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Batteries: How to keep them
alive for years and years
Lead-acid batteries are often considered to be the "weak
link" in renewable energy systems. However, todays renewable
energy batteries are better than ever, and so are the devices
that regulate and protect them. Battery failures are rarely
the fault of the batteries themselves! Follow these guidelines
to avoid the vast majority of all battery problems.
Size a battery bank and PV array properly
A battery bank should be sized (as a minimum) to a capacity
of 5 days of load. Energy use in most home power systems increases
over time, so consider sizing larger than that. Why? After
1 year of service, it is NOT advisable to enlarge a battery
bank by adding new batteries to it, because batteries' voltage
response changes with age. Stray currents flow, causing losses
and failure to equalize. A PV array, if it is the primary
energy source, should be sized to produce (on average) 30%
more energy than the load requires. This compensates for battery
losses and for less-than-average charging conditions. Luckily,
a PV array can be expanded at any time.
Buy high-quality batteries, selected
for your needs
You get what you pay for! Good deep-cycle batteries can be
expected to last for 5 to 15 years, and sometimes more. Cheap
batteries can give you trouble in half that time. Buy from
a reputable source.
Avoid multiple parallel strings
The ideal battery bank is the simplest, consisting of a single
series of cells that are sized for the job. Higher capacity
batteries tend to have thicker plates, and therefore greater
longevity. Having fewer cells will reduce the chance of randomly
occurring defects, and reduces maintenance. Suppose for example,
that you require a 700 Amp-Hour bank. You can approximate
that by using 3 parallel strings of golf-cart batteries (220
AH), or 2 strings of the larger L-16 style batteries (350
AH) or a single string of larger, industrial batteries.
Under no circumstances is it advisable to install more than
three parallel battery strings. The resulting bank will tend
to lose its equalization, resulting in accellerated failure
of any weak cells. Weak cells will be difficult to detect
because they will "steal" from the surrounding cells, and
the system will suffer as a whole and will cost you more in
the long run.
Here are some precautions to take when wiring two or more
strings of batteries in series-parallel. The goal is to maintain
all of the cells at an equal state of charge. Cells that tend
to receive less charge are likely to fail prematurely. This
can take years off of the effective life of the battery bank.
A fraction of an ohm of added resistance in one battery string
can reduce the life of the entire string.
(1) Connect the two main cables to opposite corners of the
battery bank, and maintain symmetry in wire size and lengths.
This will help to distribute current evenly through the bank.
(2) Arrange batteries to maintain even temperature distribution
throughout the bank. Avoid uneven exposure to heat sources.
Leave at least 1/2 inch of air space around each battery,
to promote even cooling.
(3) Apply a finish charge at least every 3 weeks (bring every
cell to 100% charge).
Prevent corrosion
In flooded battery installations, corrosion of terminals
and cables is an ugly nuisance that causes resistance and
potential hazards. Once corrosion gets hold, it is hard to
stop. The good news -- it is easy to prevent! Apply a non-hardening
sealant to all of the metal parts of the terminals BEFORE
ASSEMBLY. Completely coat the battery terminals, the wire
lugs, and the nuts and bolts individually. A sealant applied
after assembly will not reach all around every junction. Voids
will remain, acid spatter will enter, and corrosion will begin
as soon as your installation is finished.
Special compounds are sold to protect terminals, but you
can have perfectly good results using common petroleum jelly
(Vaseline). It will not inhibit electrical contact. Apply
a thin coating with your fingers, and it won't look sloppy.
If wire is exposed at a terminal lug, it should be sealed
airtight, using either adhesive-lined heat-shrink tubing or
submersible rubber splice tape. You can also seal an end of
stranded wire by warming it gently, and dipping it in the
petroleum jelly to liquify, and wick it into the wire.
It also helps to put the batteries over a floor drain, or
in a space without a floor, so that they can be rinsed with
water easily. Washing the battery tops (about twice per year)
will remove accumulated moisture (acid spatter) and dust.
This will further reduce corrosion, and will prevent stray
currents from stealing energy. Batteries that we have protected
by these measures show very little corrosion, even after 10
years without terminal cleaning.
Moderate the temperature
Batteries lose approximately 25% of their capacity at a temperature
of 0C (compared to a baseline of 25C). At higher temperatures,
they deteriorate faster. Thus, it is desirable to protect
them from temperature extremes. If no thermally-stable structure
is available, consider an earth-sheltered enclosure. Where
low temperature cannot be avoided, get a larger battery bank
to make up for the loss of capacity in the winter. Avoid direct
radiant heat sources that will cause some batteries to get
warmer than others.
