How to properly maintain AGM batteries during the winter

[john61ct]

Originally Posted by Eder

The AGM's (not Lifeline) in my rv have spent 10 years being stored 6 months at a time just disconnected. This includes winters in Alberta...temperatures drop as low as -40. So far no issues although my 500 AH house bank has lost a bit over the years.

Yes, allowing self-discharge past a month is not "damaging" per se.

But definitely affects longevity if otherwise well cared for.

Say if well coddled a quality deep cycle batt lasts 12 years, EoL being 65% SoH.

If stored isolated for 6 months per year without getting Floated or topped up, may lose say 2-4 years off that, depending on temperatures.

Assuming all other care factors optimized.

Each owner may well have a different cost/benefit preference, depending on the inconvenience, wealth, etc.

[john61ct]

traditionally "trickle chargers" are old school zero-stage garage units, potentially harmful.

A modern quality programmable general-usage high-amp charger will do just as good or better a job keeping your bank on Float, as

a specialized low-amp maintainer, no need to buy both.

[Maine Sail]

Originally Posted by john61ct

Yes, allowing self-discharge past a month is not "damaging" per se.

But definitely affects longevity if otherwise well cared for.

Say if well coddled a quality deep cycle batt lasts 12 years, EoL being 65% SoH.

If stored isolated for 6 months per year without getting Floated or topped up, may lose say 2-4 years off that, depending on temperatures.

Assuming all other care factors optimized.

Each owner may well have a different cost/benefit preference, depending on the inconvenience, wealth, etc.

John,

We are talking AGM's here... Can you show us a source for this info?

We have a slew of LA batteries here at the shop, many of which are too good to get rid of but not good enough to place back in service. They are often used as loaners, for testing etc..

Most of these are AGM's, some GEL's too, and many of them have sat in excess of 18 months to 2 years with no charging at all but all were fully charged when stored. I have noted changes in actual tested Ah capacity of less than .4% as far out as two years with zero storage charging.

Do you have testing data like this John? Do you own the equipment to test for this?

If I have a choice for storage, in a cool climate, to leave an AGM fully charged and fully isolated vs. throwing it on a maintainer the choice, 100% of the time, is to charge to 100% then disconnect it and let it sit. I do not see what you claim with AGM or GEL and I can and have tested for this.

Just for grins I just went out to the shop and checked on a used TPPL AGM that was last charged in Nov of 17 and it is resting at a solid 12.825V. The Lifeline AGM sitting next to it, last charged in Dec of 17, is resting at 12.816V (Fluke 289 NIST Calibrated). A Lifeline AGM battery only needs a boost charge when terminal voltage falls to 12.5V but I have yet to see that happen over any winter storage period provided the battery was charged properly in the fall.

We have numerous AGM banks out there, that have been stored disconnected for 6+ months every winter and that are in excess of 10 years old..

Usually in the spring, when I am recommissioning stored AGM's, the batteries are well in excess of 12.7V. Heck these two I just measured are both at 1 year +/- and still resting at over 12.8V... A 6 months storage for an AGM in a cool or colder climate is a cake walk.

Throwing it on any unattended charge source, during a winter storage time frame, only introduces and increases risk factors.

EnerSys:

"A fully charged ODYSSEY battery can be stored for up to two years at 77ºF (25ºC) from a full state of charge. At lower temperatures, storage times will be even longer."

For what it is worth the term "trickle charging" is the application of a low level intermittent or continuous constant-current charge intended to keep the battery at 100% SOC. Lifeline for example specifically advises against the use of "trickle" chargers with their AGM's.

[john61ct]

Thanks much for the correction Rod.

My point was that isolated is better than floating (much less trickling).

For my own banks when stored, I try to top up before they self-discharge more than 2-3%.

Is doing that just "more trouble than it's worth", or actually causing some sort of harm?

Are you saying that so many months of starting-Full then isolated storage without topping up, has zero impact on longevity?

Is it not the case that sitting at 98%, then self-discharging down to say 90% over time, is equivalent to **some** level of PSOC abuse?