Beware budget battery reliabilty
A
Saft Rechargeable Battery Systems product story
Edited by the Engineeringtalk editorial team
Aug 24, 2004
Saft has issued a warning to lighting manufacturers after laboratory tests of two Chinese-made rechargeable nickel-cadmium batteries showed them to be unsuitable for emergency lighting applications.
Saft has issued a warning to lighting manufacturers after laboratory tests of two Chinese-made rechargeable nickel-cadmium batteries showed them to be unsuitable for emergency lighting applications.
The company became concerned when engineers discovered that the inferior materials in the positive electrode and separator meant that neither battery was able to hold its charge for anything like the required minimum period.
Dave Pattison, National Sales Manager for Saft Rechargeable Battery Systems says: "We understand the pressures that emergency lighting manufacturers face to be competitive".
"Batteries account for up to 30% of the cost of the finished product, and a saving of 20% is certainly appealing".
"Nevertheless, if the batteries chosen do not last or perform as they should real damage will be done to the reputations of lighting manufacturers directly".
"We are also concerned that it will reflect badly on the battery business as a whole".
The batteries concerned are coming into Europe from China and are simply not fit for purpose in emergency lighting.
"They may be adequate for less demanding applications like torches or toys", says Pattison, "but the reliability of emergency lighting where batteries must hold their charge for at least four years is a very different matter".
"Failure at the critical moment could be literally be a matter of life and death".
"What lighting manufacturer would voluntarily take the risk of legal action or having their reputation damaged?".
To establish its case Saft has tested the permanent charge endurance of its own high temperature VT D-size battery against two Chinese-made competitors.
The first test was carried out according to the IEC60285 standard, which includes an ageing period of six months at 70C which equates to use of the battery in the application over four years at 40C.
The Saft cells retained a high level capacity - more than 2.5Ah.
The competitor's model not only showed signs of severe cell degradation, but its capacity had decreased significantly and was down to less than 0.5Ah after 6 months and showed a voltage drop on all discharge curves even after two months of overcharging.
Another "competitor" battery would not even pass the basic requirements of the less severe ICEL test let alone the more demanding requirements of IEC60285.
In two critical areas, the construction of the positive electrode and the separator, the differences are clear and show why these cells are not suitable for use in an emergency light.
At the heart of the batteries that pass the stringent safety tests, like those from Saft, is a sintered positive electrode.
This provides a stable electrical conductor, produces no capacity loss in long storage, copes well with long storage at high temperatures and is mechanically stable in permanent charge.
By contrast the poorer quality products employ foam technology that is not stable and suffers irreversible capacity loss over time.
Compare also the construction of the separator used in the Saft batteries and that used in the inferior products.
The one used by Saft retains a stable permanent charge and provides a strong barrier between the positive and negative electrodes.
The development of this separator has taken Saft many years to perfect.
Poorer quality batteries suffer from oxidation of the fibres at high temperature which, combined with low weight and bad binding, makes them vulnerable to short circuiting.
Pattison concludes: "Reputable battery manufacturers have made huge investments in technology as well as materials".
"These factors make the performance difference and ultimately it is reliability that the users of emergency lighting are paying for".
"If a cheap battery fails to deliver the power when it's needed then it could be the most expensive purchase that you will ever make".
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Contact Saft Rechargeable Battery Systems
Tel +44 1279 772550
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