Charge a Fully Discharged AGM Battery
 September 2, 2018
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      David Nash

 

Charge a Fully Discharged AGM Battery
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I learned about trick on how to charge a fully discharged AGM battery from the official Optima website.  The process shown in the video above mirrors the process recommended by the actual manufacturer.  You do need to be careful, but this is not something I cooked up in the basement.

AGM batteries are a little different than regular batteries, even deep cycle batteries should not be fully discharged.  Most battery chargers have electronics that prevent them from dangerously recharging bad batteries.  Completely dead AGM batteries can’t produce voltage to operate this safety device unlike deeply discharged traditional batteries.

What this method does is to trick your charger into thinking it is topping off a good battery while it is actually recharging a old dead one.

This does not always work, these batteries are not designed to be fully discharged and if they sit to long in this state there may be too much internal sufination to allow the electricity to recharge the battery.

Equipment Needed:

  • Battery charger
  • Jumper cables
  • Good battery (it doesn’t really matter what type as long as the voltage is the same – AGM or flooded cell just make sure both are 12 volt)
  • Deeply discharged AGM battery
  • Multimeter
  • Timer

Procedure:

  • Hook up the good battery and deeply discharged AGM battery in parallel
    • In parallel means connect positive to positive and negative to negative.
    • Do not have the charger connected to the battery yet
  • Hook up the good battery to the charger.
  • Turn on the charger.
  • Let charge for an hour
  • Check the AGM batteries.  If they seem hot to the touch their may be something wrong with the batter. Batteries normally become warm during charging so a little warmth is not something to be alarmed with.
    • Also discontinue the process if you hear the battery a hissing sound coming from the safety valves.
  • With your voltage meter, check to see if the AGM battery has charged to 10.5 volts or above. This should take less than two hours.
  • Once the “bad” battery reads 10.5 or more, disconnect the charger
  • Connect the “bad” AGM battery to the charger.
  • Turn on the charger and continue until the AGM battery reaches a full charge

In most cases, but not all, this will charge a fully discharged AGM battery.  There are professional AGM battery service centers that may be able to do more.  This is important to me as these batteries are very expensive.  I have three doe my solar set up and they cost over $300 a piece.