FAQs About PWM Charging
in Solar Systems:
Pulse Width Modulation (PWM)
is the most effective
means to achieve constant voltage battery
charging by switching the solar system controller’s power devices. When in PWM
regulation,
the current from the solar array tapers
according to the battery’s condition and recharging needs.
Charging a battery with a solar system is a
unique
and difficult challenge. In the “old days,” simple
on-off regulators were used to limit battery out-gassing when a solar panel produced
excess
energy. However, as solar systems matured it
became clear how much these simple devices interfered
with the charging process.
The history for on-off regulators has been early
battery
failures, increasing load disconnects, and growing
user dissatisfaction. PWM has recently surfaced
as the first significant advance in solar battery
charging.
PWM solar chargers use technology similar to
other
modern high quality battery chargers.
When a battery voltage reaches the regulation
setpoint,
the PWM algorithm slowly reduces the charging
current to avoid heating and gassing of the
battery, yet the charging continues to return the
maximum amount of energy to the battery in
the shortest time. The result is a higher charging
efficiency, rapid recharging, and a healthy
battery at full capacity.
In addition, this new method of solar battery
charging
promises some very interesting and unique
benefits from the PWM pulsing. These include:
1. Ability to recover lost battery capacity and
desulfate
a battery.
2. Dramatically increase the charge acceptance
of
the battery.
3. Maintain high average battery capacities (90%
to
95%) compared to on-off regulated state-of-charge levels
that are typically 55% to 60%.
4. Equalize drifting battery cells.
5. Reduce battery heating and gassing.
6. Automatically adjust for battery aging.
7. Self-regulate for voltage drops and temperature
effects in solar systems.
These benefits are discussed in more detail in the following paragraphs.
How does this technology help me?
The benefits noted above are technology driven.
The more important question is how the PWM technology
benefits the solar system user.
Jumping from a 1970’s technology into the new
millennium
offers:
• Longer battery life:
– reducing the costs of the solar system
– reducing battery disposal problems
• More battery reserve capacity:
– increasing the reliability of the solar system
– reducing load disconnects
– opportunity to reduce battery size to lower
the
system cost
• Greater use of the solar array energy:
– get 20% to 30% more energy from your
solar
panels for charging
– stop wasting the solar energy when the
battery
is only 50% charged
– opportunity to reduce the size of the solar
array
to save costs
• Greater user satisfaction:
– get more power when you need it for less
money!!
ADVANTAGES OF PWM CHARGING:
1. Ability to recover lost battery
capacity
According to the Battery Council International,
84%
of all lead acid-battery failures are due to sulfation.
Sulfation is even more of a problem in
solar systems, since “opportunity charging” differs
significantly from traditional battery charging.
The extended periods of undercharging common
to solar systems causes grid corrosion, and
the battery’s positive plates become coated with
sulfate crystals.
PWM pulse charging can deter the
formation
of sulfate deposits, help overcome the resistive
barrier on the surface of the grids, and
punch through the corrosion at the interface. In
addition
to improving charge acceptance and efficiency,
there is strong evidence that this particular
charging can recover capacity that has been
“lost” in a solar battery over time. Some research
results are summarized here.
A 1994 paper by CSIRO, a leading battery
research
group in Australia notes that
pulsed-current charging “has the ability to recover
the capacity of cycled cells.” The sulfate crystallization
process is slowed, and the inner corrosion
layer becomes thinner and is divided into
islands. The electrical resistance is reduced and
capacity is improved. The paper’s conclusion is
that pulse charging a cycled battery “can evoke a
recovery in battery capacity.”
Another paper, a Sandia National Labs study in
1996
summarizes testing of a VRLA
battery that had “permanently” lost over
20% of its capacity. Conventional constant voltage
charging could not recover the lost capacity.
Then the battery was charged with a PWM
controller, and “much of the
battery capacity has been recovered.” It
was practically proved that a battery that was “dead”
recovered much of its lost capacity after extended
charging with a PWM controller.
2. Increase battery charge acceptance
Charge acceptance is a term often used to
describe
the efficiency of recharging the battery. Since
solar batteries are constantly recharging with
a limited energy source (e.g. opportunity charging
with available sunlight), a high charge acceptance
is critical for required battery reserve capacity
and system performance.
