Winter Battery Charging Question

RCB · December 2021

At present my AGM batteries stay charged over the winter from the solar panels. They are 100%, or near as possible, charged all winter. If i did not have solar, the batteries would have been fully charged and disconnected when the trailer was stored and allowed to gradually self discharge over the winter. This was the procedure on my boats and the batteries would discharge to about 12.4 - 12.5.

My question, is continual charging or allowing gradual discharge over the winter better for battery life/performance and why ?

Thanks, and Happy Holidays to all.

pthomas745 · December 2021

This is internet rabbit hole sort of stuff. There is no "real" evidence either way that a long slow self discharge or continuous charging makes a big difference on battery life. The battery I just took out of my trailer lasted from 2016. Battery was disconnected when the trailer was stored for a couple of years. Constantly on and connected to my solar panel for the last couple of years in the back driveway. (This was a stock lead acid). In the years I used it, it rarely, if ever, approached 50 percent charge.

The main way to damage a battery and shorten its life is to bring it to a low "depth of discharge" state and leave it there.

The only thing mentioned in my favorite Battery FAQ is this note about "depth of discharge". Whether or not leaving my battery switch on most of the time with the solar connected gave me a "5 percent" discharge cycle is hard to say. And everything about a battery is very relative, as far as counting the number of "cycles" a battery has left. (The old battery is sitting in the garage, holding a charge very well. The self discharge rate "appears" to be a bit higher this winter compared to a couple of years ago, which seems expected.)

"Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film."

Horigan · December 2021

As noted above, keeping it charged, or disconnected is okay. Note that AGM batteries have much lower self discharge compared to lead acid batteries, so with the battery switch off it will remain relatively fully charged during winter storage.

ScottG · December 2021

pthomas745 said:

...
"Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film."

This is an interesting tidbit. I've heard similar things before, and have often thought about this in the context of solar charging. I keep my panels deployed full time when in camp, and I suspect that a lot of this "short cycle" charging may be occurring. Presumably, a good solar controller or other smart charger accommodates for this, but the details are beyond my pay grade.

I'm not a big fan of keeping batteries on chargers fulltime, but that is more a personal idiosyncrasy that anything based on real evidence. Plenty of people do this with solar and/or shore power with no ill effects. During the winter I monitor my batteries and charge them up only when they get below 12.5V. Even that is not a very deep discharge, but certainly above 10%.

Regardless, I got five plus seasons out of my last lead acid battery, so it doesn't owe me anything. Interestingly, that battery finally died completely only after I replaced it and started carrying it as a spare that was charged before each trip but never actually used.

Denny16 · December 2021

I leave our TaB400 AGM battery connected and on solar charging, which is reduced in winter due to location, but gets enough to keep it charged. Disconnecting a TaB400 battery is difficult and not easy to do, due to their location. Just switching off the battery disconnect switch will isolate the battery/batteries. The solar controller can be set via its phone app to not charge the battery. This puts the battery in same state as disconnecting the battery terminals.
Cheers

WayneW · December 2021

An interesting wrinkle in all of this is how Victron MPPT controllers are programmed. From a previous discussion and a video from Victron, charging from the solar panel will not begin until the output voltage from the solar panel is at least 5V above the voltage of the battery. Here in northern Idaho with the low angle of the sun in the winter, it's probably pretty rare to get any charge out of the solar panel unless the battery is a fair way below full charge.

I avoid this conundrum altogether by removing the battery and storing it in the garage. I throw a charger on it about once a month or whenever I remember.

Happy Holidays to all!

Denny16 · December 2021

Whilst that is easy to do with the smaller battery located in the front tub on the TaB320, it does not work with the TaB400, with two large, heavy batteries buried under screwed down panels under the rear bunk. The singe 200-amp AGM battery in our 2018 TaB400 weighs around 200 lbs, beyond my ability to lift it out.
cheers

RCB · December 2021

Thanks guys, but what approach to winter charging will result in the longest life of the battery…and why ?

pthomas745 · December 2021

I think the best way to store a battery is charge it fully, turn the switch off, and leave it. The battery will slowly "self discharge". An AGM is said to "self discharge" at 1 to 2 percent per month. (As you pointed out in your original post.) The ordinary "lead acid" batteries I've owned pretty much performed that way. You avoid any chance of the "short cycling" issue discussed above.

Sharon_is_SAM · December 2021

@RCB
Storing a fully charged AGM battery without or with a functioning charging system will ensure a long battery life.

The issue with storing batteries in the cold without a charge system is the risk of a depleted battery developing sulfation which can reduce battery life. You mitigate that risk by knowing the health of your battery and fully charging before storing. The typical limited self discharge rate with cold storage is inconsequential and has not been shown to impact battery AGM performance.

