More on battery management (was Unexpected pleasure from abused AGM batterie

[Joel Davidson]

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I think it is more accurate to say that - Given the intermittent 
availability of RE, it's virtually impossible
to _fully_ recharge a battery in an RE system unless it is a battery that 
can be and is equalized.

----- Original Message ----- 
From: "Windy Dankoff" <windydankoff at mac.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, May 02, 2006 3:55 PM
Subject: More on battery management (was Unexpected pleasure from abused AGM 
batteries) [


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Wrenches,
    The anonymous lurker continues ...

Windy,
Thanks for the anonymity.

Sorry for the delay replying to the post by Darryl (Thayer) asking
about batteries.  I don't check the Wrench board as often as I
should.  His post from April 15 begins below:

> Great story Windy, but what do wrenches do
> to assure complete recharge after cycling?

Given the intermittent availability of RE, it's virtually impossible
to _fully_ recharge any battery in an RE system, regardless of
chemistry or topology.  Given this, I'd like to focus on VRLA (valve
regulated lead-acid), since that's been the focus of the conversation
thus far, but also keep comments generally applicable to all
lead-acid environments as much as possible.

As batteries approach "full", charge current must taper to avoid
outgassing and generation of excessive internal heat.  Unless a
battery has been sitting unused for days (meaning already at 98%+
SOC), and the charging system (PV/wind, etc.) has been available full
time for that entire period, we simply can't get a battery to a
*true* 100% SOC.  80-95% is a common maximum.

There's also the issue of "leakage" current.  This is the current
that flows through a battery even when it's fully charged. (Sidebar
note: leakage current is not the same as self-discharge current.)  In
healthy VRLA batteries, leakage current will be 0.30-1.00 mA per A-H
rated capacity with 2.25 Volts Per Cell (VPC) applied at 77F.  This
value will vary with temperature; higher temperatures being more
conducive to higher leakage currents.  For example, if a battery is
rated 1,000 A-H, you would measure 300-1000 mA (nominal) through the
pack with 2.25 VPC at 77F when it is fully and absolutely 100%
charged.  Anything much above this value (by a factor of 3 or more,
meaning 3A or higher) indicates a leaky or possibly shorted cell.
Much lower means an open cell.  VRLA batteries, if allowed to sit in
a highly discharged state for extended periods (several months or
longer) will cause some of the lead to be leached out of the plates
by the water.  This lead, which is then forced out of solution during
the next charge cycle, deposits at the most convenient place -
usually the matte binder between the plates.  This results in higher
leakage currents and sometimes "soft" shorts.  If the former, the
condition can sometimes be corrected by a gentle, long-term float
charge.  If the latter, the cell is often ruined.

After charging, and sitting for 24 hours without charge current
applied, VRLA cell voltages shouldn’t vary more than 0.10 VPC across
the entire pack.  If this condition exists, a careful equalizing
charge (2.30VPC for 20 hours, or 2.35VPC for 8 hours) is indicated.

Overcharging a VRLA battery is worse than undercharging.  VRLA cells
have an internal gas-recombinant catalyst system similar to that
found in Hydrocaps.  This catalyst converts hydrogen and oxygen into
water .. *provided the rate at which the gasses are generated do not
exceed the rate at which the recombination can take place*.  If the
latter occurs, pressure builds up inside a cell, which opens a vent
and allows excess gas to escape.  Some vents are self-healing .. some
are not, meaning self-healing vents re-close after the pressure is
diminished.  If the former, the cell will lose *some* water in the
form of hydrogen and oxygen.  The loss of water will increase the
cell internal impedance by an amount determined by the quantity of
water lost.  If the vent is NOT self-healing, once breached, the cell
will slowly die due to continued drying (common in gel batteries).


> My situation is the battery's are required
> to supply power at night, next day they
> get recharged on a three stage charge cycle.
> (Absorb to absorb volts, limit current at absorb
> volts for time, drop volts and hold at float the
> remainder of day) After 6 months I checked the
> battery set, it was at 49.8 resting volts. So I
> increased the Absorb time, checked once a month
> until the resting volts were at the maximum of about
> 2.2 volts per cell. Which means I am slightly cooking
> the batteries. So i set the the absorb time back.

