[RE-wrenches] AGMs - Maintenance Cycling

Tue Dec 2 07:08:23 PST 2025

AGMs are generally a lead calcium chemistry battery; lead calcium batteries
are meant for a float service, which means they can sit in float for a long
time without losing much capacity, but they can lose up to 10-15% when held
in float for over 6 months, with the newer carbon additives that
recoverd lost capacity comes back easier and faster than they did just 10
years ago. . Periodic cycling helps exercise the batteries and test the
system, ensuring that when power goes out, the installer/user can verify it
works as expected.

For FLA and AGMs, you should turn off the AC input breakers every 6 months
and allow the inverter to switch to inverting mode for 2 to 4 hours.
Monitor the DC voltage at each battery to ensure the individual battery
voltages are falling uniformly,  if six volt batteries they should fall
withing .4 to .6 vdc of each other while 12v agm should be about .6 to
.8vdc of each other.    This is a quick test to ensure all cells are
healthy and loading correctly.

After the (20-30%) discharge, put the system into bypass, loosen, clean (if
necessary), and retorque your connections at all batteries, all the way
back to the inverters.   In a stationary application, these can loosen up
over 9-12 months.   In a mobile application, that's about 4-6 months...

After the connections, you can switch it back to normal mode and allow the
chargers to recharge the battery bank to full capacity.

The most common issue with AGMs is chronic undercharging, which causes
sulfation. This builds internal resistance in the battery and, over time,
leads to cell heating and swelling. Please ensure the Bulk/Absorb V is set
correctly for the full absorption timers with AGM. If you don't get the
Absorb Timers long enough, you will quickly lose top-end capacity, usually
within 2-3 months of operation. By the time it's discovered, the damage has
already been done and is very difficult to recover.

There are many inverters and chargers on the market with pre-programmed
"AGM" settings that are made up by some inverter/charger manufacturers and
will quickly kill an AGM battery bank.   Another issue is companies that
rely solely on end amps (return amps or tail current) to determine whether
an AGM battery is fully charged.     Depending on the AGM, the battery is
not fully charged until the current going into the battery falls below 2-4
% of the C20 rate for 60 mins... not the 0, 3, or 10 mins that many
manufacturers are using today.   Yes, some manufacturers will transition
from ABSORB to float immediately once the charging current falls below
2%-4% of the C20 rate. That will kill any battery, and with LFP, it could
cause massive balancing issues over time.

Maybe more than you wanted, but I hope this helps...

*Steve Higgins* *⋅* *Technical Services Manager*
*t* +1.902.597.4020   *m* +1.206.790.5840
*f* +1.902.597.8447   *e* steve at surrette.com
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On Tue, Dec 2, 2025 at 5:10 AM Mark Frye via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> OK, I am thinking the most important thing is to ensure that the batteries
> are fully charged before I put them into storage.
>
>
>
> *From:* RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> *On
> Behalf Of *James Jarvis via RE-wrenches
> *Sent:* Tuesday, December 2, 2025 4:41 AM
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> *Cc:* James Jarvis <jj at aprsworld.com>
> *Subject:* Re: [RE-wrenches] AGMs - Maintenance Cycling
>
>
>
> The life of the batteries will be longer in that service then in a deep
> cycling service.
>
>
>
> -James Jefferson Jarvis
> APRS World, LLC
> +1-507-454-2727
>
> http://www.aprsworld.com/
>
>
>
>
>
> On Tue, Dec 2, 2025 at 6:30 AM Mark Frye via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
>
>
> Thanks Guys,
>
>
>
> I guess I am wondering if the overall life of the battery will suffer if
> it is only ever cycled between 0 and 20% DOD and never deep cycled.
>
>
>
>
>
> *From:* RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> *On
> Behalf Of *Jay via RE-wrenches
> *Sent:* Monday, December 1, 2025 3:31 PM
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> *Cc:* Jay <jay.peltz at gmail.com>
> *Subject:* Re: [RE-wrenches] AGMs - Maintenance Cycling
>
>
>
> Agreed and especially if the batteries are in a cooler to cold environment
> even less an issue as the colder temps slows down any self discharge
>
>
>
> Jay
>
>
>
> On Dec 1, 2025, at 12:57 PM, James Jarvis via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
> 
>
> Is there a specific problem you are trying to resolve? If the loads are
> shut off when in storage, I don't see any issue with not messing with any
> charging what-so-ever for some months of storage.
>
>
>
> -James Jefferson Jarvis
>
> APRS World, LLC
> +1-507-454-2727
>
> http://www.aprsworld.com/
>
>
>
>
>
> On Mon, Dec 1, 2025 at 10:47 AM Mark Frye via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
> Folks,
>
>
>
> I have some AGMs that are PV charged during normal use and can be line
> charged when in storage (IOTA).
>
>
>
> The batteries will generally see 10-20% daily DOD and will generally
> complete the absorb cycle and see a fair amount of float each day.
>
>
>
> They may be deployed in this mode for many months.
>
>
>
> After that, they will go into storage for several months.
>
>
>
> Storage will include float charging at the IOTAs regular voltage.
>
>
>
> I have the LVD set at 46V.
>
>
>
> Would it make sense to occasionally turn on the load, turn off the line
> charger, let the bank run down to LVD, turn off the load, turn on the
> charger with the increased voltage jumper installed, charge overnight,
> remove jumper and resume normal float charging?
>
>
>
> Thanks,
>
>
>
> Mark
>
>
>
>
>
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