THE MARKET is being inundated with more and more electronic devices powered by batteries, writes Jesús Miranda.
Self-contained emergency luminaires have been in the market for long time with the very popular NiCd or NiMh batteries. However, Lithium batteries, widely familiar from mobile phones and computer laptops, are now penetrating the emergency lighting arena.
However, different batteries, including chemistry variations of Lithium batteries, behave differently. For this reason, each battery type must be treated differently and therefore manufacturers must have a good understanding of how the battery behaves in order to design a battery management system that protects the battery against premature capacity losses.
There are five key behavioural aspects to consider when designing the battery management system:
- State of Charge (SOC): This indicates the percentage of the battery that is charged. It’s worth remembering that battery capacity may be affected negatively when the SOC is 100 per cent. Lithium Ion batteries, due to their high energy density, don’t need to be charged to 100 per cent to provide three-hour emergency lighting. In fact, with Li-ion batteries it’s recommended that the SOC is less than 100 per cent, typically 80 per cent.
- Over-charge and over-discharge: Different batteries may lose significant capacity if they are over discharged (also known as deep discharge) or over-charged. Manufacturers set up a maximum and a minimum of SOC to protect the battery lifetime. It’s also important to take into consideration the size of the charging current which can damage the battery if it is too high.
- Self-discharge rate: Lithium batteries typically have a slower self-discharge rate (2 to 3 per cent per month) compared to NiCd and NiMh (up to 10 per cent per month) but this depends on battery construction and operating temperatures. Lithium batteries show a self-discharge curve with a downwards angle which facilitates the measurement of the output voltage of the battery, which indicates the SOC. Other batteries such as NiCd and NiMh have a discharge curve that’s flat for a large part of the self-discharge time. The challenge here is to estimate the SOC.
- Charging regime: For batteries with a flat discharge curve such as NiCd, the simple method to charging it to always over-charge the NiCd batteries to ensure the battery is kept at 100 per cent full. This can be a 24/7 trickle charge (common for simple products), or a periodical charge (three minutes an hour or so with high-specification NiMH batteries). The negative consequence of keeping the battery at 100 per cent means that their operating life is not as long as Lithium batteries. Periodical charge is used for lithium ion batteries and with a few minutes of controlled charging current of two-to-three times a day the battery is charged up to the set SOC (typically 80 per cent).
- Storage time: The self-discharge rate plays an important role during storage time. The best way of storing and shipping individual NiCd cells is actually to short it out (0V, therefore an SOC of 0 per cent). It will not be damaged by the self-discharge if it’s not connected to a load. This explains why it’s recommended that NiCd batteries are disconnected during storage time from the electronic PCB as this still consumes energy.
It’s fair to say that there are no perfect batteries. Each has its own advantages and disadvantages depending on the intended application.
Each battery performs correctly and would not fail within its lifetime as long as the battery management system has been purposely designed for the battery technology in use and the emergency luminaire is installed and maintained according to the manufacturer’s instructions.
At Z10 Safety we use Lithium Ion batteries across our portfolio which means we have based the design of the battery management on an on-going long-term ageing capacity test and key behavioural aspects of the batteries.
- Jesús Miranda is managing director of emergency lighting specialists Z10 Safety. For further information please contact firstname.lastname@example.org
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