Rob
Phillips, CEO of battery consultancy Accutronics, gives a brief overview of Smart Batteries, Smart Chargers, and the
System Management Bus, and their advantages as power solutions for portable products…
Batteries can be
viewed as living beings with their environment and treatment directly affecting
their performance. Batteries that are overcharged or exist at elevated
temperatures often exhibit a shortened life, whilst those that are treated
carefully can provide many years of maintenance free performance.
In order to charge batteries appropriately, it is
necessary to have a method of understanding the charge state of the
battery. This has to be balanced with extracting the maximum discharge and
lifetime from the battery, so as to also offer value-for-money.
Appropriate battery specification is therefore of
paramount importance to portable device designers. However one type of battery
solution; smart batteries, which have been on the market for a while now, can radically
simplify the process of battery specification, while dramatically reducing
risk.
What are smart batteries, and how do they
work?
Smart batteries are intended to be integrated
into portable devices as part of a wider ‘smart power management system’. This
will typically include a smart battery, a smart charger, and a systems
management bus (SMBus) for communicating between the different elements.
In a traditional portable device setup, any
battery featured is simply a ‘dumb’, chemical power cell. Battery metering,
assessment of remaining capacity, and any decisions taken regarding power
usage, are informed entirely by the readings ‘taken’ by the host device. Such
readings are largely guesswork, and are typically based on the amount of
voltage passing from the battery through the host device, or, (less
accurately), through readings taken through a Coulomb Counter in the host.
In a smart power management system, however, the
battery is able to ‘tell’ the host, with a high degree of accuracy, how much
power it has left, and how it wishes to be charged.
In general, the battery, smart charger and the
host device communicate with each other to maximise product safety, efficiency
and performance. For example, smart batteries only request charge when they
require it, rather than placing a constant, steady ‘drain’ on the host system.
Hence smart batteries charge more efficiently.
Smart batteries can also maximise the ‘runtime
per discharge’ cycle by telling their host device when to shut down based on
its own assessment of its remaining capacity. This method is vastly superior to
‘dumb’ systems that use a fixed voltage cut-off.
Hence host portable systems that use smart battery
technology can provide accurate, meaningful runtime information to users. This
is obviously of vital importance in mission critical devices where
power failure is not an option.
Gauging and adaptation
Smart batteries constantly track their own
capacity whether they are being charged, discharged or stored. Battery capacity is reported in
milli-ampere hours (mAh) to a resolution of 1mAh. The real capacity is reported
in both mAh and as a percentage, (of the original design capacity and of the
last time the battery was charged).
Certain correction factors are employed by the
battery fuel gauge to adjust for changes in, for example, battery temperature,
its charge rate, its discharge rate, etc. Smart battery gauges also tend to be
adaptive, modifying the adjustments they make as the battery ages and its chemical
properties and ability to hold charge change.
As a result, smart batteries can usually predict
their capacity to within ±1%,
(a major advantage when compared to the ±20% accuracy found in products
employing ‘dumb’ batteries).
Smart
batteries can also extend the useful life of a battery by modifying their charging
algorithm based on changing environmental conditions: Batteries can be damaged
if they are charged whilst they are very cold or very warm. Smart batteries
will therefore reduce the charge current while the battery is warm to reduce
the potential for damage, and prevent charging altogether if the battery is
exceptionally cold or hot.
Standardised communications and future proofing
Smart batteries maximise charge efficiency and
safety by requesting their own charge voltage and current from a compatible
smart charger. This method ensures that batteries are only charged when they
need to be, and at the most appropriate voltage and current.
Being ‘SMBus (System Management Bus) and SBDS
(Smart Battery Data Specification) compliant’ means that smart batteries comply
with an open standard that is easily accessible by OEM device developers.
The SBS (Smart Battery System) specification, and
the accompanying SMBus spec were originally created by Duracell and Intel in
1994, and involve carrying battery information over a two-wire communication
bus. Along with the SBCS (Smart Battery Charger Specification) and the SBSMC
(Smart Battery System Manager Specification), the SBS standard describes all of
the information that can be communicated between smart batteries, chargers and
host devices.
As a result of its on-going communication of
status, charge profile, etc., a smart power system is able to future proof a product’s
power supply: Should newer battery technologies be developed (with, for
example, higher storage capacities and different charging regimes), these new
batteries can simply be ‘swapped out’ without having to replace the chargers in
the field. The existing chargers will simply receive different charging
instructions from the batteries and adapt accordingly.
This simple fact frequently makes Smart Power
Systems more economical in the long run, particularly in the case of medical
equipment that is highly expensive, or for which maximised battery life is
crucial, (in the case, for example, of Anaesthesia workstations, ventilators
and remote patient monitoring).
Ease of integration
Interestingly, we’ve found that customers tend to
imagine they’ll need to do lots of work to make batteries work in their system.
The reality, however, is that electronic
component manufacturers such as Texas Instruments and Linear Technology have
produced excellent and largely reliable, highly-integrated reference designs
for their smart charger and power management ICs. These reference designs make
the integration of a smart battery pretty easy. In addition, the openness of
the various communications protocols make life easy for device software
engineers to interrogate the batteries and extract useful information, such as
remaining capacity, runtime to empty, cycle life, etc.
Powering ahead
Despite coming with a small associated cost-add,
smart batteries and smart charging systems tick the right boxes for many portable devices, with clear advantages in terms of life, charge, safety and future proofing.
The market for smart charging systems is now
fairly mature. However the option of implementing smart charging systems is overlooked
by designers surprisingly frequently. Also, the battery life and level of
adaptability that designers can expect to afford from specific smart power
technologies are always changing. I’d always, therefore, recommend that designers
engage with a credible third-party battery specialist in order to ensure that
they specify the most appropriate smart charging solution for their device.
