L-16 "cycling" requirements? [RE-wrenches]

[Joel Davidson]

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Hi Doug,
Thanks for your EV charge-discharge insight. Makes sense. I thought the 
"stiffness" was characteristic of using an electromotive battery in a 
stationary application and that equalizing (gassing - bubbling) re-mixed the 
electrolyte. What you see contradicts this. Perhaps someone with deeper 
chemistry knowledge can answer in. Do reversable chemical reactions lose 
their reversablility over time or cycling or what?
Joel Davidson

----- Original Message ----- 
From: "Doug Pratt" <dmpratt at sbcglobal.net>
To: <RE-wrenches at topica.com>
Sent: Friday, May 20, 2005 5:48 PM
Subject: RE: L-16 "cycling" requirements? [RE-wrenches]


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Ya know Todd, now that you ask, I can't point to a definitive source that
says "cycle your deep-cycle batteries periodically or lose capacity", but
that's been my understanding for lead-antimony cells. They tend to get
"stiff" chemically and won't go as deep without dropping voltage. My years
of driving an EV certainly support this. Need to do a long trip? There's
nothing like a modest warm-up cycle or two to make your batteries deliver
more kWhrs without voltage sag.

Lead-calcium is a different critter, and loves to just float and float with
most cycles no deeper than 10-15%. But as you've found, since Ma Bell quit
using 48-volt relay-based switches (in the '80s), which required monstrous
banks of cells, lead-calcium wet-cells have become rare.

Given my druthers with sealed batteries, I prefer gels. They've got more
moisture in them initially, so they're a bit more tolerant of abuses. But
they're harder to manufacture, therefore more expensive, and not always
worth the higher cost. My own Outback grid-tie system has a set of 550 Ah
Yuasa industrial gels on it ($$$!!!), and I'd sure like to see 20 years out
of them. (Ask me again in 15 years...)

My biggest beef with wet-cells on grid-tie systems is that folks will forget
to water them. Since they're used infrequently they sorta slip from memory
and the next thing you know it's been 8 to 30 months since the last
watering. I've yet to visit a grid-tie system with wet cells that didn't
have plates showing (other than brand-new). It's just inviting trouble.

Thanks for the $/kWh/yr calcs. Good info!

Cheers,
Doug Pratt


-----Original Message-----
From: Todd Cory, Mt. Shasta Energy Services
[mailto:toddcory at finestplanet.com]
Sent: Friday, May 20, 2005 11:45 AM
To: RE-wrenches at topica.com
Subject: Re: L-16 "cycling" requirements? [RE-wrenches]


I would also like to know where this idea came from that "batteries need
exercise to stay healthy"? Trojan discounts that notion. They have also
suggested that EQing a floating battery is unnecessary... the stir the
electrolyte myth doesn't wash.

As I understand it, batteries have two "clocks working toward their end of
life.
Once clock is a chemical clock... that even floated with no cycling, they
will
only last so long. The other is a cycle clock... that counts the number of
charge / discharge cycles. Also of consideration is the length of time they
are
left discharged due to the resulting sulfation that happens to the plates.

We can talk all day about optimal bulk, float and EQ voltage settings, but
evidence suggests it is best to err on the side of slightly overcharging wet
cells rather than chronically undercharging.

I have found lead antimony wet cells are really not all that horrible in
grid
intertied applications, especially as with modern inverters like the
Outback,
float current losses are minimized. With hydrocaps watering is also
minimized
and a "cheapo" like a Trojan L-16 will typically last a good 10 to 12
years...
similar to their life expectancy in an off grid system. Plus, they are very
cost
effective. Yes, a better wet cell would be a lead calcium configuration but
for
some reason (probably manufacturing quantities?) they are horribly
expensive.

As I understand it, lead calcium's are best for shallow cycling and float
service. (Perfect for battery back up grid ties.) I suppose the reason for
the
price differential is that they are not mass produced as lead antimony cells
are. Last year I did some research about the price to replace those lead
calcium
wet cells at the hydro plant I work at since their "rated life" is nearly
up.
(Guess who gets to take them home!)
I believe the cost is going to be around $9K for 12 kW.
10 kWh of AGM's are about $1.2K
By comparison 20 kW of L-16's were about $1.3K.

Price per kWh:
Wet cell lead antimony: $65.00; life span ~10 years = $6.50/kWh/year (deep
cycle)
Wet cell lead calcium: $750.00; life span ~20 years = $37.50/kWh/year
(shallow
cycle)
AGM lead calcium: $120.00; life spam ~10 years = $12.00/kWh/year (shallow
cycle)

If labor is included in the above numbers wet cell lead calciums are more
price
competitive.


I agree that each application needs the proper battery configuration and
chemistry. Grid ties with battery backup optimally need a different battery
chemistry than off grid. The problem seems to find a cost effective approach
to
this application.

So Doug what do you use? AGM's?

This is a pertinent topic for the list, as with Outback GFX inverters, grid
intertied systems with backup power with no longer have to pay the
inefficiency
penalty we had with brand X's old SW design.

Todd

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