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Solar Battery Management

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Posted by Evad Remlu on November 12, 2008, 1:39 pm
 
Here's my rules for battery management:

1. Batteries should be fully charged everyday to prevent sulfation. A really
full charge is where the charge current is C/100 @ 16 volts in a 12 volt
system. I have found that C/50 seems to work but if not held there for at
least one hour per day the sulfation will occur.

2. Parallel strings must be checked weekly with clamp-on DC ammeter for
charge and discharge balance. Weak imbalanced strings must be disconnected
and desulphated by allowing to sit at full charge for at least 24 hours. One
cell can become sulphated in one week and ruin the whole bank if allowed to
persist.

3. All batteries must be maintained at exactly the same temperature
preferably in an insulated container. A non-contact temperature probe is
handy to check each battery's temperature. Temperature imbalance will cause
charge imbalance. Never allow the sun to shine on the batteries.

4. Batteries should be watered monthly and all cells should use water
equally. Low water use indicates sulfation.

5. Solar panels should provide a full bulk charge by noon everyday. This
gives time for a long acceptance charge to reach C/50 @ 16 volts. Generator
should be used on first cloudy day to get this full bulk charge in by noon.

6. Remember, one bad cell will ruin a whole bank. You only have a week or so
to recover a cell that has gone into sulfation so constant vigilance is
necessary to keep the bank balanced.

7. As the winter days get shorter and the weather gets cloudier the need to
do bulk charging with a generator becomes more necessary and the use of
solar panels for acceptance charge only may be necessary. Completion of bulk
charge is indicated by a rapid rise in battery voltage from 15 to 16 volts.

8. If your charger won't put out at least 16 volts, then get one that will.

NM







Posted by spaco on November 13, 2008, 4:11 pm
 
What is "C/100"?  What is "C/50"?   What is "an acceptance charge"?

Pete Stanaitis
------------------

Evad Remlu wrote:


Posted by You on November 13, 2008, 6:10 pm
 

Wiki should be your friend..... but apparently isn't......

Posted by Duane C. Johnson on November 13, 2008, 6:58 pm
 Hi Pete;


 > What is "C/100"?  What is "C/50"?
 > What is "an Acceptance Charge"?

C = Amp Hour Capacity of the battery or "C".

100 should have units of hours although is often
shortened to just the number without the units.

So:
Let's say the capacity of the battery is 220Ahr, a common size.
C = 220Ahr
220Ahr / 100hr = 2.2A charging, (or discharging,) rate.
or
220Ahr /  50hr = 4.4A charging, (or discharging,) rate.

"Acceptance Charge", sometimes "Float Charge" is a
maximum charge rate that can be applied to a wet cell that
can be applied indefinitely. However, periodic watering
must be performed. This is also the charge rate that can
be used in "Trickle Chargers".

 > Pete Stanaitis

Duane

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Posted by bealiba on November 14, 2008, 2:12 am
 
From the Swinburne University of Technology's course notes for
"Introduction to Renewable Energy Technologies:

4.1 Selection and Sizing of Battery Charger
The considerations for battery charger sizing are to be able to charge
the
battery in a reasonable time, and not to charge it too fast,
especially when
near fully charged. A simple rule of thumb to size the current rating
of the
charger is to use the 10 h charge rate, i.e. 10% of C10 (the 10 h
capacity).
If the C10 rating of the battery was 500 Ah, then a charger rated at
50A would
be suitable.
The maximum charge rate should be about 15% to 25% of C20 (the 20
hour
capacity of the battery bank), so this sets the upper limit to the
charger size.
Since chargers are usually used with generator sets, the current drawn
by the
charger must not overload the generator. The charger should therefore
have a
maximum current draw at least 10% less than the generator capacity.
Smaller
battery chargers are often used but take longer times to recharge the
battery
bank.
Battery chargers must also be chosen for the correct system voltage.

also:

3.4.3 Cycle life
The term state of charge (SOC) is used to refer to the per~entage of
storage
capacity still available in the battery ie. 100 % SOC is fully charged
and 50 %
SOC is half charged. When a battery is used, it is charged and
recharged and
undergoes cycles where the SOC varies over time. A battery cycle is
defined
as a cycle of discharge and recharge from a given state of charge
(SOC), down
to a lower state of charge, and back to the original state of charge.
One deep discharge cycle would consist of :
e.g. 100% SOC to 20%SOC Discharge to Recharge 100% SOC.
One shallow discharge cycle would consist of:
e.g. 100% SOC to 80% SOC Discharge to Recharge 100% SOC.

Battery cycle life will depend on plate additives, plate thickness and
plate
porosity. Car batteries have many, thin, porous plates so that there
is a large
acid-to-plate contact area thus enabling rapid reactions that produce
very
high currents (as required by starter motors).
Batteries used for SPS (deep cycle types) don't need to meet such high
current
surges and so have fewer, thicker, less porous plates. They are
however
required to have longer life spans. Consequently their construction
incorporates thicker plates which are held together to prevent
shedding of the
active plate material over time. Automotive batteries typically can
provide up
to about 60 deep discharge cycles while SPS batteries typically range
from
300 (small applications) to 1200 to 1500 deep discharge cycles (large
applications).

Deep cycle lead acid batteries do not fail catastrophically but
experience a
more gradual decline of available capacity as they approach the end of
their
useful life. Battery life is inversely proportional to the depth of
discharge
(DOD) that the batteries experience on a regular basis (e.g. daily
DOD). Depth
of discharge is the inverse of SOC and refers to the percentage of the
capacity
of the battery that has been removed during discharge ie. if a battery
is fully
charged to start with, then 10% DOD equals 90% SOC.

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