What is the risk of storing a battery on a charging system? What happens if the controller or panels fail? Overcharge, undercharge? Maybe this method of storage requires more monitoring?

I don’t think either way proves to be superior when it comes to AGM battery life, but there are different risks.

ChanW · December 2021

Interesting thing: With our conventional lead-acid 6V GC battery pair, in keeping the Noco Genius smart battery charger/maintainer constantly connected, is that the battery-cell water levels need to be watched more closely.
The Noco Genius does a constant mini-equalization sort of cycle, if you look at the charge-cycle graph using the Victron Battery Monitor app, you can see the cycle going down to a trickle, and then gradually feeling its way up to equalization charge rates, then dropping back down to a trickle, over and over again. NOCO says that this keeps the battery healthy.
When we had the original single 12V battery, and continually charged via the Tab's onboard converter (we always leave it plugged in), I could go for a year easy without watering the battery. But in this case, I think we've heard from someone who noted that the onboard converter barely, if ever, gets to an absorption voltage, so it's no wonder it doesn't use up water.
Just an observation which makes me think I will unplug the Noco except once a month or so.
Of course this would all be different for you-all with AGM batteries.

Sharon_is_SAM said:

What is the risk of storing a battery on a charging system? What happens if the controller or panels fail? Overcharge, undercharge? Maybe this method of storage requires more monitoring?

I don’t think either way proves to be superior when it comes to battery life, but there are different risks.

Sharon_is_SAM · December 2021

I edited my response to note that it is specific to AGM batteries.

@ChanW - interesting observation regarding the battery water levels when storage charged via converter vs smart charger. So, do you think the “float” charge out of the converter gave your battery just enough and prevented water loss as a result? Or, do you think the smart charger was over kill, as demonstrated by depleting the water? What made you change to the smart charger?

ChanW · December 2021

@Sharon_is_SAM, I'm not "smart" enough to know that! I changed to the smart charger to be sure that I had a proper equalization cycle on my expensive new batteries...

I think I've read that giving the battery such a slow (and never complete) charge could add to the sulfation problem (if that 'slow charge' is actually the case - I haven't looked at the charge curve from the converter now that I have a monitor attached! I'll have to check that!)

My theory is that the converter is designed to charge at the "most common" charge curve, without cooking a battery. Kind of like the 'least common denominator', it'll work for every battery in a mediocre way, but not for any as efficiently as it should, efficiently so as to get the most out of the battery.

And in any case, it doesn't have an equalization cycle available. The Noco touts itself as 'automatic equalization', or some such. The water loss is normal when equalizing, from what I understand.

Sharon_is_SAM · December 2021

Thank you @ChanW. If you go down the rabbit hole of battery charging you will find various opinions about needing to “equalize” cycle AGM batteries. I think regular lead acid batteries do benefit. Anyway, for AGM batteries, the one good thing about the WFCO converter is that it charges slowly - something the AGMs like. We use a smart charger to charge our AGM before in-season and winter storage. Our AGMS like their winter nap sans charger😊

AnOldUR · December 2021

Sharon_is_SAM said:

If you go down the rabbit hole of battery charging you will find various opinions about needing to “equalize” cycle AGM batteries. I think regular lead acid batteries do benefit.

I contacted VMAXTANKS about this question. For my AGM's they said to not use the Victron equalization feature. May differ for other brands of AGM's?

Staff (vmax g.a.) (7/16/2021 4:11:52 PM)

You are very welcome! We do not recommend using the equalize feature, please keep this option OFF.

Please let us know if there is anything else we can help you with.

Best regards,
VMAXTANKS
Customer Service

ChanW · December 2021

@AnOldUR, that makes sense to me.

The 'equalization' process produces off-gassing, which needs to vent somewhere. An AGM has a vent, but from what I understand t it's only intended for the rare time that might occur with an overcharge...

I have no idea what an AGM does about the sulfation that the equalization-cycle helps to reduce.

Maybe someone knows this chemistry better... ?

Denny16 · December 2021

From what I have been told, AGMs are glass matt type battery, the electrolyte (acid in a flooded battery) is impregnated into the glass mat, and are less likely allow to sulfate build up, which occurres on the lead plates on lead acid/flooded batteries.

See: https://www.power-sonic.com/blog/the-complete-guide-to-agm-batteries/

Cheers

ChanW · December 2021

Thanks Denny, very informative.

pthomas745 · December 2021

I fell into the "videos about making batteries" trap a few weeks ago, and the process of AGM's was very interesting. They are pretty simple inside. About the 2 minute mark here is where the glass "mats" are stacked together.

https://www.youtube.com/watch?v=x1Ah-PKJcfM

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