot support a 80% discharge (to 20% state of charge). Other mfg. might not call a battery failed if it support 25-35% or so state of charge (65-75% of capacity).

If your batteries "dying" at 15 hours of use is unacceptable to you--Then they should be replaced.

AGMs do have other issues... They have catalysts in each cell to recombine the hydrogen and oxygen (gasses from charging) back into water... Those catalysts do fail over time (the more the batteries "gas", the faster the catalysts fail). Once the catalysts fail, the batteries will begin to outgas/vent. And since you cannot replace the lost water, the batteries will fail and die.

Note that failing batteries can overheat... So watch battery temperatures, hot/melted cell tops, noise from venting gasses, etc. It is possible to overheat and cause batteries to catch fire in worst case situations. You can see Thomas had a battery beginning to melt down:

Ruined Battery,more for the dumpster

Monitoring charging voltages, charging currents, battery temperatures/voltages across batteries/cells--Can all give you early warning of failures (check at least once per month--In my humble opinion).

When making measures on batteries--You generally have several "identical" cells/batteries... When you see identical cells/batteries operating with different voltages/temperatures (specific gravities for flooded cell), you have your early warning and need to look further for the causes of the differences.

You can also look at the current needed to float a battery... In general, if the batteries are full and you see >~2% steady state charging current (200 AH battery string, >~4 amps) during float--Then I would suspect the batteries are at end of life and should be replaced (if for safety issues, if nothing else).

-Bill