Can you trust a low-cost battery?

Master Instruments Pty Ltd
By Isidor Buchmann, Cadex Electronics
Tuesday, 05 July, 2005


The battery pack is a mystical black box that does not change size, weight and colour during its lifetime. Neither does it reveal the charge level and state-of-health. And yet, the battery plays a key role in many portable applications.

The rechargeable battery is a corrosive device that starts deteriorating as soon as it leaves the manufacturing plant. With the exception of nickel-based batteries, capacity loss is permanent.

Ageing is based on the shelf life and number of discharge/charge cycles. Depending on the battery type and depth of discharge, a rechargeable battery provides between 100 to 1000 discharge/charge cycles and the shelf life is 2-5 years.

In the first year, a new portable system works well because the batteries are new. There are very few unexpected failures attributed to batteries. During the second year, however, some batteries start losing capacity and the reliability begins to decline.

Figure 2: Good and faded battery. Battery A is half empty but can provide 100% capacity when fully charged. Battery B is fully charged and can only hold 50% capacity because of ageing.

New packs are added, and in time the battery fleet becomes a jumble of good and failing batteries. That's when the battery headaches begin.

The 'green light' on the charger does not guarantee a good battery; neither does it ensure sufficient runtime. 'Ready' only indicates that the battery is fully charged on whatever space that is available to fill.

In fact, weak batteries charge more quickly (because there is less to fill) and remain on 'ready' longer than strong packs. Bad batteries tend to gravitate to the top and become a target for the unsuspecting user. Batteries going 'ready' first may be deadwood. Ironically, these are the very batteries that may get picked in an emergency.

When fully charged, a new battery has a nominal capacity of 100%. This figure is based on a one-hour discharge at a specified load current. (For example, a 1 Ah nickel-cadmium battery can provide 1 A for one hour.) If the battery dies after 30 minutes, the available capacity is only 50%. A low runtime is caused either by a partial charge or an ageing battery.

In Figure 2 are two batteries with an assumed runtime of 30 minutes. Although both packs contain the same amount of energy, battery A is half charged but otherwise healthy, whereas battery B is fully charged but has an ageing problem.

Figure 3: Effects of internal battery resistance. A battery with low internal resistance is able to provide high current on demand, while the pack with high resistance causes the voltage to collapse and the equipment to cut off.

Unusable material (rock content) has crept in that robs the battery of its energy storage capability. To the outside, both packs appear identical. Measuring the terminal voltage or internal resistance will not give a definite clue as to the condition of the battery.

Beside capacity fade, the loss of battery performance is also manifested in the ability to deliver load current. Nickel-metal-hydride and lithium-ion batteries have an increase in internal resistance with age and cycle count.

Figure 3 illustrates the internal resistance, or load current, on hand of a free flowing and restricted water tap. Battery A can easily provide the needed current on demand, while Battery B will cause the voltage to collapse. The flow restriction will trigger a 'low battery' alert and shut down the equipment.

The symptoms manifest themselves in the battery being empty or not being able to hold a charge. High internal resistance is commonly caused by cell corrosion and cannot be reversed. Low temperature and low state-of-charge will contribute to a temporary increase in internal resistance.

A battery with high internal resistance may still perform adequately on low current appliances such as a flashlight, portable CD player or a wall clock. Medical equipment, laptops and two-way radios, on the other hand, draw heavy current bursts and the battery must have a sufficiently low internal resistance to deliver current.

The attribute of high capacity and low resistance is known as a battery having a good state-of-health.

Battery analysers as quality control

How can the state-of-health of a battery be measured and how does the user know when to replace the pack? The correct tool for this task is a battery analyser.

Over the past few years, analysers have gained steady inroads into public safety, hospitals, transport and defence. These organisations have realised the importance of knowing the status of each battery in service.

Because of the finite service life of a rechargeable battery, managers responsible for portable systems commonly service the batteries every 2-3 months and replace them when they fall below 80% capacity.

Battery labelling assists in battery maintenance. This method works by attaching a small label to each battery containing service date, due date and capacity reading.

The system is self-governing because the user will only pick a battery that has properly been serviced and labelled.

Expired batteries are segregated and serviced on an analyser. Packs that check out at 80% or above are relabelled and returned to the field; those that fall below the set target capacity are replaced with new packs or are assigned to less critical applications.

The question is often asked: "Does battery maintenance prolong battery life and save money?". The answer is yes.

With scheduled service, the life of nickel-based batteries can typically be doubled. On lithium-ion and lead acid batteries, the purpose of battery maintenance leans towards verifying performance rather than restoration.

There are three philosophies governing reliability and cost when acquiring batteries. The equipment manufacturer sets very conservative time limits in terms of battery service life. They advise the user to empty only well-known brands and recommend frequent replacements.

The purchasing agent, on the other hand, tries to cut costs by stretching the battery life as long as possible. They say: "If the battery can be used longer, why throw it out while it still has life in it?".

The third philosophy is purchasing a low-cost import, possibly from Asia. Although lower in expenditure, many users have learned that some less-known brand names can exhibit significant variations in performance. So who is right?

Cost-conscious battery users are willing to experiment and a battery analyser can satisfy all three curiosities. The aim is staying in the 80-100% operating range. Cadex analysers feature a life-test program that can cycle a battery to death. The instrument keeps score of the cycle count and stops when the capacity drops to 80% or any other preset capacity threshold.

The analyser also indicates the internal resistance in milliohms. This ensures that the battery has sufficiently high load capability.

Test results can be stored and the performance graphically displayed. One can see how the capacity and internal resistance change with continued cycling. Selecting a lower cost battery with good performance will no longer pose undue risks if its durability can be proven. This will save the organisation money without compromising reliability.

Smart batteries in portable equipment

Increasingly, 'smart batteries' are being infiltrated into high-end equipment. These batteries contain a fuel gauge that shows the state-of-charge of the battery. How accurate is this indication? Not very! Let's examine why the smart pack continues to conceal the battery mystery.

Rather than providing readout of both state-of-health and state-of-charge, the fuel gauge only shows the residual charge. The state-of-health remains unknown. The user does not know if the battery in question has 100% of its rated capacity or only a portion thereof.

If, for example, the battery state-of-health has deteriorated from 100% to 50% and the state-of-charge indicator reads 50%, the actual remaining energy is only 25%. This small energy reserve explains why the battery quits so rapidly after the fuel gauge dips below the 50% state-of-charge mark.

The fuel gauges are not standardised among manufacturers. Furthermore, most systems work by displaying the percentage of the remaining charge without reference to the actual capacity.

Such an indication is very deceptive. A battery may be fully charged but the actual reserve capacity is unknown.

Summary

Charge-and-use without scheduled maintenance does not guarantee reliable batteries. Quality control is essential in keeping a battery fleet healthy and the costs low. Not only does regular battery maintenance prolong the life of nickel-based batteries, the service weeds out the unwanted deadwood.

Detection of fading batteries is relatively simple because the performance decreases gradually over a few months. Only batteries that are allowed to deteriorate too far tend to experience a sudden death.

On routine days, weak batteries can hide comfortably among the good ones. During emergencies and heavy traffic when full performance is needed, however, these non-performers stand out like sore thumbs.

Battery failure during critical moments is not an option. To strengthen the battery fleet, organisations are beginning to take a proactive approach in terms of battery maintenance and record keeping.

The cost savings are apparent. Longer battery life, fewer unexpected downtimes and reduced repair cost are the direct result.

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