In this situation, a compromise must be achieved.  A three-state
charger should be set for bulk mode to 2.28VPC @ 77F, and MUST be
temperature compensated.  Open-circuit voltage testing is at best
only a rough approximation of state of charge.  2.2VPC open circuit,
with no charge being applied, indicates one of two conditions: either
the cells were under a charge until moments before the reading was
taken (i.e. a surface charge was measured, in which case the voltage
reading is higher than would be expected), or the cells are being
overcharged.  If the latter, the charge rate and/or time need to be
reduced.  Nominally, at 77F, a reading of 2.14VOC indicates 100% SOC
in a VRLA cell.  Ultimately, overcharging to this level is a primary
failure mechanism in VRLA batteries, caused by positive grid
corrosion.  Though it may seem contrary to common sense, a slight
undercharge condition is healthier than overcharge for VRLA cells.


> What do I do to get an automated battery charge
> reading, and get an automated correction to keep
> the batteries at full and not overfull. with flooded
> batteries I can add water and check with the
> hydrometer. By the way I have a trimetric, and I am
> unable to tell the net battery status with enough accuracy.

To get a truly meaningful automated battery reading, you’d need a
sophisticated data acquisition system - one capable of measuring
individual cell voltages as well as temperature, both cell and
ambient.  Since this is beyond the means of most, you can monitor the
entire battery voltage, with occasional visits to check cell voltages
with a quality DVM.

Charge regulator voltage limits should be set at 13.5V for a 12V
system, 27V for a 24V system, 40.5V for a 36V system, and 54V for a
48V system at 77F, which brings me to the next point.  Temperature
must also be taken to account as follows:

A location having an ambient temperature of 75°F (24°C) to 77°F
(25°C) will result in optimum battery life and performance,
irrespective of battery topology. Temperatures below 77°F (25°C)
reduce battery charge efficiency and discharge performance.
Temperatures above 77°F (25°C) will result in a reduction in battery
life.  The greater the rise above 77F average, the more rapid the
decline in battery life.  A 20F increase in average battery
temperature will reduce battery life by 50%.  To wit: a battery that
would have provided 20 years of service at 77F average will provide
only 10 years at 95F.

To attain best battery life, a charge compensation factor of .003
VPC/°F (.0055 VPC/°C) is recommended. The minimum voltage is 2.20
VPC.  The maximum is 2.35VPC.  Outside these limits, compensation
does not apply.

Battery charge rates also need to be controlled.  Many VRLA batteries
are able to accept current to a C/2 rate, or more, but this is not a
recommended practice.  C/8 to C/20 rates are preferred.  Chargers
should be of a current-limited constant-voltage design.  A three or
four state charger is not required except in the rare instances where
a controlled equalize charging is needed.  Even then, PV and wind
systems are poor candidates for providing the excess charge due to
their time-limited and/or variable output.  An equalize charge CAN be
accomplished on a "split" basis .. with half applied one day .. the
other half the next.  Bear in mind should you do this, the battery
system cannot be used in the middle of a split equalize cycle.

> Hi Darryl,
> We have discovered this lately about the Trimetric.
> It seem to become more inaccurate as the batteries
> become more sulphated - that is allowing them to drop
> below the 50% state of charge. Ralph at Bogard offered
> little help in explaining the reason for this. We are now
> not trusting the Trimetric, which we use to consider THE
> guide. We're asking customers to check the Trimetric
> accuracy with specific gravity tests.
>
> Eric Stromberger
> Mendocino Solar Service

As a battery sulphates, metered readings can become increasingly
inaccurate.  Sulphation due to chronic undercharging and long-term
disuse at low SOC results in decreased active plate area, which
generally cannot be recovered in sealed batteries.  (Flooded
batteries seem to be an exception, but that is for another day.)
Intelligent metering, thinking a battery is a particular capacity, is
often fooled when voltages fail to follow expected curves over time.
The longer a sulphated battery is used, the greater the errors
become.  This is tantamount to changing the wheels on your vehicle to
a different diameter, but not altering the mechanics used to measure
speed or distance.  Speed is in error instantly.  Conversely, the
farther you travel after the change, the greater the odometer error
becomes, but may not be immediately evident.

Hopefully this has provided some insight into batteries, with
emphasis on the newer VRLA topologies.


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