Publication date: 08 June 2010
Building on the long running success of its NPC range of batteries, Yuasa has recently introduced a new range of cyclic VRLA batteries. The new TEV range has excellent cyclic ability coupled with an added advantage of higher capacity. Here, Matt Jordan, Technical Services Manager at Yuasa Battery Sales (UK) Ltd, gives a brief overview of these new batteries and how they differ from conventional float/standby batteries.
Cyclic batteries are specially designed for use in applications where the battery is repeatedly charged and discharged. Typical applications include portable equipment, golf trolleys, wheelchairs and mobility scooters and radio controlled toys etc.
These applications are very different from float or standby applications where the battery is kept continuously connected to a specified charge voltage. In this type of application the batteries will only be discharged during maintenance or when the mains supply fails.
Due to the very demanding nature of cyclic applications a different specialist design of plates, paste material and separator AGM (Absorbent Glass Mat) is required.
There are three potential failure modes for cyclic batteries, namely electrolyte stratification, positive material softening and negative plate sulphation.
Electrolyte stratification. During charging, high density (strong) acid will diffuse from plate surfaces. If battery design is not correct then over time the strong acid will stratify to the bottom of the battery case. This strong acid will eventually cause premature plate damage and failure. The likelihood of this happening in cyclic applications is higher due to the amount of time a battery spends being charged from low states of charge.
Positive material softening. The crystalline structure of the positive plates is critical to long term cyclic ability. Due to the constant reversal of the chemical process during charging and discharging, if the structure of the plates is wrong then softening will occur which will lead to premature failure.
Negative plate sulphation. This is mainly caused by leaving batteries for long periods in a discharged state and symptoms of this include an open-circuit voltage lower than the rated value. When a battery is left in a discharged state or for prolonged periods of storage, lead sulphate crystals begin to form acting as a barrier to recharge and preventing normal operation. Depending on the degree of sulphation, a battery may be recovered from this condition by constant current charging at a higher voltage with the current limited to one tenth of the battery capacity for a maximum of 12 hours. It is important to note that the applied voltage will exceed the normal recommendation in which case the battery must be monitored and not left unattended and be removed from charge if excess heat is dissipated. The voltage required to “force” this maximum current into the battery will reduce as the battery recovers until normal charging can take place.
However, in extreme circumstances, a battery may never recover from sulphation and should be replaced.
Yuasa TEV batteries use an advanced combination of the latest materials with state-of-the-art plate barrier prevention techniques and unique separator systems to help ensure suitability for deep discharge use and thus considerably prolong the battery’s service life.
As mentioned previously, batteries left in a discharged state tend to deteriorate more quickly and cyclic batteries tend to be left in such a state more often. The design of Yuasa’s TEV series batteries ensures a low self-discharge rate of as little as 0.1% per day at 20°C, considerably extending the storage period possible.
Correct charging of any VRLA battery is essential in optimising battery performance and life. Although a constant voltage charge should be applied, optimum charging also depends on temperature (nominally 20°C), charge current (maximum ¼ battery capacity) and ripple current (minimum).
For cyclic charging, a higher charging voltage is used compared to standby/float batteries but it is essential that the charger should never be left on indefinitely as it will overcharge and destroy the battery. Excessive charge voltages force a high overcharge current into the battery which will dissipate as heat and may cause gas emission through the safety valve. Within a short period of time, overcharging will corrode the positive plate material and accelerate the battery towards end of life. Under these conditions, the heat produced inside the battery can lead to thermal runaway due to the increased electrochemical reaction within the battery. A battery may swell before failing and will be irrecoverable from this potentially dangerous state.
High temperatures should also to be avoided as excessive heat kills batteries. Elevated temperatures will reduce battery service life, often quite dramatically and in extreme cases can cause thermal runaway as described previously. Low temperatures will help to ensure a longer service life but batteries used in such conditions will have reduced capacity.
Float batteries will have a prescribed float design life of 5 or 10 years whereas a cyclic battery will reach its end-of-life after a prescribed number of cycles. The exact number is dependent on the depth of discharge (DOD) of each cycle such that a reduced DOD will result in an increased number of cycles before end of life. DOD is expressed as the percentage of the battery capacity required per duty cycle and for TEV batteries, which feature a notional design life of 500 cycles at 75% DOD.
Yuasa Battery Sales UK is fully supporting, as a ‘producer’, the new battery directive 2006/66/EC, working in partnership with G and P Batteries and Valpak to implement the requirements of this directive regarding end of life recycling.
At the forefront of lead-acid battery technology since the early part of the 20th century, Yuasa embraces new developments, applying research and investing in modern production plants to meet the increasing demands of a power hungry world. Yuasa batteries are available worldwide through a global network of Yuasa companies and appointed distributors.
Information from: Yuasa Battery Sales (UK) Ltd, Unit 22, Rassau Industrial Estate, Ebbw Vale, Gwent, NP23 5SD. Tel: 08708 500312, Fax: 08708 500317. Email: enquiries@yuasa-sales.co.uk Website: www.yuasa-battery.co.uk
