AGM Failure modes and ?Recovery?

[Captain Bill]

Recently my two lifeline 4d AGM batteries suddenly lost all of their capacity. They were six years old and I was starting to think about replacing them. About 10 days before they failed I decided to do a check on their capacity. My battery monitor said that overnight I had used 16% of their capacity and were at 84% so I disconnected them and let them sit for about 4 hours. I measured the voltage and consulted the Lifeline SOC and I was within .02 volts of 84% on the first battery and .01V on the second battery. They had not even rested the suggested 24 hours. I thought that was pretty good for 6 year old batteries. Ten days later, much to my surprise I got up in the morning and the voltage was sitting at just over 10 volts! My battery monitor said my minimum voltage overnight was 9.79. My understanding was that AGM batteries tended to die a slow death, losing capacity as they aged and that sudden failures were not characteristic. I charged them up to full SOC by running my engines all day on a long passage and the next day I was back down to 10V. Obviously these batteries had fallen off a cliff. After getting the back to full charge (accepting .005c amps at 14.4V) I went and bought an FLA battery to allow me to limp back to the states. I suspect that the problem was overcharging in that my solar controller would not go into float mode and was charging for many more hours than recommended for my batteries. I did not understand a setting in the controller called Float Cancel and left it where it was on the default AGM setting which was supposed to be 11.5 volts. When I dumped the program from the controller I found it was set at 12.5V, not 11.5. The effect was that everytime the voltage dropped below 12.5 volts the float for the next day was canceled and it stayed in absorption mode. The lifeline manual says that if discharge is less that 30% your controller should stay in absorption 2 hours, but I have never seen my amperage at .005C after two hours which is what it says it should be for 100% charge. My current thinking is that overcharging has depleted the electrolyte in these sealed batteries and there is no way to recover from this condition. Lifeline batteries can take a conditioning charge, i.e. equalization. My understanding is that this is to remove sulfation from the plates and will only exacerbate the problem from depletion of the electrolyte.

I would like to hear from some battery experts If I have likely interpreted the failure correctly and that a conditioning charge will not likely help me recover any capacity. The FLA battery I was forced to get is a hybrid type deep cycle 4D battery from NAPA which had been sitting on the shelf for over a year in Georgetown Exuma and it does not have much capacity. (I'm below 12V every morning under a load of 5a) I have been on this battery for two weeks and I measured my AGMs today and one is sitting at 12.9V the other at 12.7 but I don't believe they have any real capacity left. They are 210 AH rated ea. yet were at 10V after only about 70 AH was used from them.

Bill

[LeaseOnLife]

Hi Bill

I have Lifeline AGMs too, older than yours, my solar controllers are set to never go into float but stick with 14.4V. Even then, a good solar day barely charges the batteries to full (defined by 0.005C). A night motoring, with a successive sunny day will do.

The absorption time suggested in the manual seems to me a minimum time, to be verified by the 0.005C current at absorption voltage.

I think a conditioning cycle is in order, and even if above conclusion is wrong, you can't fill electrolyte back into these batteries, so the risk is low. Just make sure to disconnect your electronics when putting 15.x Volts to your batteries for 8 hours.