Robust battery [RE-wrenches]

[Dan Duffield]

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I have tried to make this short, but my robust battery of choice is an AGM 
and this is why.
I like a premium Absorbed Glass Mat like the GNB AbsolyteIIP or 
XL  especially in climates with cold winters - winter is just around the 
corner too.

AGM Basic strengths:
1200 cycles @ 80% D.O.D. -  a true deep cycle.
No gassing to surrounding electronics.
No watering- ? did i put enough distilled water in this cell or too much? 
what about last time?
No equalization of individual cells - normally - but it can still be done; 
though much finer regulation process is involved.
No obvious reason/need to isolate battery bank outside, garage, shed or 
crawl space (hate that fiberglass).
Normally can't be spilled.
Stout package container design.
Actual extended warranty in PV application when stacked vertically and 
temperature is logged and within specified limits.
Battery chemistry is fixed at time of cell sealing,  each individual cell 
(among many) can't be routinely or accidentally contaminated.
Little sedimentation of the electrolyte and overuse of the bottom areas of 
the plates - much less of an issue in stationary appl. than wet electrolyte 
type.
THEY DON'T SMELL. THEY do not routinely liberate gases, that is - if in a 
confined limited area cannot support combustion- so they are probably safer 
that wet batteries (IMHO) - given that a charge controller used with either 
battery type functions properly.
Self-discharge rate about 8-9% better per week than an industrial wet 
lead-acid.
No acid/electrolyte spit on battery container  on skin, on clothes, and the 
floor, on shoes, on pets, on carpet, ON CLIENT. etc
The "XL" models are available in some pretty humongous amp-hour ranges, and 
so it is easier to avoid multiple strings at whatever system voltage you 
are using, because you can use one of these large bad-boys; up to 6000 A-H 
per 2v cell.
Elimination of passive or active venting of compartmentalized battery 
box/room or failure of venting system.

Disadvantages:
High cost & expensive to replace - Harder to Sell
Very Heavy & hard to move - we use a our own custom lifting hoist (winch 
powered) $$$ -this also limits sites to ones with road access
Heavy weights means possible injury or strain to people who move these units
Must charged with a temperature compensated regulator
         If one grounds the negative dc conductor- one must be VERY careful 
when touching/tightening the positive terminals because batteries are in 
powder coated steel containers (personally i believe it is far safer not 
ground the DC side and to use the properly rated overcurrent protection in 
both DC conductors IMHO) . I think not grounding either DC conductor - but 
providing overcurrent protection on both DC conductors is normal or policy 
on U.S. Naval vessels: at least high voltage DC strings.

  The single most significant issue to me is the SMELL
         People, like myself and my clients in New Mexico have cold winters 
and many sites at elevations above 5000'. I have been to  many, many sites 
where batteries were NOT welcome in the home because of nothing more than 
their SMELL.  These were or are left in the garage or crawl space or shed - 
that will not or has not been a heated living space; kept at the same 
temperature as living space.
Because the industrial AGM don't smell, I recommend and my clients don't 
mind or have an issue with bringing these batteries into the house, the 
nice warm - temperature controlled environment of a house, because they 
don't smell or gas or spit at all.

         More importantly, temperature is the key to good performance of 
the weakest link (battery) in a pv system.
I use the following 5 points when trying to justify these more expensive 
batteries to my clients.

Quintuple whammy.
1). During the winter, the available hours of sun per day is the least - 
system typically is at it's lowest array to load ratio.
2). The sun's rays are at a very low angle and pass through more of the 
atmosphere (edgewise) this produces less watts per square meter of 
available power.
3). As a battery bank's ambient temperature lowers (in winter -unheated 
area) the overall capacity of the bank is (temporarily) reduced. Instead of 
having 5 days of reserve, suddenly I have a drastic reduction in reserve 
capacity. Robbed!
4). The individual charge cycle efficiency is drastically reduced too, when 
batt's are significantly colder than 77F.
         (I put 100 units watt-hours, I expect to get around 80+ units for 
a wet lead acid and above 90+ units for an industrial AGM back out - if my 
batteries are around 77F. These efficiency values are for new or rather new 
batteries.
         However,. if my batteries are in a unheated crawl space and the 
temperature falls to 25 degrees ambient, instead getting 80+ or 90+out, 
having put in 100, I will be lucky to get back 60 watt-hours. Yikes! My 
energy bucket went from a leaky, to gushing - somebody get a mop. 
Insulation slows this process but does not stop it - once a gets cold 
battery it stays cold.
5). The cumulative effect of the 4 items above: equals, that a smelly (but 
more economical) wet battery bank in an unheated area will experience a 
harder cycling of the plates of the battery, and over it's lifetime 
experience a greater AVERAGE depth of discharge and a reduced number of cycles.
         Items labeled 3) & 4). are only temporary - When the batteries 
chemical reactions are slowed by cold temperatures, they will be OK again 
at room temperature -= come spring or summer.
We can't do much about items 1) and 2), but we can do something about 3). & 
4). & 5).
3 & 4 can be eliminated if a battery was available that could be brought 
indoors safely and say placed in a cozy little 70 degree closet or the 
expense could be justified to heat a dedicated battery area and vent it 
properly.

  Unfortunately most people trying to get up and running and off grid do 
not prioritize battery banks to a heated space- they are doing a septic and 
a well and fencing and .......
Result : Poor wintertime performance of whole PV system.
Locally, this is too often is blamed on lack of peak hours of sun and not 
on a COLD battery storage area.

AGM's or any sealed long life battery wins again because their sealed 
nature allows them - their niche in the heated living space.
Too me- an wet lead acid battery is like a fiesty Chow or high strung 
Rotweiler and an AGM is like a Labrador Retriever.  All of them are strong 
and easier to keep outside and all are a mess, if they leak - but my lab is 
a true deepcycle dog that doesn't fart - just like my AGM's.

Now, the only other real issue (IMHO) with these AGM's batteries is a 
technical one.
         It is not usually seen it whole house PV systems, but it can be 
seen it little special PV applications. If you charge an AGM battery bank's 
total capacity in a short time, lets say in less than 5 hours, a C/5 rate, 
you are charging FAST. The AGM battery's voltages tends to rise very 
quickly after 2.5 - 3 hours - when charging at a C/5 rate or better. This 
will not happen often on a big remote home battery bank, because it is a 
BIG Bank with lots of capacity and most remote owners charge at a C/20 rate 
and if they have lots of nice pv panels closer to a C/10 rate.
         However,  on a very small battery banks, the lowest 40-50% of 
capacity will charge in say perhaps in 2 to 2.5 hours. After 2.5 hours the 
battery's voltage climes VERY fast at this high charge rate. The remainder 
may take proportionally much longer  (longer than the rest of the sun that 
day) because of the voltage ceiling of 14.1 is reached soon and the 
charging rate must be tapered or valved by the proper controller 
(temperature compensated). You may have good sun and plenty of panels, but 
the battery itself will limit how fast you can charge it and whether you 
can even fill it -in the time remaining till sundown.

I would whole-heartedly recommend AGM batteries, my home is at 6910' feet 
and my sealed batteries are a nice thermal mass too!
Of course if you live in an area where it never gets cold like HAWAII, it's 
not an issue and your warmer and a whole smarter than me.

That's my 2 cents on Absolytes (AGM's) versus wet lead acid batteries.
My home system runs on Absolytes and I have been very happy with them,  I 
am happily spoiled with these units, but yes - one day i will have to 
replace them, cha-ching.
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