TechArchive

Original article date: June 1996

Many people will have experienced some problems in the use of rechargeable batteries in both their professional and personal life. KEVIN HALLIGEN at DSB Special Batteries reviews the position.

The majority of difficulties encountered with rechargeable batteries in portable applications are a function of poor management crude charging and incorrect cell selection. Providing a complete solution to the user’s power management requirements negates ‘fuzzy’ interfaces and aids better understanding of overall system requirements.

One of the major concerns of the professional user of battery-powered communications or safety equipment is to find that in an emergency situation a battery failure renders it inoperative. It is for this reason that many critical applications still use primary batteries.

Users require the following features of a secondary battery:

• Fast-charge without a reduction in cycle life.
• Optimum power-to-weight ratio by utilising new battery chemistries
• Visible state-of-charge and remaining usable capacity via an electronic indicator or fuel gauge.

Lead-acid batteries are typically used in heavy duty or stand-by power applications. They are therefore outside the scope of this article. Nickel-cadmium (Ni-Cd) has for some time been the backbone chemistry for the majority of rechargeable applications. Ni-Cd can be a reliable and very forgiving technology that allows both fast charge rapid discharge and some degree of battery abuse without significant long-term damage.

Over the last few years Nickel metal hydride (Ni-Mh) cells have been introduced spurred on by a desire for a more environmentally friendly solution which offers similar or better energy density to Ni-Cd batteries. However whilst Ni-Mh is able to offer a higher capacity per unit volume than Ni-Cd it is more prone to damage on charging and is not suited to high rate discharge applications. In addition self discharge is high meaning Ni-Mh is really only suitable for cyclic applications where standing time between charges is kept to a minimum.

A number of major problems have been experienced by the professional battery user. These include:-

• Conventional charge termination causing repeated overcharge which stresses the battery and reduces cycle life due to the ‘venting’ of the electrolyte.
• Early charge termination due to unstable battery characteristics during charge. This is particularly common with inflection point termination technology.
• Memory effect – caused by crystalline build-up on the battery plates as a result of incomplete discharge cycles coupled with continued overcharge.
• Inefficient and unreliable charger designs due to the use of linear constant current regulators in conjunction with heavy transformer-fed power supplies.

The introduction of better battery charge controllers has meant that most of these problems have been either completely eliminated or reduced to a more acceptable level. However the lack of a strong inflexion point (-dV characteristic) of Ni-Mh can cause a charger to fail to terminate due to the absence of a significant voltage drop and hence cause the battery to overcharge. If this is repeated over a number of cycles it will significantly reduce battery performance capacity and life.

Battery management rather than the rechargeable battery itself is the cause of many of the problems being encountered. There are many different types of battery chargers on the market ranging from simple linear chargers in a wall block through to sophisticated fast chargers which are able to monitor the condition of the battery during the charge cycle and optimise the process.

Whilst sophisticated fast chargers are obviously more expensive they offer the user a number of valuable long term benefits that can make the system a cost-effective option and reduce ownership costs. These include:-

• Extended battery life due to accurate charge termination. Independent tests have shown battery cycle life can be improved by up to 50% when compared to -dV charge regimes.
• A conditioning profile can extend the life of ‘tired’ batteries. By cycling a faltering pack with the reflex charging technique the capacity can be increased.
• Ni-Cd batteries can be safely charged at the 4C rate (15 minutes). Ni-Mh cells need to be handled more gently but a 2C (30 minute) charge has proved to be achievable using advanced charging techniques while still providing adequate battery cycle life and usable capacity.
• Deep negative discharge pulses (characteristic of reflex charging techniques) assist the breakdown of ‘dendrites’ on the cell plates. This reduces the so-called ‘memory effect’ without the need to fully discharge the battery on every cycle. Batteries charged using this technique from new are unlikely to show memory effect older batteries may require a number of discharge/charge cycles before a significant improvement in performance is perceived. This improvement can normally be measured in terms of internal impedance of the cell.

DSB can incorporate a range of advanced electronics within battery packs. These range from simple capacity gauges with basic displays to complex battery diagnostics in-the-pack charger modules or output conditioning.

These systems are usually designed for OEMs and military users who are looking to further integrate their battery-powered equipment and to optimise its performance at the same time.

Some of the available packages include:-

• Battery capacity monitor for battery packs or in-system installation that maintains an accurate record of a battery’s available charge. The circuit monitors a voltage drop across a sense resistor connected in series with the negative battery terminal and determines the charge status and discharge activity. Self-discharge of Ni-Mh and Ni-Cd batteries is estimated based on an internal timer and temperature sensor. Compensations for battery temperature and rate of charge or discharge are applied to charge discharge and self discharge calculations to provide ‘available capacity’ information across a wide range of operating conditions.
• Battery event and time remaining indicators that display charge/discharge data battery abuse data and cycle count. These are available with a range of functionality from basic LEDs to on-screen displays giving accurate real-time information on battery status.
• Output voltage converters which provide pre-regulated battery voltages which improve system efficiency and reduce complexity.

For rechargeable battery users who are experiencing problems and do not have access to advanced charging technology there are a number of useful tips that if used over a period of time will improve the service life of their battery.

• Fully discharge a battery before charging it as this will reduce the so called ‘memory effect’. If it is not practical to do this every time then the battery should be taken out of service periodically and fully cycled.
• Ensure that the rechargeable cells used are up to the application. For example cell manufacturers produce a range of cell types to cover specific applications. Example include:- high temperature cells for use in emergency lighting and high rate cells for use in power tools.
• Try to establish a regime whereby batteries are not left ‘on charge’ for long periods of time. Even with trickle chargers this will cause the battery to be overcharged and potentially overheated.
• Batteries will tend to self-discharge if left on the shelf for any period Ni-Mh cells self-discharge at a more rapid rate than Ni-Cd cells. Therefore charge the battery just before you need it!

Rechargeable battery chemistries are currently being introduced to replace the traditional Ni-Cd and relatively new Ni-Mh systems. These are lithium-Ion lithium-polymer and lithium-metal technologies which offer a much higher energy density than Ni-Cd or Ni-Mh and – in the case of lithium-polymer – a very low profile design which is attractive to mobile telephone manufacturers.

These technologies although still in development are far from common. It is likely that in 1-2 years they will be readily available in the market place at a comparable price.

• DSB Special Batteries
• Tel: 01293 611930
• Contact: Kevin Halligen

June 1996