UPS Battery Charging – Important and not easy
Batteries play a crucial role in UPS installations; providing a source of power for short-duration ride through for a short power failure, or until a back up generator starts up. They can also be linked together into battery strings offering extended run time for longer duration mains power cuts. The most commonly installed UPS battery in use today is VRLA (Valve Regulated Lead Acid).
VRLA batteries incorporate thin plates engineered in parallel to provide the greatest conducting surface possible to deal with high rate currents provided by attached loads. Their design, however, ensures that they have a finite design life of five to ten years for UPS batteries and the way they are charged and maintained can have a significant impact on that. One failed or weak battery in a battery string can compromise the whole UPS system.
As they approach the end of their design life, there is often a need to mix new UPS batteries with old in a battery string and this can cause problems if they are not correctly monitored and maintained.
In a transformer-based UPS, the charging current supplied by the rectifier is load dependent. The UPS can provide additional current for charging where its output power is less than its nominal rating and within the limits of its maximum charging current. However, high recharge currents are known to damage batteries so the rectifier/charger will:
- Limit the maximum current for recharging to 15% of the battery Ah rating.
- Provide 80% of the recharging power with limited current and increased voltage and the remaining 20% at a lower constant voltage, known as float voltage.
This is the classic and recommended way to charge UPS batteries. But many UPS battery failures are caused by over charging, which results in battery dry out. Even a small float overvoltage or high rate charging current can lead to increased gas pressure in the battery, which will result in electrolyte escaping through relief valves. Once it’s escaped, this electrolyte cannot be replaced.
(note: the above is a paragraph taken from The Power Protection Guide - the design, installation and operation of uninterruptible power supplies, which can be purchased from our estore or Amazon UK)
UPS battery chargers rely on a constant temperature (25 degrees centigrade) to provide a constant floating voltage. So the ambient temperature around where the batteries are located must remain constant. In fact, the whole ethos of UPS battery charging is to maintain everything at a constant, constant current, constant voltage, constant temperature.
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