is a set of notes from a literature review intending to lead to the
development of a charging algorithm for a photovoltaic battery power
management system. The information may not be accurate.
Lead-Acid batteries present a number of difficult issues to deal with when it comes to charging for long life
use. The following will briefly review battery operation and methods for fast,
efficient charging. Most algorithms are applicable to
situations where an unlimited amount of current is available. In
photovoltaic systems the current available is variable and limited, so
the algorithms need to be re-evaluated for this purpose.
"smart" algorithms are considered, that is, those that are implemented
using computer control and suitable electronics hardware support. A
number of algorithms relate to low cost commercial implementations
dating from earlier times which do not necessarily address significant
needs of the batteries. A good description of some of the more important algorithms is given in .
scenario is a photovoltaic or aeolian system in which the batteries are
used intermittently and recharged in when power is available.
lead acid battery chemistry is described well in many available
publications (e.g. the introduction in ). The issues to be noted are:
overcharge reactions will take place at normal cell voltages but at a
quite low rate. Battery plate composition can be adapted to reduce this
self discharge rate to a minimum. The so called
gassing voltage is a compromise between the rate of battery charging
and the loss of electrolyte due to electrolysis.
discharge lead sulphate is formed at both plates and can form
insulating deposits if the battery is not fully recharged, resulting in
loss of capacity over time.
- During overcharge the positive plate can erode.
overcharge molecular oxygen can be released at the positive plate
and, under extreme overcharge, hydrogen at the negative plate. These
gases are released in wet cells with accompanying loss of water, while
in VRLA cells the oxygen is allowed to diffuse to the negative plate where it is
recombined with hydrogen ions back to water in an oxygen recombination
reaction. This reaction is exothermic and can cause water loss and
thermal runaway if care is not taken. The reaction can take up an
increasing amount of the charging current as the cells age, resulting
eventually in an inability to recharge the battery.
temperatures increase the rate of reactions and can result in irreplaceable loss of water in VRLA cells,
particularly during charging. Charging is usually recommended to not take place
at temperatures above 50oC.
Battery charging algorithms are reviewed separately as follows:
term refers to the uncontrolled increase in temperature in a lead-acid
battery as a result mainly of the exothermic oxygen recombination
reaction. It can occur during overcharge, typically in the float phase,
when the ambient temperature is high and the float charge current has
not been reduced accordingly. Other factors such as inadequate
ventilation of the battery case can contribute to the phenomenon. Oxygen recombination
increases with battery age so it can be difficult to predict when
runaway will occur; it is preferable to detect the beginnings of
runaway and disconnect the charger.
from the runaway phenomenon which can cause catastrophic fire damage,
overheating could also cause internal damage and premature battery
the best way to detect thermal runaway is to monitor the battery
temperature. Alternatively the current could be monitored at a fixed
voltage. When the battery is fully charged all current is used by the
oxygen generation and recombination processes. During charging in the
absorption and float phases, current should decrease as the battery
approaches full charge and remain low. If it begins to increase rapidly
after this, it may indicate possible runaway. In any case there is no
value in continuing to put more current into the battery as it is
achieving no useful recharge.
of pulsed or interrupted current charging has been reported to improve
the cycle life of batteries, with the ICC algorithm being proposed to
ensure that the battery is fully charged. However the studies to hand
use experimental results and
do not give an explanation of the electrochemical processes behind the
success of this method. In order to make an intelligent decision about
appropriate parameters for these algorithms, some insights are needed.
A mathematical model developed in  aims to do just this, taking into
account the double layer.
rest period after each of the charge cycles allows the concentrations
of reaction products that build up near the plates during charge, to
diffuse away. This enables more charge to be put into the battery while
maintaining the terminal voltage below the gassing voltage. As such the
rest period would need to be of the order of the relaxation time of the
diffusion processes. Some experimental results show that the total time
to complete fuil charge is actually less than that which would be
achieved using a constant terminal voltage in the absorption phase of
the three stage method. This could be due to the lower level of gassing
reactions that absorb current, and possibly the reduction of the
shielding effect of the double layer.
Application to Photovoltaic Systems
systems are characterised by current and voltage limited sources that
vary with available sunlight. Aeolian systems have the same nature. As
such it isn't possible to rely on unlimited current levels for fast
charging. Nevertheless the ICC method has applicability here
particularly if more than one battery is being managed, as other
batteries could be charged during the rest period of a particular
to adapt the algorithm would be to attempt to put a fixed amount of
charge into the battery during the charge cycle, while keeping the rest
period constant. Thus the charge cycles will vary in duration as the
current varies. This is only approximate as the battery characteristics
are very nonlinear. Indeed if the current is low enough there may not
need to be any rest period required.
possibility would be to continue to put charge into the battery until
the voltage reaches a given level. If this level is the gassing voltage
then we are almost back to the intermittent charge (IC) algorithm
although here we are keeping the rest period constant. The difficulty
with this and with IC is that it is difficult to identify an
end-of-charge point as the cycles will vary in length with available
current from the PV source.
proposal is to charge the battery during a fixed time and allow it to
rest while other batteries are being charged. The rest period will as a
minimum be twice that of the charge period. This interrupted mode can be used also during the bulk charge phase if there are other batteries being charged. The
current should be limited in systems where the source current can be
high, but typically the overall system design will provide its own
appropriate limit. If this is not the case then the ICC method may not
be suitable as excess power would simply be wasted. This algorithm only
differs from the ICC in that the currents are variable over time.
- "Charge regimes for valve-regulated lead-acid batteries: Performance overview inclusive of temperature compensation." Y.S. Wong, W.G. Hurley, W.H. Wölfle. Journal of Power Sources 183 (2008) 783–791.
Algorithms for Increasing Lead Acid Battery Cycle Life for Electric
Vehicles." Matthew A. Keyser, Ahmad Pesaran, Mark M. Mihalic, Bob
Nelson, 17 Electric Vehicle Symposium Montreal, CANADAOctober 16-18, 2000
charging of lead/acid batteries - a possible means for overcoming
premature capacity loss?" L.T. Lam *, H. Ozgun, O.V. Lim, J.A.
Hamilton, L.H. Vu, D.G. Vella, D.A.J. Rand, Journal of Power Sources 53 (1995) 215-228.
- “Mathematical modeling of current-interrupt and pulse operation of valve-regulated lead acid cells,” V. Srinivasan, G. Q. Wang, and C. Y. Wang, J. Electrochem. Soc. 150, A316–A325, 2003.
- "A New Approach to Intermittent Charging of Valve-Regulated
Lead–Acid Batteries in Standby Applications", M. Bhatt, W.G. Hurley,
W.H. Wölfle, IEEE Trans. Ind. Electron. 52 (2005) 1337–1342.