Mysterious Surrette Behavior [RE-wrenches]

Rob Harlan mendosol at mcn.org
Fri May 12 17:33:26 PDT 2006


Wrenches,
Thank you for this discussion on off-grid battery behavior..
The Surrette installation in this case is charged by a 1KW solar array and a
SW4024/5.5K genset combo. The absorption times have been one hour
(non-adjustable) for the C-40 and two hours (adjustable) for the SW.  The
charge rates are C-30 and a C-9 respectively.  Ezra and Travis's  points are
well taken here.  It sounds like  both the charge rates and absorption times
need to be increased for this size of a battery bank (1060ah @24v) to combat
sulfation.

I still don't get the consistently off (way off) Trimetric readings.  Simon
at Surrette seems to think it is caused by the fact that the batteries  have
higher resistance to charge due to "greater plate thicknesses and higher
paste densities" in their batteries.  That in combination with sulfation
caused by low absorption times (batteries not being completely charged)
yields a higher resistance which means that the voltage is going up and
amperage is tapering off too soon.  This would corroborate the theory that
harder charging is needed.

Surrette is also saying that a 20 point SG spread between cells is
acceptable which doesn't correspond at all with what I have always been
told.  I guess I don't believe it.  Interestingly enough they are also
saying that a
1/2" of added water can yield as much as a 0.010 difference in readings.

If the problem is truly a lack of sufficient charging, this particular
installation will probably have to have: .1) An increase in the SW
absorption time setting.  2) An MX-60 to allow for longer absorption times
and a sizable solar driven EQ capability. (If the owners ever go on
vacation).3) An increase in array size. All of this is a n expensive fix,
and hopefully, a
genuine solution.

For the record this has all reminded me that I need to get baseline specific
gravity and voltage readings fresh off of the delivery truck, size the
bank conservatively based on charging capability and make sure that the
owner (and the installer) is not overfilling the bats.

I am reimpressed with the complexity and subtlety of battery relations.
Cook your pasta fast and well out there.

-Rob
Harlan Electric
NABCEP Certified
(780) 487-6448
mendosol at mcn.org

 Original Message ----- 
From: "Ezra Auerbach, DragonSun Consulting" <ezra at lasqueti.net>
To: <RE-wrenches at topica.com>
Sent: Friday, May 12, 2006 1:55 PM
Subject: RE: Mysterious Surrette Behavior [RE-wrenches]


>
> Hi,
>
> I'd like to add some general comments about big batteries and off grid
> systems. I've had occasions to discuss the matter of oversized batteries
(or
> undersized charging systems) with a few battery manufacturers. They have
> been consistent in telling me that the ability to apply significant
amounts
> of minimum current to batteries is key to successfully charging them. As a
> general rule of thumb they specify a minimum of the C10 rate for initial
> (max bulk amps) charge rate and C5 if you can perhaps deliver it. In my
> various jobs over the last twenty years I've also had the opportunity to
see
> countless battery systems which were oversized for the available charging
> system, a very high percentage underperformed with respect to longevity
> expectations. I think the battery manufacturers are correct in demanding
> high capacity charging systems for their large batteries - especially
those
> offering long warranties.
>
> I've lived with large capacity batteries for years (the same set for the
> last fourteen) and I can attest that the only time I really get them fully
> charged from renewable is on those bright sunny days when the wind is
> howling as well - I can get pretty close to the C10 rate of my batteries
in
> those conditions and they respond very nicely to the extra charge current.
> I've even been known to start the generator early in one of these perfect
> weather charge cycles to add an extra 80 amps or so to the initial rate.
>
> Bottom line for me is that battery charging is a lot like cooking. Bring
the
> electrolyte up to temperature with current and the magic transformation of
> water into acid occurs. Without sufficient current the electrolyte never
> gets sufficiently warm for a full charge process to occur and the
batteries
> remain essentially "under cooked". I often compare battery charging to
> cooking a pot of pasta. You can put the pasta in a pot of cold water and
> leave it on the stove on low all day. At the end of the day you'll have
> tepid mush but it won't be cooked pasta. You need to apply sufficient heat
> to boil the water to cook the darn stuff, much like you have to apply
> sufficient current to charge the batteries.
>
> Best regards,
>
> Ezra
>
>
> Ezra Auerbach
> DragonSun Consulting
> Morewater Road
> Lasqueti Island, B.C. V0R 2J0
>
> Tel 250.333.8650
> Email: ezra at lincsat.com
>


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