[RE-wrenches] Couple of Qustions about AGM batteries

Exeltech exeltech at yahoo.com
Mon Feb 1 06:10:36 PST 2010



AGM
batteries in good condition and fully charged will consume approximately 1
milliamp per rated amp-hour in "float" mode at 25C.  Thus, your 250
amp-hour battery will require 250 milliamps at the manufacturer's rated float
voltage (including temperature compensation).



Discussion now turns to whether or not it's preferable to leave AGM batteries
connected to a source of current in float mode.  Continuous float current
flow with the battery at 100% state of charge, and supplied over a long period
of time (typically months to years), leads to a condition called "positive
plate corrosion" (PPC). In this process, material in each positive plate
is gradually diminished, being leached into the electrolyte and/or deposited on
the negative plate.  Once PPC has occurred, the lost material cannot be
recovered and re-deposited back.  Reduction of positive plate material
leads to a decreased ampere-hour rating for the battery suffering from PPC,
which then tends to aggravate the corrosion situation over time.



The rate of PPC can in of itself be reduced by a *slight* reduction in float
voltage at 100% SOC, and of a value to reduce the float current from 1 mA per
rated battery A-H to 0.5 mA per A-H.



The problem in doing so: most equipment on the market isn't designed to nor is
capable of monitoring battery float voltage and current sufficiently accurately
to be of benefit.



You  didn't say at what temperature the batteries are kept.  Cold
batteries have a self-discharge rate lower than when hot.  AGM batteries
lose less than 1% per month at 25C (77F) through self-discharge.  Thus if
your batteries are new/healthy, you'll be down less than 2% in six weeks if
charged to 100%, then disconnected.



If your batteries are consuming 2 amps at 26.4 Vdc and 21C, either they're not
100% fully charged .. or something else is amiss.  Incidentally, battery
"float" current increases with age from the 1 m-A per A-H stated
above.



That being the case, I'd agree with Warren.  At 2 amps, either you have
unknown load(s) consuming power .. or something else is wrong (could be as
simple as the batteries not being at 100% SOC).



My recommendation: First determine why the batteries are consuming 2
amps.  Once you resolve that issue .. six weeks of current under proper
float conditions (as defined by the manufacturer) won't significantly help or
hurt the batteries.  If the batteries are truly drawing 2 A at
26.4 V (presuming that's the proper temperature-compensated voltage for
those batteries), it's a troubling sign - and I'd leave the batteries connected
to your current source, as it indicates a high rate of self-discharge.  If you're able to reduce the float current to
the 250 mA range at the manufacturer recommended float voltage .. once you
reach that current level, I'd disconnect them for the six week period.



Finally .. I also agree with Warren.  Sealed AGM are not intended in the
sense of the word for "equalization".  The manufacturers often
call their equivalent to this a "freshening" charge.  It's not
as rigorous as a flooded-cell equalization due to the sealed nature of AGM batteries. 
A true "equalize" would likely over-gas the cells, potentially
opening the safety vent on the batteries.  When this happens, if it's a
self-sealing vent, you'll have lost some of the available H2O.  If it's
not a self-sealing vent .. you'll have destroyed the battery.





Batteries are a chemical and mechanical complexity that often defy easy
answers, particularly in a forum such as this.



Hope this helps.



(By the way - I'm on the road, and if you post any reply/questions, I won't be able to answer for at least a week or more.)
Dan









--- On Mon, 2/1/10, Warren Lauzon <windsun at wind-sun.com>
wrote:



From: Warren Lauzon <windsun at wind-sun.com>

Subject: Re: [RE-wrenches] Couple of Qustions about AGM batteries

To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>

Date: Monday, February 1, 2010, 5:05 AM

The answer to systems not being used is nearly
always leave them on float. 

However, 2 amps seems a bit high for AGM batteries unless you have some kind 

of phantom load.



Equalization for AGM's is different and for a different purpose than with 

flooded. The current should be quite a bit less than you would need for a 

flooded, since the purpose is not to stir the liquid up but simply to get 

all cells in the series up to the same state of charge. AGM's don't usually 

need much equalization.



..................................................................................................

Northern Arizona Wind & Sun - Electricity From The Sun Since 1979

Solar Discussion Forum: http://www.wind-sun.com/ForumVB/

..................................................................................................

----- Original Message ----- 

From: "Peter Parrish" <peter.parrish at calsolareng.com>

To: <re-wrenches at re-wrenches.org>

Sent: Sunday, January 31, 2010 4:19 PM

Subject: [RE-wrenches] Couple of Qustions about AGM batteries





> We have put together a 850 Wdc portable stand-alone PV system. It uses

> a Blue Sky 3024DiL charge controller, a pair of SunXtender 250 A-hr AGM

> batteries (24 volt configuration), and a Xantrex Prosine 1000 inverter.

>

> We have been using this system for powering "events" and getting
some

> good marketing from the activities. Everything has worked out well, and

> the system is not being used now and probably won't until March or April

> sometime.

>

> We have let the system fully charge the batteries through bulk/absorption

> and now float. The float conditions are 26.4 V and 2.0 A at a battery

> temperature of 21 deg-C.

>

> My first question: is it alright to let the batteries float every day for

> the next 6 to 8 weeks, or should I disconnect them from the rest of the

> system until we are ready to use the system again?

>

> My second question: since the 3024iL can perform an equalization,

> is there a safe equalization procedure for these AGM batteries?

>

> - Peter

>

> Peter T. Parrish, Ph.D., President

> California Solar Engineering, Inc.

> 820 Cynthia Ave., Los Angeles, CA 90065

> CA Lic. 854779, NABCEP Cert. 031806-26

> peter.parrish at calsolareng.com

> Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

 




      
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