tom-slattery avatar image
tom-slattery asked

AGM Super Cycle Four Bank Significant Reduced Capacity

I installed 4 x 145ah Super Cycle AGM's 18 months ago (Still under Warranty). They were marvelous giving us days service before needing charge. Usual discharge to 12 volts.

This week suddenly we have extremely low capacity and using our normal charge methods (Sterling shorepower 40A charger set to AGM) or the boat engine alternator. Absorption charging at 14.6V and 14.4V respectively. The shorepower drops to 13.6V for float.

I only seem to be able to get 50ah storage into batteries (using NASA Marine Clipper BM2 battery monitor) and then the charge current is down to 1 or 2 Amps within 4 hours or so.

Why would this have happened to four new batteries and so suddenly?

Help very much appreciated.

battery chargingAGM Batterybattery capacity
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Tom Slattery

It is possible it is not all four that are a problem but the two on the ends. They are often the ones who work the hardest in the bank. They may be pulling down the whole bank. But from what you are describing they are at end of service life.

In agm batteries closer to 18 months sulfation is always and issue. Sulphation is caused by a high depth of discharge and not being charged up with enough absorption time to reverse the battery chemistry and bring it to true 100%. These are the two main problems I see in customers agms banks.

At 12v assuming no heavy load drawing your batteries down to around 25-35% capacity possibly even lower. Agms and gel like to be charged everyday with a long absorption time, so they would have sulfated causing the capacity drop.

On the datasheet you get 700 cycles at 40% at a cycle a day that is just under two years. You are drawing lower than that so the life span will be much less. At lower DOD 300 cycles or less than a year.

We always recommend lithium batteries for this reason, as their cost per cycle makes them cheaper than gel or agm even though their initial outlay seems more expensive.

This book makes for interesting reading if you like the technical stuff.

A short video that sums it up nicely.

Or this article will help you understand more about battery care.

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tom-slattery avatar image tom-slattery commented ·

Thank you Alexandra. I will disconnect each battery and see if one is dragging all down.

The warranty is for 24months and Victron say expected lifespan is 7 to 10 years which is quite different than what you are saying.

They were definitely not mischarged. They are on a charger for AGMs with Absorption, float and storage automatic outputs. This seems to be working fine!

I am reading the references you sent thank you very much.

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neoneddy avatar image
neoneddy answered ·

On the charging side, how are you determining full charge? You want to see .05 C tail off current. Here is a good video showing that and why.

Also, if you don't have temp compensating chargers, 14.6v could be too high if the batteries are too warm. You could have cooked off all the water in the glass mat.

Here is what I'd do under the assumption that right now you don't have much to lose.

1. Disconnect all batteries and get a sense for if any of them are bad. Check OC Voltage.. at full charge you want 12.8 or more. Put a load on the battery for a little bit, then recheck.

2. Attempt equalize charge , usually not recommended for AGM, but it might revive them. You might have a few low cells in the 12v battery. So charge up at 15- 15.5v .. you might hear bubbling / gurgling. Watch for smells, and batteries getting warm, don't want that.

3. Attempt to re-water. You can pop some of the sealed caps / covers off and replenish water. It won't be like an FLA, but the moisture will work it's way back in there.

I have 22kw or so of AGM Here is my bank, the main part has been going strong for over 18 months. They are good for 7-10 years, maybe more. Just don't let them sit discharged for long periods of time. (I try and make sure they hit absorb voltage once a week minimum, usually every 3 days.

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tom-slattery avatar image tom-slattery commented ·


Thank you.

The charger is temperature compensating.

Initial disconnection and looking at OC volts showed all ok. But now am just disconnecting them again and looking to charge them individually to see if there is a variation.

As they are still under warranty I will not interfere with the batteries themselves.

For now I would love to nail it down to one battery as it just seems too much for all four to be gone at once!

Thank you for input!


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billknny avatar image
billknny answered ·

If you are routinely discharging them down to 12.0 Volts, that is a REALLY deep discharge. Maybe down to 20% SOC? (without knowing the draw-down rate corresponding the that voltage measure it is hard to be sure). If you have done that every day for 18 months, you might just be at the end of the normal cycle life of the batteries.

You might get 7 years out of these if you cycle them from 100% to 70%. Not between 90% and 20%

Especially if you only recharge to a tail current of 1 to 2 amps. That's not full.

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