FACTORS AFFECTING THE LENGTH OF BATTERY LIFE
Temperature High ambient temperature is generally considered to be the most common cause of premature battery failure. The higher the temperature, the quicker the chemical reaction, which increases water loss and corrosion.
Sealed lead-acid batteries have a rated capacity based on an optimum operating temperature of 20- 25°C. It is generally accepted that expected service life will reduce by 50% for every 10°C constant increase above the recommended temperature.
Short-term fluctuations in temperature have little effect on battery life. Adjusting the float voltage depending on the temperature can reduce the effect of higher temperatures, but only marginally. After a UPS runs on battery during a power failure, it recharges so it’s ready for use again – this is known as a discharge/charge cycle.
Every battery is designed with a finite number of discharge and recharge cycles. Each discharge and subsequent recharge slightly reduces the capacity of the battery, in proportion to the depth of discharge – a battery that is only partially discharged can sustain more cycles than one that is fully discharged every time.
Operational voltages Every battery manufacturer states the appropriate charging voltage rates for their specific cell designs. Charging outside of these recommended parameters can cause significant damage and reduce expected service life. Overcharging can lead to excessive hydrogen and oxygen gases. This can cause internal dry out or in extreme cases thermal runaway, which can result in failure or even fire and explosion. Undercharging or too low voltage can cause sulphate crystals to form on the plates, which over time will harden and reduce the battery capacity.
Ripple current AC ripple generated by the rectifier, charger, or inverter is one of the biggest causes of overheating, which speeds up the deterioration of battery poles and will lead to premature failure.
Poor storage of unused batteries Even if UPS batteries sit unused, their lifespan begins to decrease as they automatically discharge small amounts of energy.
If batteries need to be stored for a sustained period, it’s advisable to top-up charge them according to the manufacturer’s guidelines. Storing unused batteries at a temperature of 10°C or below will also help prolong their life.
Period of time batteries are left in a discharged state If the battery is fully discharged to 0%, it is vital not to leave it in this state for any prolonged period as it can result in lasting damage. It is one of the main causes of sulphation (see below), which inhibits recharging and normal battery operation.
Incorrect battery application Because of their specific role, UPS batteries are designed to deliver high rates of energy for a short time – generally a few minutes, although this can be stretched in certain applications. There are other batteries, for example to back up telecoms or switchgear, that are designed to provide several hours’ autonomy.
Using a battery designed for telecoms in a UPS system won’t perform as well as one manufactured specifically for such an application. The UPS topology itself can also impact on battery performance. Uninterruptible power supplies run in battery mode in two scenarios: firstly, if the mains power goes off; secondly, if the mains power input goes out of tolerance (i.e. voltage is too high/low or there is a frequency anomaly when running off a generator).
Some UPS topologies – online UPS – can handle wider input voltage and/or frequency windows, which means that the UPS will only run off the batteries in the case of emergency. This prolongs battery life. If any of the problems highlighted above aren’t promptly detected and resolved, it could kickstart a ‘domino effect’ that speeds up the failure of other batteries within the same system, even if they’re in perfectly good condition. For example, if one battery is overheating, it’s likely to heat the surrounding cells, which could eventually cause them to fail too. Similarly, if a battery’s impedance becomes a problem, the voltage applied to all the other cells within the system may need to increase, speeding up their rate of failure.