Solar PV systems have a history of problems due
to
poor battery charge acceptance. For example, a
study of four National Forest Service lighting systems
using on-off shunt controllers
clearly demonstrated the problems caused
by low charge acceptance. The batteries remained
at low charge states and went into LVD every
night, but the battery typically accepted only
about one-half the available solar energy the
next day during charging. One system only accepted
10% of the energy available from the array
between 11:00 AM and 3:00 PM! After
extensive study, it was determined that “the
problem is in control strategy, not in the battery.”
Further, “the battery was capable of accepting
that charge, but it wasn’t being charged.”
Later a system “similar in all respects” except
using a constant voltage charge controller was
studied. In this case, the “battery is being maintained
in an excellent state of charge.” The
PWM controller puts 20% to 30%
more of the energy generated by the solar
array into the battery than the on-off
regulator.
3. Maintain high average battery
capacities
A high battery state-of-charge (SOC) is important
for battery health and for maintaining
the reserve storage capacity so
critical for solar system
reliability. An FSEC Test Report noted that “the life of a
lead-acid battery is proportional
to the average state-of-charge,” and that
a battery maintained above 90% SOC “can provide
two or three times more charge/discharge
cycles than a battery allowed to reach
50% SOC before recharging.”
However, many solar
controllers interfere with the recharging of the
battery. The FSEC study noted at the end of the
report that the “most significant conclusion is
that some controllers did not maintain the battery
SOC at a high level, even when loads were
disconnected.” In addition, a
comprehensive 23 month study of
SOC factors was reported by Sandia in 1994. It was learned that
the regulation setpoint has little effect on long-term
SOC levels, but the reconnect voltage is
strongly correlated to SOC.
Sandia concluded that the number of times
a
system cycles off and on during a day in regulation
has a much stronger impact on battery
state-of-charge than other factors within any
one cycle. Our PWM will “cycle”in regulation many times per second. It would be expected that batteries
charged with PWM algorithm
will maintain a very high
average battery state-of-charge in a typical
solar system. In addition to providing a greater
reserve capacity for the system, the life of
the battery will be significantly increased according
to many reports and studies.
4. Equalize drifting battery cells
Individual battery cells may increasingly vary
in
charge resistance over time. An uneven acceptance
of charge can lead to significant capacity
deterioration in weaker cells. Equalization is
a process to overcome such unbalanced cells. The
increased charge acceptance and capacity recovery
capabilities of PWM pulse charging will
also occur at lower charging voltages.
PWM pulse charging will hold the
individual battery cells in better balance where
equalization charges are not practical in
a solar system.
5. Reduce battery
heating and gassing
Clearly battery heating/gassing and charge
efficiency
go hand in hand. A reduction in transient
gassing is a characteristic of pulse charging.
PWM will complete the recharging job more
quickly and more efficiently, thereby minimizing
heating and gassing. The ionic
transport in the battery electrolyte is more
efficient with PWM. After a charge pulse, some
areas at the plates become nearly depleted of
ions, whereas other areas are at a surplus. During
the off-time between charge pulses, the
ionic diffusion continues to equalize the concentration
for the next charge pulse. In
addition, because the pulse is so short, there is
less time for a gas bubble to build up. The gassing
is even less likely to occur with the down pulse,
since this pulse apparently helps to break up
the precursor to a gas bubble which is likely a
cluster of ions.
6. Automatically adjust for battery
aging
As batteries cycle and get older, they become
more resistant to recharging. This is primarily
due to the sulfate crystals that make the plates
less conductive and slow the electro-chemical
conversion.
However, age does not affect PWM charging.
The PWM charging will always
adjust in regulation to the battery’s needs. The
battery will optimize the current tapering
according to its internal resistance, recharging
needs, and age. The only net effect of age with
PWM charging is that gassing may begin earlier.
7. Self-regulate for voltage drops
and temperature effects
With PWM charging, the critical
finishing charge will taper per the equation I = Ae-t. This provides a self-regulating final charge that
follows the general shape of this equation. As such, external system factors such as voltage
drops in the system wires will not distort the critical final charging stage. The voltage drop
with tapered charging current will be small fractions of a volt. In contrast, an on-off
regulator will turn on full current with the full voltage drop throughout the recharging cycle
(one reason for the very poor charge efficiency common to on-off regulators).
In summary, the PWM
charge controller will provide the recharging current according to what the battery needs and
takes from the controller. This is in contrast to simple on-off regulators that impose an external
control of the recharging process which is generally not responsive to the battery’s
particular needs.