Use temperature compensation
When batteries are cold, they require an increase in the
charge voltage limit, in order to reach full charge. When
they are warm, they require a reduction in the voltage limit
in order to prevent overcharge. Temperature compensation is
a feature in many charge controllers and power centers, as
well as in the back-up chargers in some inverters. To use
this feature, order the accessory temperature probe for each
charging device, and attach it to any one of the batteries.
Use low-voltage disconnects
Discharging a battery to exhaustion will cause immediate,
irreversable loss of capacity and life expectancy. Your system
should employ low voltage disconnect (LVD) in the load circuits.
Most inverters have this feature, and so do many charge controllers
and power centers. Don't depend on human behavior to prevent
over-discharge. It can be caused easily by accident or by
an irresponsible user. Again, most inverters have LVD built-in
but if there are DC loads on the system, please incorporate
an LVD device.
Bring batteries to a full state-of-charge
at least every 3 weeks
Bring the batteries to a full state-of-charge (SOC) at least
every 3 weeks. This reduces internal corrosion and degradation,
and helps to insure equalization, so that any weaker cells
do not fall continually farther behind. A full SOC may occur
naturally during most of the year, but do not hesitate to
run a generator when necessary, to bring the batteries up.
Information like this should be posted at the power center.
For more details, refer to the instructions for the inverter/charger
and for the batteries.
How do you know when a battery is
100% charged?
The "charged" indicator on most PV charge controllers means
only that battery voltage is relatively high. The SOC may
be approaching full, but is not necessarily near 100% A voltmeter
reading gets you closer, but it is not a certain indicator.
It varies to much with current flow, temperature and time,
to give a clear indication.
For flooded batteries, a hydrometer is the definitive indicating
device, although not a convenient one. With it, you can measure
every cell individually. Obtain one from a battery or automotive
supplier. Even the cheapest hydrometer is fine. Rinse it after
use, and keep it clean. An amp-hour meter is the most informative
and user-friendly way to monitor SOC. For sealed batteries,
it is the ONLY definitive method. See next paragraph.
Install a System Monitor
Would you drive a car with no dashboard? Metering is not
just "bells and whistles". It is necessary to help you to
read the status of the sytem. Many charge controllers have
indicator lights and readouts built-in. For a full-scale remote
home, consider the addition of a digital monitor, like Trace
TM-500, Tri-Metric, E-Meter or Omni-Meter. These devices monitor
voltage and current, record amp-hours, and accurately display
the state-of-charge of the battery bank. They also record
more detailed information that can be useful for troubleshooting.
The monitor may be mounted in another room or building, for
handy viewing.
How to Read a Hydrometer
A hydrometer will help you to determine whether the battery
bank is getting fully charged, and whether any individual
cells are falling behind. You should be aware that a hydrometer
will give you false readings under the following conditions.
(1) After adding water: For pure water to mix throughout
the cell, it takes time and some bubbling during finish charge.
A hydrometer will show a greatly reduced reading until the
fluid mixes.
(2) Low temperature: As battery temperature drops, the fluid
becomes more dense. A temperature compensating hydrometer
is best. Otherwise, for every 10C below 25C, subtract 3.5
points from the reading.
(3) Time lag during recharge: As the battery recharges, the
fluid becomes more dense down between the plates. The hydrometer
reads the fluid above the plates. You will get a delayed reading
until the fluid is mixed by the movement of bubbles during
finish charge. The voltage will rise steadily, providing an
indication that something is happening.
During discharge, you will get a true hydrometer reading
because the fluid becomes less dense and will circulate to
the top. Any time a hydrometer indicates a fully charged cell,
you KNOW it is fully charged. WARNING
BATTERY ACID IS HAZARDOUS. When working around batteries,
wear safety glasses. Get a rugged plastic bottle to keep with
your service tools, and fill it with a sodium bicarbonate
(baking soda) and water. Use it to neutralize accidental splash
or spills and to clean normal acid spatter from battery tops.
Finally, don't wear your favorite blue jeans!
Just add water
Note: This applies only to "flooded batteries", not to "sealed
batteries". The plates of every cell in your battery bank
must be submerged at all times. Never add any fluid to a battery
except distilled water, deionized water, or very clean rainwater
collected in plastic containers. Most batteries require addition
of water every 6 to 12 months. There is no need to fill them
more frequently than needed to submerge the plates. Fill them
only to the level recommended by the manufacturer, generally
about an inch below the top, otherwise they may overflow during
finish-charging.
Conclusion
Batteries are the heart of your power system. They may demand
your attention occasionally, but your relationship with them
need not be a struggle. With a proper installation, a little
understanding and some simple maintenance, your batteries
will live a long and healthy life.
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