AGM Batteries ‘Depth of Discharge’?

Since purchasing and installing four new ‘AGM’ batteries for our domestic bank I have been puzzling as to what their level of charge, or discharge is. They came with a 5 year warranty, but I suspect that is conditional on how they are maintained, or abused!

Firstly at what point are they requiring to be re-charged, or in other words ‘How Low Can You Go’…?

The supplier states that:

AGM Leisure batteries can be drained using 80% of their capacity. They can also be re-charged almost 3 times as fast as a conventional standard wet flooded leisure battery.”

But what is 80% capacity, and how is it measured?

The state of charge is measured by the voltage of the battery bank, and this chart shows battery condition vs voltage reading.

We are recommended to:

Remain in the ‘Green’ zone to maximise the batteries life although occasionally dropping down to 11.66V is permissible.” 

Secondly what is the maximum rate of charging they will accept…?

Fast re-charging is very useful, but at what rate? I am advised to set the charge controller as follows:

  • Max charge current 30A
  • Max voltage 14.30V
  • (set charge controller to AGM or GEL batteries)

That’s all there is too it… so I am told.

Since their installation our battery overnight voltage, has been held above 12.10V; despite demands from lighting, TV, water pumping, and fridge freezer.

Long may it continue.

4 thoughts on “AGM Batteries ‘Depth of Discharge’?

  1. With 2 onboardsolar.co.uk solar panels I barely give a thought to battery levels. Just checked at 7am and my 3 year old bog standard batteries are at 12.8v.
    Even this early 0.6v going in and it’s a bit misty.
    Since I switched the panels back on in late February the lowest my SmartMeter has shown was 77% first thing in the morning after a day when it had been very wet. Normally fully charged again well before lunch and never been close to needing to run the engine.

    • Hi Steve, thanks for your response about our AGM battery bank. Wow you have a great power system to hold your set at fully charged overnight (12.8V)
      Our boat is drawing between 4~8amps during the evening when TV, Lights and Fridge Freezer are running. Daytime our 3x150a/h solar panels do raise the charge level but on the recent wet/dull days we needed to run the engine to raise the charge level to 12.8V
      John

  2. Hi both, glad your AGMs are performing as expected.
    My 4o0ah bank charges at around 60a for about half and hour after first starting the engine in the morning, that’s about 15%. The rule of thumb is 10% of capacity, but after the half hour the rate is dropping to below 30a so I don’t worry. On deeply discharged batteries I’ve seen a charge rate of 90 amps, but that was rare. Charge controller is set at 14.4v

    • Hi Geoff,
      Thanks for your comment, pleased to see you are no longer tethered to the ‘Gollen………We are keeping a close eye on our battery condition, looking forward to some solar energy to support the level of charge!
      John

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