[Shop-talk] AGM batteries: Protect from freezing?

[Bob Spidell]

Interesting question. I'm not a hexpert, but I have a lot of cars and 
farm/garden equipment that need batteries--including a new EV--so I 
spend a lot of time thinking about batteries. I have AGMs in both my 
Austin-Healeys; in one because it originally had two 6V batteries in 
parallel and replacements in an appropriate size are all but NLA, and in 
one because they're just flat-ass better (and I got tired of 
neutralizing/cleaning acid spills in the boot--'trunk' for you Yankees (:)).

IMO, AGM are better because they seem to generally be better 
constructed, and because the electrolyte--concentrated sulfuric acid--is 
constrained by the fibrous 'mat.' Otherwise, the chemistry is the same 
as any other lead-acid (note 'SLA,' or 'sealed lead acid,' aka 
'maintenance-free' batteries seem to be generally overall quality and I 
buy them when I can).

Since this question was new to me, I did some digging:

https://discoverbattery.com/support/learning-center/battery-101/what-is-the-freezing-point-of-battery-acid-electrolyte

This is something I hadn't known; that the freezing point varies with 
state-of-charge so, first off, if your battery is going to endure 
freezing temps make sure it's fully charged. I have several battery 
minders/tenders, and bought one especially for AGMs, and I use them (the 
tractors sometimes sit for weeks and, when I need them, I need them).

Here's another thing I hadn't considered, but I live in the California 
Central Valley and we rarely get below-freezing weather (TL;DR: cold 
weather bad: "Cold weather accelerates wear and tear on batteries"):

https://www.relionbattery.com/blog/top-4-factors-to-consider-for-a-cold-weather-battery

So, yeah, freezing conditions are not good for lead-acid batteries, 
including AGM.  I've heard the "don't store batteries on concrete" 
theory, but I can't offer any explanation except that the concrete is 
likely to transfer more cold to the battery (I think this is largely an 
"old wives tale"). If you think your AGM is going to experience severe 
cold for any length of time, store it somewhere warmer with a smart 
charger (definitely not a 'trickle' charger).

Bob



On 11/13/2025 11:55 PM, Philip Ethier wrote:
>
>
> On Fri, Nov 14, 2025 at 1:34 AM Philip Ethier <pethier7 at gmail.com> wrote:
>
>     Absorbed Gas Mat batteries:
>
>
> OOPS. Absorbent glass mat
>
>
>     If you leave a vehicle outside all winter in a cold climate
>     without charging the AGM batteries, will they be ruined by the cold?
>
>     I know that regular lead-acid batteries can freeze up, but I don't
>     know about AGM batteries.
>
>
>     -- 
>     Phil Ethier, West Side, Saint Paul Minnesota, USA
>     2006 Gulf Stream Conquest Super C, 8.1, white
>     2011 Cayman 2.9 PDK, white
>     2021 Ford F-150 3500cc twin turbo, white
>     1991 Caterham 1700 Super Sprint, not white
>     www.mnautox.com
>     <http://www.mnautox.com/>
>
>
>
> -- 
> Phil Ethier, West Side, Saint Paul Minnesota, USA
> 2006 Gulf Stream Conquest Super C, 8.1, white
> 2011 Cayman 2.9 PDK, white
> 2021 Ford F-150 3500cc twin turbo, white
> 1991 Caterham 1700 Super Sprint, not white
> www.mnautox.com
> <http://www.mnautox.com/>
>
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