14-02-2013, 09:20 AM
A new pulse charging methodology for lead acid batteries
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ABSTRACT
Lead acid battery cells have low energy density and relatively low life-cycle, yet
because of their cost effectiveness they are still considered the preferred choice by
many electric vehicle (EV) developers and are likely to continue to be so for the
next 5-10 years. One method of improving the performance of a battery powered
EV is to improve the battery charging methodology since EV performance and
range is largely determined by the capacity, weight and charge/discharge
characteristics of the on-board batteries. This paper describes a method for fast
charging lead acid batteries using current pulses of controllable magnitude and
duty called ‘pulse charging’. It is used together with constant voltage/current
profiles to increase charge acceptance, improve the charging time, and to
potentially increase the life cycle of lead acids cells.
Introduction
Lead acid batteries are one of the oldest electro-chemical storage cells. Since their discovery by Plante
in 1859 their energy density, charge and discharge characteristics have been improved greatly but the
basic cell elements are still the same. They are available in a variety of sizes from 1 to over 1000
Ampere-hours (battery capacity) and are almost always the least expensive storage battery for any
application [1]. There are three common types available, namely; flooded, absorbed glass mat and gell,
where the latter two are generally termed sealed or valve regulated lead acid (VRLA). Lead acid
batteries are used extensively for energy storage, emergency power as well as for engine starting,
vehicle lighting and engine ignition. Because of their cost and reliability, lead acid batteries (and
VRLA in particular) are still considered to be the dominant storage battery in the near to medium term
[2] especially in the EV industry where battery technology is undergoing considerable development.
Yet despite this, lead acid batteries still exhibit low energy density, require high maintenance, and are
slow to charge.
Pulse charging of lead acid batteries
Lead acid chemistry
The chemical process of a lead acid battery consists of two electrodes - the negative electrode made of
metallic lead (Pb), and the positive lead-oxide (PbO2) electrode, immersed in a sulphuric acid solution
(H2SO4) as shown in Figure 3.
On discharge both the electrodes build up lead sulphate PbS04(s) and the electrolyte is converted to
H2O, whilst the opposite occurs during charging [1].
Invariant pulse charging
Research overseas has shown that the hydrogen and oxygen gas development in a battery is not
immediate but has a time constant relating to the state of charge of the battery [5]. Therefore if an
applied current pulse is short enough, most of the current will be consumed by the charge reaction
rather than producing hydrogen gas. This is the principle of pulse charging - applying relatively large
currents into a battery at periodic intervals with a defined pulse width to reduce or avoid gassing and
thus increase charge acceptance and efficiency. An additional advantage is that this principle can even
be applied to almost fully charged batteries.
Prototype charger
To evaluate the performance and practicality of pulse charging, a prototype pulse charger was
developed. The prototype consists of a micro-controlled current source which is connected to a host
personal computer (PC) as shown in Figure 5. The main components are the power electronics which
form a synchronous rectifier, supervisory microprocessor and the personal computer which interfaces
to the user. Together it is designed to supply up to 100 amp current pulses for either charging or
discharging of the lead acid cells. It is also able to provide constant charge or discharge currents but of
a much lesser magnitude due to the heating of the semiconductors. To generate the current pulses, the
power electronics form a synchronous rectifier shown in Figure 6.
Charging performance
To evaluate the performance of the charger and the charging method proposed in [4], two 28 Amperehour
VRLA gell lead acid batteries were acquired with approximately the same capacity. One was
fitted with pressure and temperature sensors for monitoring, while the other (with slightly higher
capacity) was left as supplied.
In Figure 8 a selected result of a typical charging profile on the modified battery using conventional
charging techniques is shown. The charging current of 7 amps represents a typical overnight charging
rate for this sized battery, and was the limit of the conventional charger supplied. As illustrated here,
once the terminal voltage reaches the charging voltage (usually between 13.5-14.7volts) the charging
rate is dictated by the charge acceptance of the battery and contributes to as much as 80% of the
charging time. After 3 hours, 50% of capacity is returned and the rate of charge for the remaining 50%
is very slow, taking in excess of 30 hours. Attempts to speed up the battery charge-rate are limited. In
the constant current region either the capacity of the power source or thermal characteristics within the
battery determine this limit. In the constant voltage region a larger applied voltage or an alternative
constant current charge scheme only increases the production of H2 and O2 gassing of the battery and
does little to improve the charging time. It is in the latter region where pulse charging can provide
clear benefits in charge rate as described in section 3.
Further improvements for charge acceptance
As the battery state of charge increases the concentration of active materials near the grids within the
battery begins to decline and is one of the factors that results in lower charge acceptance. This
concentration is lowest near these grids but increases as the distance from the plate increases. If a
small discharge pulse is applied immediately following each charging pulse the concentration
differential attempts to reverse. This reaction partially restores the concentration of active materials
thereby improving the charge acceptance of the next applied current pulse. This effect can be seen in
the monitored battery terminal voltage which begins to drop immediately after the positive current
pulse has finished. This voltage should ideally be allowed to decrease to around 13-13.5 volts,
indicating sufficient restoration of the concentration gradients surrounding the plates to ensure good
charge efficiency in the next delivered pulse. A modified pulse charging strategy is therefore proposed
(Figure 14) in order to investigate the effectiveness of such a strategy.
Conclusions
To validate the advantages of pulse charging, a working prototype has been developed. This is a
general purpose charger capable of generating large current pulses for charging or discharging the
battery while a connection of a PC allows a simple interface between the user and the charger which is
capable of changing the charging profiles, and storing the received data.
A simple ‘unintelligent’ invariant pulse charging approach was shown to dramatically decrease
charging time but is also capable of gassing the battery and must be applied with care. In this
technique specific pulse widths and/or magnitudes must be chosen carefully to fit each lead-acid battery
and may need modification with battery age.
A new pulse charging approach is proposed with variable pulse width controlled by feedback
resistance free battery voltage, presenting an average current to the battery that is similar to
conventional charging profiles. This approach is shown to dramatically improve the charge rate
outside of the constant current region (typically around 50% state of charge). In addition, it appears to
eliminate gassing at these high states of charge, and does so without the need of additional sensors. A
practical observation is that high currents applied during the initial part of the charging cycle cause
consistent but limited temperature and pressure rises that may require monitoring (particularly for
sealed VRLA lead-acid cells). If the battery is placed at elevated temperatures then higher currents
than specified by the battery manufacturer should be used with care.